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Solutions Manual to accompany Principles of Electronic Materials and Devices Second Edition S.O. Kasap University of Saskatchewan Boston Burr Ridge, IL Dubuque, IA Madison, WI New York San Francisco St. Louis Bangkok Bogotб Caracas Kuala Lumpur Lisbon London Madrid Mexico City Milan Montreal New Delhi Santiago Seoul Singapore Sydney Taipei Toronto Solutions Manual to accompany PRINCIPLES OF ELECTRONIC MATERIALS AND DEVICES, SECOND EDITION S.O. KASAP Published by McGraw-Hill Higher Education, an imprint of The McGraw-Hill Companies, Inc., 1221 Avenue of the Americas, New York, NY 10020. Copyright © The McGraw-Hill Companies, Inc., 2002, 1997. All rights reserved. The contents, or parts thereof, may be reproduced in print form solely for classroom use with PRINCIPLES OF ELECTRONIC MATERIALS AND DEVICES, Second Edition by S.O. Kasap, provided such reproductions bear copyright notice, but may not be reproduced in any other form or for any other purpose without the prior written consent of The McGraw-Hill Companies, Inc., including, but not limited to, network or other electronic storage or transmission, or broadcast for distance learning. www.mhhe.com Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 Second Edition ( 2001 McGraw-Hill) Chapter 1 1.1 The covalent bond Consider the H2 molecule in a simple way as two touching H atoms as depicted in Figure 1Q1-1. Does this arrangement have a lower energy than two separated H atoms? Suppose that electrons totally correlate their motions so that they move to avoid each other as in the snapshot in Figure 1Q1-1. The radius ro of the hydrogen atom is 0.0529 nm. The electrostatic potential energy PE of two charges Q1 and Q2 separated by a distance r is given by Q1Q2/(4πεor). Using the Virial Theorem stated in Example 1.1 (in textbook) consider the following: a. Calculate the total electrostatic potential energy (PE) of all the charges when they are arranged as shown in Figure 1Q1-1. In evaluating the PE of the whole collection of charges you must consider all pairs of charges and, at the same time, avoid double counting of interactions between the same pair of charges. The total PE is the sum of the following: electron 1 interacting with the proton at a distance ro on the left, proton at ro on the right, and electron 2 at a distance 2ro + electron 2 interacting with a proton at ro and another proton at 3ro + two protons, separated by 2ro, interacting with each other. Is this configuration energetically favorable? b. Given that in the isolated H-atom the PE is 2 × (–13.6 eV), calculate the change in PE with respect to two isolated H-atoms. Using the Virial theorem, find the change in the total energy and hence the covalent bond energy. How does this compare with the experimental value of 4.51 eV? Solution 2 e– e– Nucleus Nucleus ro ro 1 Hydrogen Hydrogen Figure 1Q1-1 A simplified view of the covalent bond in H : a snap shot at one instant in time. The electrons correlate their motions and avoid each other as much as possible. 2 a Consider the PE of the whole arrangement of charges shown in the figure. In evaluating the PE of all the charges, we must avoid double counting of interactions between the same pair of charges. The total PE is the sum of the following: Electron 1 interacting with the proton at a distance ro on the left, with the proton at ro on the right and with electron 2 at a distance 2ro + Electron 2 on the far left interacting with a proton at ro and another proton at 3ro + Two protons, separated by 2ro, interacting with each other 1.1 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) PE = − Chapter 1 e2 e2 e2 − + 4 πε o ro 4 πε o ro 4 πε o (2ro ) − e2 e2 − 4 πε o ro 4 πε o 3ro + e2 4 πε o 2ro substituting and calculating we find PE = –1.0176 × 10-17 J or -63.52 eV The negative PE for this particular arrangement indicates that this arrangement of charges is indeed energetically favorable compared with all the charges infinitely separated (PE is then zero). b The potential energy of an isolated H-atom is –2 × 13.6 eV or 27.2 eV. The difference between the PE of the H2 molecule and two isolated H-atoms is, ∆PE = –(63.52 eV) – 2(–27.2) eV = - 9.12 eV We can write the last expression above as changes in the total energy as ∆ E = 12 ∆PE = 12 ( −9.12 eV) = −4.56 eV This is the change in the total energy which is negative. The H2 molecule has lower energy than two H-atoms by 4.56 eV which is the bonding energy. This is very close to the experimental value of 4.51 eV. (Note: We used an ro value from quantum mechanics - so the calculation was not totally classical). 1.2 Ionic bonding and NaCl The interaction energy between Na+ and Cl- ions in the NaCl crystal can be written as 4.03 × 10 −28 6.97 × 10 −96 E(r ) = − + r r8 where the energy is given in joules per ion pair, and the interionic separation r is in meters. Calculate the binding energy and the equilibrium ionic separation in the crystal; include the energy involved in electron transfer from Cl- to Na+. Also estimate the elastic modulus Y of NaCl given that 1 Y≈ 6ro d2E 2 dr r = ro Solution The PE curve for NaCl is given by −4.03 × 10 −28 6.97 × 10 −96 E(r ) = + r r8 where E is the potential energy and r represents interionic separation. We can differentiate this and set it to zero to find the minimum PE, and consequently the minimum (equilibrium) separation (ro). 1.2 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 dE(ro ) =0 dro 1 1 ∴ −55.76 isolating ro: r o = 2.81 × 10-10 m or 2.81 Å 96 10 ro + 4.03 9 28 10 ro 2 =0 The apparent “ionic binding energy” (Eb) for the ions alone, in eV, is: Eb = − −4.03 × 10 −28 6.97 × 10 −96 1 E(ro ) = − + q ro ro 8 q ∴ −4.03 × 10 −28 6.97 × 10 −96 1 Eb = − + 8 −19 −10 −10 J/eV m ) (2.81 × 10 m ) 1.602 × 10 (2.81 × 10 ∴ E b = 7.83 eV Note however that this is the energy required to separate the Na+ and Cl- ions in the crystal and then to take the ions to infinity, that is to break up the crystal into its ions. The actual bond energy involves taking the NaCl crystal into its constituent neutral Na and Cl atoms. We have to transfer the electron from Cl- to Na+. The energy for this transfer, according to Figure 1Q2-1, is -1.5 eV (negative represents energy release). Thus the bond energy is 7.83 - 1.5 = 6.33 eV as in Figure 1Q2-1. Potential energy E(r), eV/(ion-pair) 6 Cl –6 –6.3 Cl – r=∞ 1.5 eV 0.28 nm Cohesive Energy 0 – Cl r=∞ + Na Separation, r Na + Na ro = 0.28 nm Figure 1Q2-1 Sketch of the potential energy per ion pair in solid NaCl. Zero energy corresponds to neutral Na and Cl atoms infinitely separated. If r is defined as a variable representing interionic separation, then Young’s modulus is given by: Y= 1 d dE(r ) 6r dr dr ∴ −8.06 1 + 501.84 1 1 10 28 r 3 10 96 r10 Y= r 6 ∴ Y = 8.364 × 10 −95 1 1 − 1.3433 × 10 −28 4 11 r r 1.3 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 Substituting the value for equilibrium separation (ro) into this equation (2.81 × 10-10 m), Y = 7.54 × 1010 Pa = 75 GPa out. This value is somewhat larger than about 40 GPa in Table 1.2 (in the textbook), but not too far *1.3 van der Waals bonding Below 24.5 K, Ne is a crystalline solid with an FCC structure. The interatomic interaction energy per atom can be written as σ 6 σ 12 E(r ) = −2ε 14.45 − 12.13 r r (eV/atom) where ε and σ are constants that depend on the polarizability, the mean dipole moment, and the extent of overlap of core electrons. For crystalline Ne, ε = 3.121 × 10-3 eV and σ = 0.274 nm. a. Show that the equilibrium separation between the atoms in an inert gas crystal is given by ro = (1.090)σ. What is the equilibrium interatomic separation in the Ne crystal? b. Find the bonding energy per atom in solid Ne. c. Calculate the density of solid Ne (atomic mass = 20.18 g/mol). Solution a Let E = potential energy and x = distance variable. The energy E is given by 6 12 σ σ E( x ) = −2ε 14.45 − 12.13 x x The force F on each atom is given by 11 5 σ σ σ σ x x dE( x ) 86 7 F( x ) = − = 2ε 145.56 − . dx x2 x2 σ 12 σ6 F( x ) = 2ε 145.56 13 − 86.7 7 x x When the atoms are in equilibrium, this net force must be zero. Using ro to denote equilibrium separation, ∴ F(ro ) = 0 ∴ σ 12 σ6 2ε 145.56 13 − 86.7 7 = 0 ro ro ∴ 145.56 σ 12 σ6 = 86 . 7 ro13 ro 7 1.4 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ ro13 145.56 σ 12 = ro 7 86.7 σ 6 ∴ r o = 1.090σ Chapter 1 For the Ne crystal, σ = 2.74 × 10-10 m and ε = 0.003121 eV. Therefore, r o = 1.090(2.74 × 10-10 m) = 2.99 × 10-10 m for Ne. b Calculate energy per atom at equilibrium: 6 12 σ σ E(ro ) = −2ε 14.45 − 12.13 ro ro 6 2.74 × 10 -10 m 14 . 45 2.99 × 10 -10 m J/eV) 12 -10 −12.13 2.74 × 10 m 2.99 × 10 -10 m ∴ E(ro ) = −2(0.003121 eV)(1.602 × 10 −19 ∴ E (r o ) = -4.30 × 10-21 J or -0.0269 eV Therefore the bonding energy in solid Ne is 0.027 eV per atom. c To calculate the density, remember that the unit cell is FCC, and density = (mass of atoms in the unit cell) / (volume of unit cell). There are 4 atoms per FCC unit cell, and the atomic mass of Ne is 20.18 g/mol. 2R a a a (a) FCC Unit Cell a (b) (c) Figure 1Q3-1 (a) The crystal structure of copper which is face-centered-cubic (FCC). The atoms are positioned at well-defined sites arranged periodically, and there is a long-range order in the crystal. (b) An FCC unit cell with close-packed spheres. (c) Reduced-sphere representation of the FCC unit cell. Examples: Ag, Al, Au, Ca, Cu, γ-Fe (>912 °C), Ni, Pd, Pt, Rh. Since it is an FCC crystal structure, let a = lattice parameter (side of cubic cell) and R = radius of atom. The shortest interatomic separation is ro = 2R (atoms in contact means nucleus to nucleus separation is 2R (see Figure 1Q3-1). R = ro/2 and 2a2 = (4R)2 1.5 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ r a = 2 2 R = 2 2 o = 2 (2.99 × 10 −10 m ) 2 ∴ a = 4.228 × 10-10 m Chapter 1 Therefore, the volume (V) of the unit cell is: V = a3 = (4.228 × 10-10 m)3 = 7.558 × 10-29 m3 The mass (m) of 1 Ne atom in grams is the atomic mass (Mat) divided by NA, because NA number of atoms have a mass of Mat. m = Mat / NA ∴ m= (20.18 g/mol)(0.001 kg/g) = 3.351 × 10 −26 kg 23 -1 6.022 × 10 mol There are 4 atoms per unit cell in the FCC cell. The density (ρ) can then be found by: ρ = (4m) / V = [4 × (3.351 × 10-26 kg)] / (7.558 × 10-29 m3) ρ = 1774 kg/m3 ∴ In g/cm3 this density is: 1774 kg/m 3 3 ρ= 3 × (1000 g/kg) = 1.77 g/cm (100 cm/m) The density of solid Ne is 1.77 g cm-3. 1.4 Kinetic Molecular Theory Calculate the effective (rms) speeds of the He and Ne atoms in the He-Ne gas laser tube at room temperature (300 K). Solution Gas atoms Figure 1Q4-1 The gas molecules in the container are in random motion. 1.6 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 Concave mirror (Reflectivity = 0.985) Flat mirror (Reflectivity = 0.999) Very thin tube Laser beam He-Ne gas mixture Current regulated HV power supply Figure 1Q4-2 The He-Ne gas laser. To find the root mean square velocity (vrms) of He atoms at T = 300 K: The atomic mass of He is (from Periodic Table) Mat = 4.0 g/mol. Remember that 1 mole has a mass of Mat grams. Then one He atom has a mass (m) in kg given by: 4.0 g/mol Mat = × (0.001 kg/g) = 6.642 × 10 −27 kg N A 6.022 × 10 23 mol −1 m= From kinetic theory: 1 3 2 m(vrms ) = kT 2 2 1.381 × 10 -23 J K -1 )(300 K ) ( kT = 3 = 3 m (6.642 × 10-27 kg) ∴ vrms ∴ v rms = 1368 m/s The root mean square velocity (vrms) of Ne atoms at T = 300 K can be found using the same method as above, changing the atomic mass to that of Ne, Mat = 20.18 g/mol. After calculations, the mass of one Ne atom is found to be 3.351 × 10-26 kg, and the root mean square velocity (vrms) of Ne is found to be v rms = 609 m/s. *1.5 Vacuum deposition Consider air as composed of nitrogen molecules N2. a. What is the concentration n (number of molecules per unit volume) of N 2 molecules at 1 atm and 27 °C? b. Estimate the mean separation between the N2 molecules. c. Assume each molecule has a finite size that can be represented by a sphere of radius r. Also assume that l is the mean free path, defined as the mean distance a molecule travels before colliding with another molecule, as illustrated in Figure 1Q5-1a. If we consider the motion of one N 2 molecule, with all the others stationary, it is apparent that if the path of the traveling molecule crosses the crosssectional area S =π(2r)2, there will be a collision. Since l is the mean distance between collisions, there must be at least one stationary molecule within the volume S l , as shown in Figure 1Q5-1a. Since n is the concentration, we must have n(Sl) = 1 or l =1/(π4r2n). However, this must be corrected for the fact that all the molecules are in motion, which only introduces a numerical factor, so that 1.7 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 1 2 4πr 2 n Assuming a radius r of 0.1 nm, calculate the mean free path of N2 molecules between collisions at 27 °C and 1 atm. d. Assume that an Au film is to be deposited onto the surface of an Si chip to form metallic interconnections between various devices. The deposition process is generally carried out in a vacuum chamber and involves the condensation of Au atoms from the vapor phase onto the chip surface. In one procedure, a gold wire is wrapped around a tungsten filament, which is heated by passing a large current through the filament (analogous to the heating of the filament in a light bulb) as depicted in Figure 1Q5-1b. The Au wire melts and wets the filament, but as the temperature of the filament increases, the gold evaporates to form a vapor. Au atoms from this vapor then condense onto the chip surface, to solidify and form the metallic connections. Suppose that the source (filament)-to-substrate (chip) distance L is 10 cm. Unless the mean free path of air molecules is much longer than L, collisions between the metal atoms and air molecules will prevent the deposition of the Au onto the chip surface. Taking the mean free path l to be 100L, what should be the pressure inside the vacuum system? (Assume the same r for Au atoms). l= 1/ 2 S= (2r)2 Semiconductor Any molecule with center in S gets hit. (a) v Metal film Evaporated metal atoms (b) Molecule Molecule Vacuum Hot filament Vacuum pump Figure 1Q5-1 (a) A molecule moving with a velocity v travels a mean distance l between collisions. Since the collision cross-sectional area is S, in the volume Sl there must be at least one molecule. Consequently, n(Sl) = 1. (b) Vacuum deposition of metal electrodes by thermal evaporation. Solution Assume T = 300 K throughout. The radius of the nitrogen molecule (given approximately) is r = 0.1 × 10-9 m. Also, we know that pressure P = 1 atm = 1.013 × 105 Pa. Let N = total number of molecules , V = volume and k = Boltzmann’s constant. Then: PV = NkT (Note: this equation can be derived from the more familiar form of PV = ηRT, where η is the total number of moles, which is equal to N/NA, and R is the gas constant, which is equal to k × NA.) The concentration n (number of molecules per unit volume) is defined as: n = N/V Substituting this into the previous equation, the following equation is obtained: P = nkT a What is n at 1 atm and T = 27 °C + 273 = 300 K? 1.8 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 n = P/(kT) ∴ 1.013 × 10 5 Pa n= (1.381 × 10-23 J K-1 )(300 K) ∴ n = 2.45 × 1025 molecules per m3 b What is the mean separation between the molecules? Consider a crystal of the material which is a cube of unit volume, each side of unit length as shown in Figure 1Q5-2. To each atom we can attribute a portion of the whole volume which for simplicity is a cube of side d. Thus, each atom is considered to occupy a volume of d3. Volume of crystal = 1 Length = 1 Length = 1 Length = 1 d d Each atom has this portion of the whole volume. This is a cube of side d. . d Interatomic separation = d Figure 1Q5-2 Relationship between interatomic separation and the number of atoms per unit volume. The actual or true volume of the atom does not matter. All we need to know is how much volume an atom has around it given all the atoms are identical and that adding all the atomic volumes must give the whole volume of the crystal. Suppose that there are n atoms in this crystal. Then n is the atomic concentration, number of atoms per unit volume. Clearly, n atoms make up the crystal so that n d 3 = Crystal volume = 1 Remember that this is only an approximation. The separation between any two atoms is d. Thus, d= 1 n ∴ c d = 1 3 (2.45 × 10 25 m −3 ) d = 3.44 × 10-9 m or 3.4 nm Assuming a radius, r, of 0.1 nm, what is the mean free path, l, between collisions? l= ∴ 1 3 1 = 2 ⋅ 4πr 2 n 2 ⋅ 4π (0.1 × 10 1 −9 m ) (2.45 × 10 25 m −3 ) 2 l = 2.30 × 10-7 m or 230 nm We need the new mean free path, l ′= 100L, or 0.1 m × 100 (L is the source-to-substrate distance) l = 10 m 1.9 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 The new l ′ corresponds to a new concentration n′ of nitrogen molecules. 1 2 ⋅ 4πr 2 n′ l′ = ∴ n′ = 1 2 1 2 = 5.627 × 1017 m-3 = 2 2 − 9 8 πr l′ 8 π (0.1 × 10 m ) (10 m ) This new concentration of nitrogen molecules requires a new pressure, P′: P′ = n′kT = (5.627 × 1017 m-3)(1.381 × 10-23 J K-1)(300 K) = 0.00233 Pa In atmospheres this is: 0.00233 Pa = 2.30 × 10 −8 atm 1.013 × 10 5 Pa/atm In units of torr this is: P′ = P ′ = (2.30 × 10 −8 atm )(760 torr/atm) = 1.75 × 10 −5 torr There is an important assumption made, namely that the cross sectional area of the Au atom is about the same as that of N2 so that the expression for the mean free path need not be modified to account for different sizes of Au atoms and N2 molecules. The calculation gives a magnitude that is quite close to those used in practice , e.g. a pressure of 10-5 torr. 1.6 Heat capacity a. Calculate the heat capacity per mole and per gram of N 2 gas, neglecting the vibrations of the molecule. How does this compare with the experimental value of 0.743 J g-1 K-1? b. Calculate the heat capacity per mole and per gram of CO2 gas, neglecting the vibrations of the molecule. How does this compare with the experimental value of 0.648 J K-1 g-1? Assume that CO2 molecule is linear (O-C-O), so that it has two rotational degrees of freedom. c. Based on the Dulong-Petit rule, calculate the heat capacity per mole and per gram of solid silver. How does this compare with the experimental value of 0.235 J K-1 g-1? d. Based on the Dulong-Petit rule, calculate the heat capacity per mole and per gram of the silicon crystal. How does this compare with the experimental value of 0.71 J K-1 g-1? Solution a N2 has 5 degrees of freedom: 3 translational and 2 rotational. Its molar mass is Mat = 2 × 14.01 g/mol = 28.02 g/mol. Let Cm = heat capacity per mole, Cs = specific heat capacity (heat capacity per gram), and R = gas constant, then: Cm = ∴ 5 5 R = (8.315 J K -1 mol -1 ) = 20.8 J K −1 mol −1 2 2 C s = C m/ Mat = (20.8 J K-1 mol-1)/(28.02 g/mol) = 0.742 J K-1 g-1 This is close to the experimental value. b CO2 has the linear structure O=C=O. Rotations about the molecular axis have negligible rotational energy as the moment of inertia about this axis is negligible. There are therefore 2 rotational degrees of 1.10 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 freedom. In total there are 5 degrees of freedom: 3 translational and 2 rotational. Its molar mass is Mat = 12.01 + 2 × 16 = 44.01 g/mol. Cm = ∴ 5 5 R = (8.315 J K −1 mol −1 ) = 20.8 J K −1 mol −1 2 2 Cs = Cm/ Mat = (20.8 J K-1 mol-1)/(44.01 g/mol) = 0.47 J K-1 g-1 This is smaller than the experimental value 0.648 J K-1 g-1, because vibrational energy was neglected in the 5 degrees of freedom assigned to the CO2 molecule. c For solid silver, there are 6 degrees of freedom: 3 vibrational KE and 3 elastic PE terms. Its molar mass isMat = 107.87 g/mol. Cm = ∴ 6 6 R = (8.315 J K −1 mol −1 ) = 24.9 J K −1 mol −1 2 2 C s = C m/ Mat = (24.9 J K-1 mol-1)/(107.87 g/mol) = 0.231 J K-1 g-1 This is very close to the experimental value. For a solid, heat capacity per mole is 3R. The molar mass of Si is Mat = 28.09 g/mol. d Cm = ∴ 6 6 R = (8.315 J K −1 mol −1 ) = 24.9 J K −1 mol −1 2 2 C s = C m/ Mat = (24.9 J K-1 mol-1)/(28.09 g/mol) = 0.886 J K-1 g-1 The experimental value is substantially less and is due to the failure of classical physics. One has to consider the quantum nature of the atomic vibrations and also the distribution of vibrational energy among the atoms. The student is referred to modern physics texts (under heat capacity in the Einstein model and the Debye model of lattice vibrations). 1.7 Thermal expansion a. If λ is the thermal expansion coefficient, show that the thermal expansion coefficient for an area is 2λ. Consider an aluminum square sheet of area 1 cm2. If the thermal expansion coefficient of Al at room temperature (25 °C) is about 24 × 10-6 K-1, at what temperature is the percentage change in the area +1%? b. The density of silicon at 25 °C is 2.329 g cm-3. Estimate the density of silicon at 1000 °C given that the thermal expansion coefficient for Si over this temperature range has a mean value of about 3.5 × 10-6 K-1. c. The thermal expansion coefficient of Si depends on temperature as, λ = 3.725 × 10 −6 {1 − exp[ −0.00588(T − 124)]} + 5.548 × 10 −10 T where T is in Kelvins. The change δρ in the density due to a change δT in temperature is given by δρ = − ρ0α V δT = −3ρ0 λδT By integrating this equation , calculate the density of Si at 1000 °C and compare with the result in b. Solution a Consider an rectangular area with sides xo and yo. Then at temperature T0, 1.11 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 A0 = x0 y0 and at temperature T, A = [ x0 (1 + λ ∆T )][ y0 (1 + λ ∆T )] = x0 y0 (1 + λ ∆T ) 2 that is [ ] A = x0 yo 1 + 2 λ ∆T + (λ ∆T ) . 2 We can now use that A0 = x0 y0 and neglect the term (λ ∆T ) because it is very small in comparison with the linear term λ ∆T (λ<<1) to obtain 2 A = A0 (1 + 2 λ ∆T ) = A0 (1 + α A ∆T ) So the area expansion coefficient is α A = 2λ The area of the aluminum sheet at any temperature is given by [ ] A = A0 1 + 2 λ (T − T0 ) where Ao is the area at the reference temperature T0 . Solving for T we receive T = T0 + 1 A / A0 − 1 1 (1.01) − 1 = 25 o C + = 233.3 °C . 2 2 24 × 10 −6 o C −1 λ b. As it is shown in Example 1.6 (in textbook) the volume expansion with temperature leads to density reduction ρ= ρ0 ≈ ρ0 1 − 3λ (T − T0 ) = 1 + 3λ (T − T0 ) [ ] [ ] = (2.329 g cm −3 ) 1 − 3(3.5 × 10 −6 o C −1 )(1000 o C − 25 o C) = 2.305 g cm- 3 c. By integrating the equation we receive ρ T ρo T0 ∫ dρ = −3ρ0 ∫ λ (T )dT which gives the following relation T ρ = ρ0 1 − 3 ∫ λ dT To Substituting 300 K for T0, 1273 K for T and λ with the given expression, and carrying out the integration, which is straightforward, we receive ρ(1273 K ) = 2.302 g cm −3 which is very close to the value in b. * 1 . 8 Thermal fluctuations The cross section of a typical moving-coil ammeter is shown in Figure 1Q8-1. The rectangular moving coil (extended into the paper) is placed in the air gap between the poles of a permanent magnet and a 1.12 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 fixed, cylindrical iron core. The iron core ensures a strong magnetic field through the coil. The coil is hinged to rotate in the air gap and around the iron core. When a current I is passed through the coil, it experiences a torque and tries to rotate, but the rotation is restricted by the spring. A needle attached to coil indicates the amount of rotation. The deflection θ of the needle is proportional to the torque τ, by virtue of Hooke's Law θ = Kτ where, from electromagnetic theory, the torque is proportional to the product of the current I and the magnetic field B. Thus, θ measures the current I. A given ammeter has a coil inductance of 0.1 mH and a coil resistance of 100 Ω. The spring constant K is 1011 rad N-1 m-1 and the length of the needle is 10 cm. a. What is the rms fluctuation in the position of the ammeter needle? b. Assume the ammeter is placed in a closed circuit where the dc current is zero (I = 0), as shown in Figure 1Q8-1. Sketch the instantaneous current i(t) versus time. What is the minimum current this ammeter can measure (noise in the current)? How can this be improved? Solution We can assume a room temperature of T = 300 K, and we are given the ammeter’s characteristics. The spring constant (K) in this case relates θ and τ (torque) by the equation: θ = Kτ Spring constant: K = 1011 N-1 m-1 (Remember that rads are dimensionless units, and do not need to be carried through calculations) Length of pointer: d = 0.1 m a Inductance of coil: L = 0.0001 H θ is the rms (root mean square) value of the angle fluctuations. It must be in radians. The average energy fluctuations in the spring must be of the order of (1/2)kT. 1 1 2 1 θ = kT 2K 2 ∴ θ = TKk = (300 K )(1011 N −1 m −1 )(1.381 × 10 −23 J K −1 ) ∴ θ = 2.04 × 10-5 rad As the needle rotates randomly left and right about its equilibrium position, its tip is displaced randomly. These random fluctuations in the displacement have an rms value of (where d is the length of the needle): xrms = θ d = (2.04 × 10-5 rad)(0.10 m/rad) = 2.04 × 10-6 m or 2 µ m Certainly this extent is visible under a microscope, so if you tried to measure the position of the needle accurately by focusing a microscope on it to obtain the current accurately, you’d be wasting time. 1.13 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 R (Resistance of the coil) 0 i(t) Needle Scale Fixed iron core Permanent magnet 0.1 mH Moving Coil I=0 Ammeter N S Spring Figure 1Q8-1 Left: The ammeter. Right: The ammeter short circuited. It should be reading a zero current at all times (short circuit). But is it? b Suppose that the ammeter is short circuited as in Figure 1Q8-1 and the current through the ammeter is monitored by an invisible, non-intrusive, second noiseless ammeter. Let i be the instantaneous current in the circuit. Its rms value is then irms. The average energy fluctuation due to inductance in the coil must be in the order of (1/2)kT: 1 1 Lirms 2 = kT 2 2 (300 K )(1.381 × 10-23 J K -1 ) Tk = L (0.0001 H) ∴ irms = ∴ i rms = 6.44 × 10-9 A or 6 nA Noise in the current measurements is 6 nA. If you were to examine the current in the circuit you would see the following: i(t) iaverage = 0 irms = 6 nA t Figure 1Q8-2 Noise fluctuations of current in ammeter. Assumption: When we considered the random fluctuations, we assumed that the mechanical motion containing the spring (energy storage element) was decoupled from the electrical circuit containing the inductance (energy storage element). An exact analysis is quite difficult but this first order treatment provides rough values on the limits of the system. We should calculate how much current, denoted in, is needed to provide a deflection that is equal to xrms. For this approach we have to know more about the electromagnetic operation of the ammeter coil. For a more rigorous calculation see Electricity and Magnetism, Third Edition, B. I. Bleaney and B. Bleaney (Oxford University Press, Oxford, 1976), Ch. 23. 1.14 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 1.9 Electrical noise Consider an amplifier with a bandwidth B of 5 kHz, corresponding to a typical speech bandwidth. Assume the input resistance of the amplifier is 1 MΩ. What is the rms noise voltage at the input? What will happen if the bandwidth is doubled to 10 kHz? What is your conclusion? Solution Bandwidth: B = 5 × 103 Hz Input resistance: Rin = 106 Ω Assuming room temperature: T = 300 K The noise voltage (or the rms voltage, vrms) across the input is given by: vrms = 4 kTRin B = 4(1.381 × 10 -23 J K -1 )(300 K )(10 6 Ω)(5 × 10 3 Hz) ∴ v rms = 9.10 × 10-6 V or 9.1 µ V If the bandwidth is doubled: B′ = 2 × 5 × 103 = 10 × 103 Hz ∴ vrms ′ = 4kTRin B′ = 1.29 × 10 −5 V or 12.9 µV The larger the bandwidth, the greater the noise voltage. 5-10 kHz is a typical speech bandwidth and the input signal into the amplifier must be much greater than ∼13 µV for amplification without noise added from the amplifier itself. Voltage, v(t) vrms Time Figure 1Q9-1 Random motion of conduction electrons in a conductor results in electrical noise. 1.10 Thermal activation A certain chemical oxidation process (e.g., SiO2) has an activation energy of 2 eV atom-1. a. Consider the material exposed to pure oxygen gas at a pressure of 1 atm. at 27 °C. Estimate how many oxygen molecules per unit volume will have energies in excess of 2 eV? (Consider numerical integration of Equation 1.17, listed in textbook) b. If the temperature is 900 °C, estimate the number of oxygen molecules with energies more than 2 eV. What happens to this concentration if the pressure is doubled? 1.15 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 Solution 1: Method of Estimation The activation energy (EA) of one atom is 2 eV/atom, or: EA = (2 eV/atom)(1.602 × 10-19 J/eV) = 3.204 × 10-19 J/atom a We are given pressure P = 1 atm = 1.013 × 105 Pa and temperature T = 300 K. If we consider a portion of oxygen gas of volume V = 1 m3 (the unit volume), the number of molecules present in the gas (N) will be equal to the concentration of molecules in the gas (n), i.e.: N = n. And since we know η = N/NA, where η is the total number of moles and NA is Avogadro’s number, we can make the following substitution into the equation PV = ηRT: PV = isolating n, n RT NA 23 −1 5 3 N A PV (6.022 × 10 mol )(1.013 × 10 Pa )(1 m ) 1 n= = m3 RT ( 8.315 m3 Pa K-1 mol-1 )(300 K) ∴ n = 2.445 × 1025 m-3 Therefore there are 2.445 × 1025 oxygen molecules per unit volume. For an estimation of the concentration of molecules with energy above 2 eV, we can use the following approximation (remember to convert EA into Joules). If nA is the concentration of molecules with E > EA, then: nA E = exp − A kT n 3.204 × 10 −19 J ) ( EA 25 −3 nA = n exp − = (2.445 × 10 m ) exp − ∴ -23 -1 kT (1.381 × 10 J K )(300 K ) ∴ n A = 6.34 × 10-9 m- 3 However, this answer is only in the right order of magnitude. For a better calculation we need to use a numerical integration of n(E) from EA to ∞. b At T = 900 °C + 273 = 1173 K and P = 1 atm, the same method as above can be used to find the concentration of molecules with energy greater than EA. After calculations, the following numbers will be obtained: n = 6.254 × 1024 m-3 n A = 1.61 × 1016 m- 3 This corresponds to an increase by a factor of 1025 compared to a temperature of T = 300 K. Doubling the pressure doubles n and hence doubles n A. In the oxidation of Si wafers, high pressures lead to more rapid oxidation rates and a shorter time for the oxidation process. Solution 2: Method of Numerical Integration To find the number of molecules with energies greater than EA = 2 eV more accurately, numerical integration must be used. Suppose that N is the total number of molecules. Let y = nE/N where nE is the number of molecules per unit energy, so that nE dE is the number of molecules in the energy range dE. 1.16 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 Define a new variable x for the sake of convenience: x= ∴ E kT E = Tkx where E is the energy of the molecules. Using partial differentiation: dE = Tk dx The energy distribution function is given by: 3 1 E 2 1 2 y = 1 E 2 exp − kT kT π2 3 substitute: 1 2 exp( − x ) y = 2 kT Tkx π exp( − x ) x π Tk Integration of y with respect to E can be changed to that with respect to x as follows: simplify: y=2 ∫ ydE = 2 ∴ ∫ ydE = 2 ∫ exp(− x ) xdE ∫ exp(− x ) xdx π Tk π We need the lower limit xA for x corresponding to EA: (substitute dE = Tk dx) EA 3.204 × 10 -19 J xA = = = 77.36 kT (1.381 × 10 -23 J K -1 )(300 K ) This is the activation energy EA in terms of x. We also need the upper limit, xb which should be ∞, but we will take it to be a multiple of xA: xb = 2 × xA = 154.72 We do this because the numerical integration is difficult with very small numbers, e.g. exp(-xA) = 2.522 × 10-34. The fraction, F, of molecules with energies greater than EA (= xA) is: ∞ F= xB exp( − x ) x −33 dx = 2.519 × 10 π xA ∫ ydE = ∫ 2 EA At T = 300 K, P = 1 atm = 1.013 × 105 Pa, and V = 1 m3, the number of molecules per unit volume is n: n PV = RT NA 1.17 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Chapter 1 N A PV = 2.445 × 10 25 m −3 RT The concentration of molecules with energy greater than EA (nA) can be found using the fraction F : n= n A = nF = (2.445 × 1025 m-3)(2.519 × 10-33 ) = 6.16 × 10-8 m-3 If we estimate nA by multiplying n by the Boltzmann factor as previously: E nA = n exp − A = 6.34 × 10 −9 m −3 kT The estimate is out by a factor of about 10. b The concentration of molecules with energy greater than EA (nE) can be found at T = 900 °C + 273 = 1173 K using the same method as in part a. After calculations, the following values will be obtained: F = 1.3131 × 10-8 n = 6.254 × 1024 m-3 n A = 8.21 × 1016 m- 3 If we compare this value to the one obtained previously through estimation (1.61 × 1016 m-3), we see the estimate is out by a factor of about 5. As stated previously, doubling the pressure doubles n and hence doubles nA. In the oxidation of Si wafers, high pressures lead to more rapid oxidation rates and a shorter time for the oxidation process. 1.11 Diffusion in Si The diffusion coefficient of boron (B) atoms in a single crystal of Si has been measured to be 1.5 ×10 -18 m2 s-1 at 1000 °C and 1.1 ×10 -16 m2 s-1 at 1200 °C. a. What is the activation energy for the diffusion of B, in eV/atom? b. What is the preexponential constant Do? c. What is the rms distance (in micrometers) diffused in 1 hour by the B atom in the Si crystal at 1200 °C and 1000 °C? d. The diffusion coefficient of B in polycrystalline Si has an activation energy of 2.4-2.5 eV atom-1 and Do = (1.5-6) × 10-7 m2 s -1. What constitutes the diffusion difference between the single crystal sample and the polycrystalline sample? Solution Given diffusion coefficients at two temperatures: T1 = 1200 °C + 273 = 1473 K D1 = 1.1 × 10-16 m2/s T2 = 1000 °C + 273 = 1273 K D2 = 1.5 × 10-18 m2/s a The diffusion coefficients (D1 and D2) at certain temperatures (T1 and T2) are given by: E q D1 = Do exp − A kT1 E q D2 = Do exp − A kT2 where EA is the activation energy in eV/atom. Since we know the following: exp( −u) = exp( w − u) exp( − w ) 1.18 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 we can take the ratio of the diffusion coefficients to express them in terms of the activation energy (EA): E q Do exp − A q[T − T ]E E q E q kT1 D1 = = exp A − A = exp 1 2 A kT1 T1T2 k D2 E q kT2 Do exp − A kT2 D T1T2 k ln 1 D2 EA = q(T1 − T2 ) ∴ ∴ EA = ∴ E A = 3.47 eV/atom b 1.1 × 10 −16 m 2 /s 1.5 × 10 −18 m 2 /s J/eV)(1473 K − 1273 K ) (1473 K )(1273 K )(1.381 × 10 −23 J K −1 ) ln (1.602 × 10 −19 To find Do, use one of the equations for the diffusion coefficients: E q D1 = Do exp − A kT1 (1.1 × 10 −16 m 2 /s) D1 = E q (3.47 eV)(1.602 × 10 −19 J/eV) exp − A exp − −23 kT1 J K −1 )(1473 K ) (1.381 × 10 ∴ Do = ∴ D 0 = 8.12 × 10-5 m2 / s Given: time (t) = (1 hr) × (3600 s/hr) = 3600 s c At 1000 °C, rms diffusion distance (L1000 °C) in time t is given by: L 1000 °C = 2( D2 t ) = 2(1.5 × 10 −18 m 2 /s)(3600 s) L 1000 °C = 1.04 × 10-7 m or 0.104 µ m ∴ At 1200 °C: L 1200 °C = = 2( D1t ) = 2(1.1 × 10 −16 m 2 /s)(3600 s) ∴ L 1200 °C = 8.90 × 10-7 m or 0.89 µ m (almost 10 times longer than at 1000 °C) d Diffusion in polycrystalline Si would involve diffusion along grain boundaries, which is easier than diffusion in the bulk. The activation energy is smaller because it is easier for an atom to break bonds and jump to a neighboring site; there are vacancies or voids and strained bonds in a grain boundary. 1.12 Diffusion in SiO2 The diffusion coefficient of P atoms in SiO2 has an activation energy of 2.30 eV atom-1 and Do = 5.73 × 10-9 m2 s-1. What is the rms distance diffused in one hour by P atoms in SiO2 at 1200 °C? 1.19 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 Solution Given: Temperature: T = 1200 °C + 273 = 1473 K Damping constant: Do = 5.73 × 10-9 m2/s Activation energy: EA = 2.30 eV/atom Time: t = (1 hr) × (3600 s/hr) = 3600 s Using the following equation, find the diffusion coefficient D: E q D = Do exp − A kT ∴ (2.30 eV)(1.602 × 10 −19 J/eV) D = (5.73 × 10 −9 m 2 /s) exp − −23 J K −1 )(1473 K ) (1.381 × 10 ∴ D = 7.793 × 10-17 m2/s The rms diffusion distance (L1200 °C) is given as: L 1200 °C = 2( Dt ) = 2(7.793 × 10 −17 m 2 /s)(3600 s) L 1200 °C = 7.49 × 10-7 m or 0.75 µ m ∴ Comment: The P atoms at 1200 °C seems to be able to diffuse through an oxide thickness of about 0.75 µm. 1.13 BCC and FCC Crystals a. Molybdenum has the BCC crystal structure, has a density of 10.22 g cm-3 and an atomic mass of 95.94 g mol-1. What is the atomic concentration, lattice parameter a, and atomic radius of molybdenum? b. Gold has the FCC crystal structure, a density of 19.3 g cm-3 and an atomic mass of 196.97 g mol-1. What is the atomic concentration, lattice parameter a, and atomic radius of gold? Solution a. Since molybdenum has BCC crystal structure, there are 2 atoms in the unit cell. The density is ρ= Mass of atoms in unit cell Volume of unit cell = ( Number of atoms in unit cell) × (Mass of one atom) M 2 at NA that is, ρ= . a3 Solving for the lattice parameter a we receive 1.20 Volume of unit cell Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) a=3 Chapter 1 2 (95.94 × 10 −3 kg mol −1 ) 2 Mat =3 = 3.147 × 10-10 m −3 −1 3 23 ρ NA (10.22 × 10 kg m )(6.022 × 10 mol ) The Atomic concentration is 2 atoms in a cube of volume a3, i.e. nat = 2 2 22 = cm-3 = 6.415 × 1028 m- 3 3 = 6.415 × 10 3 − 10 a (3.147 × 10 m) For a BCC cell, the lattice parameter a and the radius of the atom R are in the following relation (listed in Table 1.3 in the textbook): R= a 3 4 i.e. R= (3.147 × 10 −10 m) 3 4 = 1.363 × 10-10 m b. Gold has the FCC crystal structure, hence, there are 4 atoms in the unit cell (as shown in Table 1.3 in the textbook). The lattice parameter a is a=3 4 (196.97 × 10 −3 kg mol −1 ) 4 Mat 3 = = 4.077 × 10-10 m −3 −1 3 23 ρ NA (19.3 × 10 kg m )(6.022 × 10 mol ) The atomic concentration is nat = 4 4 22 = cm-3 = 5.901 × 1028 m- 3 3 = 5.901 × 10 3 −10 a (4.077 × 10 m) For an FCC cell, the lattice parameter a and the radius of the atom R are in the following relation (shown in Table 1.3 in the textbook): R= a 2 4 i.e. (4.077 × 10 R= −10 4 m) 2 = 1.442 × 10-10 m *1.14 BCC and FCC crystals a. Consider iron below 912 °C, where its structure is BCC. Given the density of iron as 7.86 g cm-3 and its atomic mass as 55.85 g/mol, calculate the lattice parameter of the unit cell and the radius of the Fe atom. b. At 912 °C, iron changes from the BCC (α-Fe) to the FCC (γ-Fe) structure. The radius of the Fe atom correspondingly changes from 0.1258 nm to 0.1291 nm. Calculate the density of γ-Fe and explain whether there is a volume expansion or contraction during this phase change. 1.21 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 Solution Given: Density of iron at room temperature: ρ = 7.86 × 103 kg/m3 Atomic mass of iron: Mat = 55.85 g/mole a For the BCC structure, the density is given by: 2 Mat × (10 −3 kg/g) NA a3 Thus the lattice parameter a is: ρ= 1 1 Mat 3 a= (500 g/kg) N A ρ 1 ∴ 3 1 (55.85 g/mol) a= 23 −1 3 3 (500 g/kg) (6.022 × 10 mol )(7.86 × 10 kg/m ) ∴ a = 2.87 × 10-10 m The radius of the Fe atom, R, and the lattice parameter, a, are related. a 3 = 4R 1 1 3a = 3 (2.87 × 10 −10 m ) 4 4 ∴ R= ∴ R = 1.24 × 10-10 m b Fe has a BCC structure just below 912 °C (α-Fe). An Fe atom in the α-Fe state has a radius of RBCC = 0.1258 × 10-9 m. The density of α-Fe is therefore: 2 ρ BCC = ∴ Mat × (10 −3 kg/g) NA 3 4 RBCC 3 2 = (55.85 g/mol)(10 −3 kg/g) (6.022 × 10 4(0.1258 × 10 23 mol −1 ) −9 3 m) 3 ρBCC = 7564 kg/m3 (less than at room temperature) Fe has a FCC structure just above 912 °C (γ-Fe). An Fe atom in the γ-Fe state has a radius of RFCC = 0.1291 × 10-9 m (Remember that for a FCC structure, a 2 = 4 RFCC ). The density of γ-Fe is therefore: 4 ρ FCC = Mat × (10 −3 kg/g) NA 3 4 RFCC 2 4 = (55.85 (6.022 × 10 4(0.1291 × 10 23 1.22 g/mol)(10 −3 kg/g) 2 mol −1 ) −9 m) 3 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 ρ FCC = 7620 kg/m3 ∴ As the density increases, the volume must contract (the Fe retains the same mass). 1.15 Planar and surface concentrations Niobium (Nb) has the BCC crystal with a lattice parameter a = 0.3294 nm. Find the planar concentrations as the number of atoms per nm2 of the (100), (110) and (111) planes. Which plane has the most concentration of atoms per unit area? Sometimes the number of atoms per unit area nsurface on the surface of a crystal is estimated by using the relation nsurface = nbulk2/3 where nbulk is the concentration of atoms in the bulk. Compare nsurface values with the planar concentrations calculated above and comment on the difference. [Note: The BCC (111) plane does not cut through the center atom and the (111) has 1/6th of an atom at each corner] Solution Planar concentration (or density) is the number of atoms per unit area on a given plane in the crystal. It is the surface concentration of atoms on a given plane. To calculate the planar concentration n(hkl) on a given (hkl) plane, we consider a bound area A. Only atoms whose centers line on A are involved in n(hkl). For each atom, we then evaluate what portion of the atomic cross section cut by the plane (hkl) is contained within A. For the BCC crystalline structure the planes (100), (110) and (111) are drawn in Figure 1Q15-1. (111) a√2 (100) a (110) a√2 a a (100), (110), (111) planes in the BCC crystal Figure 1Q15-1 Consider the (100) plane. Number of atoms in the area a × a , which is the cube face = (4 corners) × (1/4th atom at corner) = 1. Planar concentration is n(100 ) 1 4 1 18 atoms m- 2 = 42 = 2 = 9.216 × 10 −10 a . m 3 294 × 10 ( ) The most populated plane for BCC structure is (110). Number of atoms in the area a × a 2 defined by two face-diagonals and two cube-sides = (4 corners) × (1/4th atom at corner) + 1 atom at face center = 2 Planar concentration is 1.23 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) n(110 ) Chapter 1 1 4 + 1 2 = 1.303 × 1019 atoms m- 2 = 42 = 2 − 10 a 2 (3.294 × 10 m) 2 The plane (111) for the BCC structure is the one with rarest population. The area of interest is an equilateral triangle defined by face diagonals of length a 2 (Figure 1Q15-1). The height of the triangle 1 3 3 a2 3 is a so that the triangular area is × a 2 × a = . An atom at a corner only contributes a 2 2 2 2 fraction (60°/360°=1/6) to this area. So the planar concentration is 1 (3) 1 1 6 = 5.321 × 1018 atoms m- 2 n(111) = 2 = 2 = 2 − 10 a 3 a 3 (3.294 × 10 m ) 3 2 For the BCC structure there are two atoms in unit cell and the bulk atomic concentration is nbulk = 2 number of atoms in unit cell 2 28 = 3 = atoms m- 3 3 = 5.596 × 10 −10 vovolume of the cell a (3.294 × 10 m) and the surface concentration is 2 2 nsurface = (nbulk ) 3 = (5.596 × 10 28 m −3 ) 3 = 1.463 × 1019 atoms m- 2 1 . 1 6 Diamond and zinc blende Si has the diamond and GaAs has the zinc blende crystal structure. Given the lattice parameters of Si and GaAs, a = 0.543 nm and a = 0.565 nm, respectively, and the atomic masses of Si, Ga, and As as 28.08 g/mol, 69.73 g/mol, and 74.92 g/mol, respectively, calculate the density of Si and GaAs. What is the atomic concentration (atoms per unit volume) in each crystal? Solution atoms: Referring to the diamond crystal structure in Figure 1Q16-1, we can identify the following types of 8 corner atoms labeled C, 6 face center atoms (labeled FC) and 4 inside atoms labeled 1,2,3,4. The effective number of atoms within the unit cell is: (8 Corners) × (1/8 C-atom) + (6 Faces) × (1/2 FC-atom) + 4 atoms within the cell (1,2,3,4) = 8 1.24 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) C Chapter 1 C FC C FC 4 1 FC a 2 FC FC 3 C a FC C a C Figure 1Q16-1 The diamond crystal structure. The lattice parameter (length of a cube side) of the unit cell is a. Thus the atomic concentration in the Si crystal (nSi) is nSi = 8 8 = = 5.0 × 1028 atoms per m- 3 3 a (0.543 × 10 −9 m)3 If Mat is the atomic mass in the Periodic Table then the mass of the atom (mat) in kg is (1) mat = (10-3 kg/g)Mat/NA where NA is Avogadro’s number. For Si, Mat = MSi = 28.09 g/mol, so then the density of Si is ρ = (number of atoms per unit volume) × (mass per atom) = nSi mat or 8 (10 −3 kg/g) MSi ρ = 3 a NA i.e. (10 −3 kg/g)(28.09 g mol -1 ) 8 ρ= 3 23 −1 (0.543 × 10 −9 m ) (6.022 × 10 mol ) calculating, ρ = 2.33 × 103 kg m-3 or 2.33 g cm- 3 In the case of GaAs, it is apparent that there are 4 Ga and 4 As atoms in the unit cell. The concentration of Ga (or As) atoms per unit volume (nGa) is nGa = 4 4 = = 2.22 × 10 28 m -3 3 a (0.565 × 10 −9 m )3 Total atomic concentration (counting both Ga and As atoms) is twice nGa. n Total = 2n Ga = 4.44 × 1028 m-3 There are 2.22 × 1028 Ga-As pairs per m3. We can calculate the mass of the Ga and As atoms from their relative atomic masses in the Periodic Table using Equation (1) with Mat = MGa = 69.72 g/mol for Ga and Mat = MAs = 74.92 g/mol for As. Thus, 4 (10 −3 kg/g)( MGa + MAs ) ρ = 3 a NA or 4 (10 −3 kg/g)(69.72 g/mol + 74.92 g/mol) ρ= −9 3 6.022 × 10 23 mol −1 (0.565 × 10 m ) i.e. ρ = 5.33 × 103 kg m-3 or 5.33 g cm- 3 1.25 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 1.17 Zinc blende, NaCl and CsCl a. InAs is a III-V semiconductor that has the zinc blende structure with a lattice parameter of 0.606 nm. Given the atomic masses of In (114.82 g mol-1) and As (74.92 g mol-1) find the density. b. CdO has the NaCl crystal structure with a lattice parameter of 0.4695 nm. Given the atomic masses of Cd (112.41 g mol-1) and O (16.00 g mol-1) find the density. c. KCl has the same crystal structure as NaCl. The lattice parameter a of KCl is 0.629 nm. The atomic masses of K and Cl are 39.10 g mol-1 and 35.45 g mol-1 respectively. Calculate the density of KCl. Solution a For zinc blende structure there are 8 atoms per unit cell (as shown in Table 1.3 in the textbook). In the case of InAs, it is apparent that there are 4 In and 4 As atoms in the unit cell. The density of InAs is then dInAs M M 4 at In + 4 at As NA N A 4 Mat In + Mat As = = = 3 a N A a3 ( = ) 4 (114.82 + 74.92) × (10 −3 kg mol −1 ) (6.022 × 10 23 mol −1 )(0.606 × 10 −9 m) = 5.663 × 103 kg m-3 = 5.663 g cm- 3 3 b For NaCl crystal structure, there are 4 cations and 4 anions per unit cell. For the case of CdO we have 4 Cd atoms and 4 O atoms per unit cell and the density of CdO is dCdO M M 4 at Cd + 4 at O NA N A 4 Mat Cd + Mat O = = = 3 a N A a3 ( = c ) 4 (112.41 + 16.00) × (10 −3 kg mol −1 ) (6.022 × 10 23 mol −1 )(0.4695 × 10 −9 m) = 8.241 × 103 kg m-3 = 8.241 g cm- 3 3 Analogously to b, for the density of KCl we receive dKCl M M 4 at K + 4 at Cl NA N A 4 Mat K + Mat Cl = = = 3 a N A a3 ( = ) 4 (39.1 + 35.45) × (10 −3 kg mol −1 ) (6.022 × 10 23 mol −1 )(0.629 × 10 −9 m) 3 = 1.99 × 103 kg m-3 = 1.99 g cm- 3 1.18 Crystallographic directions and planes Consider the cubic crystal system. a. Show that the line [hkl] is perpendicular to the (hkl) plane. b. Show that the spacing between adjacent (hkl) planes is given by d= a h + k 2 + l2 2 1.26 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 Solution This problem assumes that students are familiar with three dimensional geometry and vector products. Figure 1Q18-1(a) shows a typical [hkl] line, labeled as ON, and a (hkl) plane in a cubic crystal. ux, uy and uz are the unit vectors along the x, y, z coordinates. This is a cubic lattice so we have Cartesian coordinates and ux⋅ux = 1 and ux⋅uy = 0 etc. (a) uz (b) az1 C N al az1 C uy ak B ay1 B ay1 D O O ah ax1 A ax1 ux A Figure 1Q18-1 Crystallographic directions and planes a Given a = lattice parameter, then from the definition of Miller indices (h = 1/x1, k = 1/y1 and l = 1/z1) , the plane has intercepts: xo = ax1 =a/h; yo = ay1 = a/k; zo = az1 = a/l. The vector ON = ahux + akuy + aluz If ON is perpendicular to the (hkl) plane then the product of this vector with any vector in the (hkl) plane will be zero. We only have to choose 2 non-parallel vectors (such as AB and BC) in the plane and show that the dot product of these with ON is zero. AB = OB – OA = (a/k)uy – (a/h)ux ON•AB = (ahux + akuy + aluz) • ( (a/k)uy – (a/h)ux) = a2 – a2 = 0 Remember that: uxux = uyuy =1 and uxuy = uxuz = uyuz = 0 Similarly, ON•BC = (ahux + akuy + aluz) • ( (a/l)uz – (a/k)uy) = 0 Therefore ON or [hkl] is normal to the (hkl) plane. b Suppose that OD is the normal from the plane to the origin as shown in Figure 1Q18-1(b). Shifting a plane by multiples of lattice parameters does not change the miller indices. We can therefore assume the adjacent plane passes through O. The separation between the adjacent planes is then simply the distance OD in Figure 1Q18-1(b). Let α, β and γ be the angles of OD with the x, y and z axes. Consider the direction cosines of the line OD: cosα = d/(ax1) = dh/a; cosβ = d/(ay1) = dk/a; cosγ = d/(az1) = dl/a But in 3 dimensions, (cosα)2 + (cosβ)2 + (cosγ)2 = 1 Thus, (d2h2/a2) + (d2k2/a2) + (d2l2/a2) = 1 1.27 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 d2 = a2 / [h2 + k2 + l2] d = a / [h 2 + k 2 + l 2 ] 1 / 2 Rearranging, or, 1.19 Si and SiO2 a. Given the Si lattice parameter a = 0.543 nm, calculate the number of Si atoms per unit volume, in nm-3. b. Calculate the number of atoms per m2 and per nm2 on the (100), (110) and (111) planes in the Si crystal as shown on Figure 1Q19-1. Which plane has the most number of atoms per unit area? c. The density of SiO2 is 2.27 g cm-3. Given that its structure is amorphous, calculate the number of molecules per unit volume, in nm-3. Compare your result with (a) and comment on what happens when the surface of an Si crystal oxidizes. The atomic masses of Si and O are 28.09 g/mol and 16 g/mol, respectively. a a a (100) plane (110) plane (111) plane Figure 1Q19-1 Diamond cubic crystal structure and planes. Determine what portion of a black-colored atom belongs to the plane that is hatched. Solution a Si has the diamond crystal structure with 8 atoms in the unit cell, and we are given the lattice parameter a = 0.543 × 10-9 m and atomic mass Mat = 28.09 × 10-3 kg/mol. The concentration of atoms per unit volume (n) in nm-3 is therefore: n= ∴ 8 1 8 1 3 = 3 3 3 9 9 9 − a (10 nm/m ) (0.543 × 10 m) (10 nm/m) n = 50.0 atoms/nm3 If desired, the density ρ can be found as follows: Mat 28.09 × 10 −3 kg/mol 8 −1 23 N ρ = 3 A = 6.022 × 10 −9 mol3 a (0.543 × 10 m) 8 ∴ ρ = 2331 kg m-3 or 2.33 g cm-3 b The (100) plane has 4 shared atoms at the corners and 1 unshared atom at the center. The corner atom is shared by 4 (100) type planes. Number of atoms per square nm of (100) plane area (n) is shown in Fig. 1Q19-2: 1.28 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) B A a D Ge B A Chapter 1 (100) E a a E C a a D C Figure 1Q19-2 The (100) plane of the diamond crystal structure. The number of atoms per nm2, n100, is therefore: n100 ∴ 1 1 4 + 1 4 + 1 4 1 1 = 42 2 = 2 2 9 9 9 − a (10 nm/m) (0.543 × 10 m) (10 nm/m) n 100 = 6.78 atoms/nm2 or 6.78 × 1018 atoms/m2 The (110) plane is shown below in Fig. 1Q19-3. There are 4 atoms at the corners and shared with neighboring planes (hence each contributing a quarter), 2 atoms on upper and lower sides shared with upper and lower planes (hence each atom contributing 1/2) and 2 atoms wholly within the plane. B a√2 A A B (110) a D (110) C C D Figure 1Q19-3 The (110) plane of the diamond crystal structure. The number of atoms per nm2, n110, is therefore: n110 1 1 4 + 2 + 2 2 1 = 4 9 2 aa 2 (10 nm/m ) [ ( )] 1 1 4 + 2 + 2 4 2 1 9 2 2 (10 nm/m ) ∴ n110 = ∴ n 110 = 9.59 atoms/nm2 or 9.59 × 1018 atoms/m2 [(0.543 × 10 −9 ( m ) (0.543 × 10 −9 m ) )] This is the most crowded plane with the most number of atoms per unit area. The (111) plane is shown below in Fig. 1Q19-4: 1.29 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 A A a√3 √2 a√2 30° 30° 60° D a√2 C 60° C D B B a√2 2 a√2 2 Figure 1Q19-4 The (111) plane of the diamond crystal structure The number of atoms per nm2, n111, is therefore: n111 = 60 1 3 + 3 360 2 1 9 2 1 2 3 (10 nm/m ) 2 a a 2 2 2 60 1 3 + 3 360 2 ∴ n111 = ∴ n 111 = 7.83 atoms/nm2 or 7.83 × 1018 atoms/m2 c 1 2 −9 −9 2 (0.543 × 10 m ) (0.543 × 10 m ) 2 2 1 9 2 3 (10 nm/m ) 2 Given: Molar mass of SiO2: Mat = 28.09 × 10-3 kg/mol + 2 × 16 × 10-3 kg/mol = 60.09 × 10-3 kg/mol Density of SiO2: ρ = 2.27 × 103 kg m-3 Let n be the number of SiO2 molecules per unit volume, then: ρ=n Mat NA ∴ 23 −1 3 −3 N A ρ (6.022 × 10 mol )(2.27 × 10 kg m ) n= = Mat (60.09 × 10 −3 kg/mol) ∴ n = 2.27 × 1028 molecules per m3 Or, converting to molecules per nm3: n= 2.27 × 10 28 molecules/m 3 (10 9 nm/m ) 3 = 22.7 molecules per nm3 Oxide has less dense packing so it has a more open structure. For every 1 micron of oxide formed on the crystal surface, only about 0.5 micron of Si crystal is consumed. 1.30 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 1.20 Vacancies Estimate the equilibrium vacancy concentration in the Si crystal at room temperature and at 1200 °C, given that the energy of vacancy formation is 3.6 eV. Solution To find the vacancy concentration in Si at T = 27 °C + 273 = 300 K: Si has a diamond crystal structure (as shown in Table 1.3 in the textbook) and therefore the concentration of atoms per unit volume (n) is given as: n= 8 8 28 atoms/m 3 = 3 = 4.997 × 10 3 −9 a (0.543 × 10 m) where a = 0.543 nm is the lattice parameter of Si, given in Q1.19. We are given that the energy of vacancy formation (EV) in Si is 2.4 eV, or: EV = (2.4 eV/atom)(1.602 × 10-19 J/eV) = 3.845 × 10-19 J/atom The concentration of vacancies (nV) is then given by: E nV = n exp − V kT ∴ 3.845 × 10 −19 J nV = ( 4.997 × 10 28 m 3 ) exp − -23 −1 (1.381 × 10 J K )(300 K ) ∴ n V = 2.47 × 10-12 vacancies / m3 (at room temperature) At T′ = 1200 °C + 273 = 1473 K: E nV′ = n exp − V kT ′ ∴ 3.845 × 10 −19 J nV = ( 4.997 × 10 28 m -3 ) exp − -23 −1 (1.381 × 10 J K )(1473 K ) ∴ n V = 3.09 × 1020 vacancies / m3 (at 1200 °C) Note: Strictly we should use the concentration of Si (n) at 1473 K by taking into account the unit cell expansion and recalculating n = 8/a3 to get a better value for n′. Given that nV = n exp(-EV/kT) has the entropic pre-exponential term missing (i.e. it is already an approximation) the calculation above is more than sufficient. 1.21 Pb-Sn solder Consider the soldering of two copper components. When the solder melts, it wets both metal surfaces. If the surfaces are not clean or have an oxide layer, the molten solder cannot wet the surfaces and the soldering fails. Assume that soldering takes place at 250 °C, and consider the diffusion of Sn atoms into the copper (the Sn atom is smaller than the Pb atom and hence diffuses more easily). 1.31 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 a. The diffusion coefficient of Sn in Cu at two temperatures is D = 1.69 × 10-9 cm2 hr-1at 400 °C and D = 2.48 × 10-7 cm2 hr-1 at 650 °C. Calculate the rmsdistance diffused by an Sn atom into the copper, assuming the cooling process takes 10 seconds. b. What should be the composition of the solder if it is to begin freezing at 250 °C? c. What are the components (phases) in this alloy at 200 °C? What are the compositions of the phases and their relative weights in the alloy? d. What is the microstructure of this alloy at 25 °C? What are weight fractions of the α and β phases assuming near equilibrium cooling? Solution a Given information: Temperatures: T1 = 400 °C + 273 = 673 K T2 = 650 °C + 273 = 923 K Diffusion coefficients: D1 = 1.69 × 10-9 cm2/hr = (1.69 × 10-9 cm2/hr)(0.01 m/cm)2 / (1 hr) × (3600 sec/hr) D1 = 4.694 × 10-17 m2/s D2 = 2.48 × 10-7 cm2/hr = (2.48 × 10-7 cm2/hr)(0.01 m/cm)2 / (1 hr) × (3600 sec/hr) D2 = 6.889 × 10-15 m2/s The diffusion coefficients at certain temperatures are given by: E q D1 = Do exp − A kT1 E q D2 = Do exp − A kT2 where EA is the activation energy in eV/atom. We can take the ratio of the diffusion coefficients to express them in terms of the activation energy (EA): ∴ ∴ E q Do exp − A q[T − T ]E E q E q kT1 D1 = = exp A − A = exp 1 2 A kT1 T1T2 k D2 E q kT2 Do exp − A kT2 D T1T2 k ln 1 D2 EA = q(T1 − T2 ) 4.694 × 10 -17 m 2 /s 6.889 × 10 -15 m 2 /s J/eV)(673 K − 923 K ) (673 K )(923 K )(1.381 × 10-23 J K -1 ) ln ∴ EA = ∴ EA = 1.068 eV/atom (1.602 × 10 -19 Now the diffusion coefficient Do can be found as follows: E q D1 = Do exp − A kT1 1.32 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 D1 4.694 × 10 -17 m 2 /s = E q (1.068 eV)(1.602 × 10 -19 J/eV) exp − A exp − -23 -1 kT1 (1.381 × 10 J K )(673 K ) ∴ Do = ∴ Do = 4.638 × 10-9 m2/s To check our value for Do, we can substitute it back into the equation for D2 and compare values: (1.068 eV)(1.602 × 10 -19 J/eV) EA q -9 2 D2 = Do exp − = ( 4.638 × 10 m /s) exp − -23 -1 kT2 (1.381 × 10 J K )(923 K ) ∴ D2 = 6.870 × 10-15 m2/s This agrees with the given value of 6.889 × 10-15 m2/s for D2. Now we must calculate the diffusion coefficient D3 at T3 = 250 °C + 273 = 523 K (temperature at which soldering is taking place). (1.068 eV)(1.602 × 10 -19 J/eV) E q D3 = Do exp − A = ( 4.638 × 10 -9 m 2 /s) exp − -23 -1 kT3 (1.381 × 10 J K )(523 K ) ∴ D3 = 2.391 × 10-19 m2/s The rms distance diffused by the Sn atom in time t = 10 s (Lrms) is given by: Lrms = 2 D3t = 2(2.391 × 10 -19 m 2 /s)(10 s) ∴ L rms = 2.19 × 10-9 m or 2 nm b From Figure 1Q21-1, 250 °C cuts the liquidus line approximately at 33 wt.% Sn composition (C o). ∴ c C o = 0.33 (Sn) For α-phase and liquid phase (L), the compositions as wt.% of Sn from Figure 1Q21-1 are: Cα = 0.18 CL = 0.56 The weight fraction of α and L phases are: Wα = CL − Co 0.56 − 0.33 = = 0.605 or 60 wt.% α -phase CL − Cα 0.56 − 0.18 WL = Co − Cα 0.33 − 0.18 = = 0.395 or 39.5 wt.% liquid phase CL − Cα 0.56 − 0.18 1.33 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 400 A LIQUID Temperature ( C) 300 CL +L C B L+ 200 61.9% 97.5% + 100 Co C C 0 0 Pure Pb 20 40 60 80 Composition in wt. % Sn 100 Pure Sn Figure 1Q21-1 The equilibrium phase diagram of the Pb-Sn alloy. d The microstructure is a primary α-phase and a eutectic solid (α + β) phase. There are two phases present, α + β. See Figure 1Q21-2. Primary Eutectic Figure 1Q21-2 Microstructure of Pb-Sn at temperatures less than 183°C. Assuming equilibrium concentrations have been reached: C′α = 0.02 C ′β = 1 The weight fraction of α in the whole alloy is then: Wα′ = Cβ′ − Co Cβ′ − Cα′ = 1 − 0.33 = 0.684 or 68.4 wt.% α -phase 1 − 0.02 The weight fraction of β in the whole alloy is: Wβ′ = Co′ − Cα 0.33 − 0.02 = = 0.316 or 31.6 wt.% β -phase Cβ′ − Cα′ 1 − 0.02 1.22 Pb-Sn solder Consider the 50% Pb-50% Sn solder. a. Sketch the temperature-time profile and the microstructure of the alloy at various stages as it is cooled from the melt. b. At what temperature does the solid melt? c. What is the temperature range over which the alloy is a mixture of melt and solid? What is the structure of the solid? 1.34 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 d. Consider the solder at room temperature following cooling from 182 °C. Assume that the rate of cooling from 182 °C to room temperature is faster than the atomic diffusion rates needed to change the compositions of the α and β phases in the solid. Assuming the alloy is 1 kg, calculate the masses of the following components in the solid: 1. The primary α. 2. α in the whole alloy. 3. α in the eutectic solid. 4. β in the alloy (Where is the β-phase?). e. Calculate the specific heat of the solder given the atomic masses of Pb (207.2) and Sn (118.71). Solution a 50% Pb-50% Sn L T Proeutectic (primary) L Proeutectic solidifying Eutectic ( + ) solidifying Eutectic L L (61.9% Sn) (19% Sn) ~210 C L+ 183 C L+ + + t Figure 1Q22-1 Temperature - time profile and microstructure diagram of 50% Pb-50% Sn. All compositions are in weight %. b When 50% Pb-50% Sn is cooled from the molten state down to room temperature, it begins to solidify at point A at about 210 °C. Therefore at 210 °C, the solid begins to melt. c Between 210 °C and 183 °C, the liquid has the eutectic composition and undergoes the eutectic transformation to become the eutectic solid. Below 183 °C, all the liquid has solidified, and the structure is a combination of the solid α-phase and the eutectic structure (which is composed of α and β layers). d At 182 °C, the composition of the proeutectic or primary α is given by the solubility limit of Sn in α : 19.2% Sn. 1.35 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 1 The primary or proeutectic α (pro-α) exists just above and below 183 °C (eutectic temperature), i.e. it is stable just above and below 183 °C. Thus the mass of pro-α at 182 °C is the same as at 184 °C. Applying the lever rule (at 50% Sn): Wpro− α = CL − Co 61.9 − 50 = = 0.279 CL − Cα 61.9 − 19.2 Assume that the whole alloy is 1 kg. The mass of the primary or proeutectic α is thus 27.9% of the whole alloy; or 0.279 kg. The mass of the total eutectic solid (α + β) is thus 1 - 0.279 = 0.721 or 72.1% or 0.721 kg. solid: If we apply the lever rule at 182 °C at 50% Sn, we obtain the weight percentage of α in the whole Wα = Cβ − Co Cβ − Cα = 97.5 − 50 = 0.607 97.5 − 19.2 Thus the mass of α in the whole solid is 0.607 kg. Of this, 0.28 kg is in the primary (proeutectic) α phase. Thus 0.607 - 0.279 or 0.328 kg of α is in the eutectic solid. Since the total mass of α in the solid is 0.607 kg, the remainder of the mass must be the β-phase. Thus the mass of the β-phase is 1 kg - 0.607 kg or 0.393 kg. The β-phase is present solely in the eutectic solid, with no primary β, because the solid is forming from a point where it consisted of the α and liquid phases only. e Suppose that nA and nB are atomic fractions of A and B in the whole alloy, nA + nB = 1 Suppose that we have 1 mole of the alloy. Then it has nA moles of A and nB moles of B (atomic fractions also represent molar fractions in the alloy). Suppose that we consider 1 gram of the alloy. Since wA is the weight fraction of A, wA is also the mass of A in grams in the alloy. The number of moles of A in the alloy is then wA/MA where MA is the atomic mass of A. Thus, Number of moles of A = wA/MA. Number of moles of B = wB/MB. Number of moles of the whole alloy = wA/MA + wB/MB. Molar fraction of A is the same as nA. Thus, nA = wA / MA w A wB + M A MB and nB = wB / MB wA w + B MA MB We are given the molar masses of Pb and Sn: MSn = 118.71 g/mol MPb = 207.2 g/mol Let n = mole (atomic) fraction and W = weight fraction. Then WPb = weight fraction of Pb in the alloy. We know this to be 0.5 (50% Pb). The mole fractions of Pb and Sn are nPb and nSn and are given by: nPb = − MSn WPb ( MPb − MSn )WPb − MPb 1.36 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ nPb = − Chapter 1 (118.71 g/mol)(0.5) = 0.3642 (207.2 g/mol − 118.71 g/mol)(0.5) − 207.2 g/mol nSn = 1 - nPb = 1 - 0.3642 = 0.6358 The mole fraction of Pb is 0.3642 and that of Sn is 1 - 0.3642 = 0.6358. The heat capacity of a metal is 3R per mole, where R is the gas constant. We want the specific heat capacity Cs (heat capacity per gram of alloy). 1 mole of the alloy has a mass MPbSn: MPbSn = nPb MPb + nSn MSn Thus the specific heat capacity Cs (i.e. heat capacity per gram) is: Cs = 3R 3R = MPbSn nPb MPb + nSn MSn 3(8.315 J K -1 mol -1 ) ∴ Cs = ∴ C s = 0.165 J K-1 g- 1 (0.3642) (207.2 g/mol) + (0.6358) (118.71 g/mol) "If your experiment needs statistics, you ought to have done a better experiment." Ernest Rutherford (1871-1937) 1.37 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 Second Edition ( 2001 McGraw-Hill) Chapter 2 2.1 Electrical conduction Na is a monovalent metal (BCC) with a density of 0.9712 g cm-3. Its atomic mass is 22.99 g mol–1. The drift mobility of electrons in Na is 53 cm2 V-1 s-1. a. Consider the collection of conduction electrons in the solid. If each Na atom donates one electron to the electron sea, estimate the mean separation between the electrons? b. What is the approximate mean separation between an electron (e-) and a metal ion (Na+), assuming that most of the time the electron prefers to be between two neighboring Na+ ions. What is the approximate Coulombic interaction energy (in eV) between an electron and an Na+ ion? c. How does this electron/metal-ion interaction energy compare with the average thermal energy per particle, according to the kinetic molecular theory of matter? Do you expect the kinetic molecular theory to be applicable to the conduction electrons in Na? If the mean electron/metal-ion interaction energy is of the same order of magnitude as the mean KE of the electrons, what is the mean speed of electrons in Na? Why should the mean kinetic energy be comparable to the mean electron/metal-ion interaction energy? d. Calculate the electrical conductivity of Na and compare this with the experimental value of 2.1 × 107 Ω-1 m-1 and comment on the difference. Solution a If D is the density, Mat is the atomic mass and NA is Avogadro's number, then the atomic concentration nat is nat = i.e. DN A (971.2 kg m -3 )(6.022 × 10 23 mol −1 ) = (22.99 × 10 −3 kg mol −1 ) Mat nat = 2.544 × 1028 m-3 which is also the electron concentration, given that each Na atom contributes 1 conduction electron. If d is the mean separation between the electrons then d and nat are related by (see Chapter 1 Solutions, Q1.5; this is only an estimate) d≈ 1 nat 1/ 3 = 1 = 3.40 × 10-10 m or 0.34 nm (2.544 × 10 28 m −3 )1/3 b Na is BCC with 2 atoms in the unit cell. So if a is the lattice constant (side of the cubic unit cell), the density is given by M 2 at NA (atoms in unit cell)(mass of 1 atom) D= = a3 volume of unit cell 1/ 3 1/ 3 isolate for a: 2 Mat a= DN A so that a = 4.284 × 10-10 m or 0.4284 nm 2(22.99 × 10 −3 kg mol −1 ) = 3 23 -3 −1 (0.9712 × 10 kg m )(6.022 × 10 mol ) 2.1 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 For the BCC structure, the radius of the metal ion R and the lattice parameter a are related by (4R)2 = 3a , so that, 2 R = (1/4)√[3a2] = 1.855 × 10-10 m or 0.1855 nm If the electron is somewhere roughly between two metal ions, then the mean electron to metal ion separation delectron-ion is roughly R. If delectron-ion ≈ R, the electrostatic potential energy PE between a conduction electron and one metal ion is then ( −e)( + e) ( −1.602 × 10 −19 C)( +1.602 × 10 −19 C) PE = = 4πε o delectron − ion 4π (8.854 × 10 −12 F m −1 )(1.855 × 10 −10 m ) ∴ (1) P E = -1.24 × 10-18 J or -7.76 eV c This electron-ion PE is much larger than the average thermal energy expected from the kinetic theory for a collection of “free” particles, that is Eaverage = KEaverage = 3(kT/2) ≈ 0.039 eV at 300 K. In the case of Na, the electron-ion interaction is very strong so we cannot assume that the electrons are moving around freely as if in the case of free gas particles in a cylinder. If we assume that the mean KE is roughly the same order of magnitude as the mean PE, KEaverage = 12 meu 2 ≈ PE average = −1.24 × 10 −18 J (2) where u is the mean speed (strictly, u = root mean square velocity) and me is the electron mass. 1/ 2 Thus, 2 PEaverage u≈ me so that u = 1.65 × 106 m/s 1/ 2 2(1.24 × 10 −18 J) = −31 kg) (9.109 × 10 (3) There is a theorem in classical physics called the Virial theorem which states that if the interactions between particles in a system obey the inverse square law (as in Coulombic interactions) then the magnitude of the mean KE is equal to the magnitude of the mean PE. The Virial Theorem states that: 1 KEaverage = - PEaverage 2 Indeed, using this expression in Eqn. (2), we would find that u = 1.05 × 106 m/s. If the conduction electrons were moving around freely and obeying the kinetic theory, then we would expect (1/2)meu2 = (3/2)kT and u = 1.1 × 105 m/s, a much lower mean speed. Further, kinetic theory predicts that u increases as T1/2 whereas according to Eqns. (1) and (2), u is insensitive to the temperature. The experimental linear dependence between the resistivity ρ and the absolute temperature T for most metals (non-magnetic) can only be explained by taking u = constant as implied by Eqns. (1) and (2). d If µ is the drift mobility of the conduction electrons and n is their concentration, then the electrical conductivity of Na is σ = enµ. Assuming that each Na atom donates one conduction electron (n = nat), we have σ = enµ = (1.602 × 10 −19 C)(2.544 × 10 28 m −3 )(53 × 10 −4 m 2 V -1 s-1 ) i.e. σ = 2.16 × 107 Ω -1 m- 1 which is quite close to the experimental value. Nota Bene: If one takes the Na+-Na+ separation 2R to be roughly the mean electron-electron separation then this is 0.37 nm and close to d = 1/(n1/3) = 0.34 nm. In any event, all calculations are only approximate to highlight the main point. The interaction PE is substantial compared with the mean thermal energy and we cannot use (3/2)kT for the mean KE! 2.2 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 2.2 Electrical conduction The resistivity of aluminum at 25 °C has been measured to be 2.72 × 10-8 Ω m. The thermal coefficient of resistivity of aluminum at 0 °C is 4.29 × 10-3 K–1. Aluminum has a valency of 3, a density of 2.70 g cm-3, and an atomic mass of 27. a. Calculate the resistivity of aluminum at –40 °C. b. What is the thermal coefficient of resistivity at –40 °C? c. Estimate the mean free time between collisions for the conduction electrons in aluminum at 25 °C, and hence estimate their drift mobility. d. If the mean speed of the conduction electrons is ∼1.5 × 106 m s -1, calculate the mean free path and compare this with the interatomic separation in Al (Al is FCC). What should be the thickness of an Al film that is deposited on an IC chip such that its resistivity is the same as that of bulk Al? e. What is the percentage change in the power loss due to Joule heating of the aluminum wire when the temperature drops from 25 °C to –40 °C? Solution a Apply the equation for temperature dependence of resistivity, ρ(T) = ρo[1 + αo(T-To)]. We have the temperature coefficient of resistivity, αo, at To where To is the reference temperature. The two given reference temperatures are 0 °C or 25 °C, depending on choice. Taking To = 0 °C + 273 = 273 K, ρ(-40 °C + 273 = 233 K) = ρo[1 + αo(233 K - 273 K)] ρ(25 °C + 273 = 298 K) = ρo[1 + αo(298 K - 273 K)] Divide the above two equations to eliminate ρo, ρ(-40 °C)/ρ(25 °C) = [1 + αo(-40 K)] / [1 + αo(25 K)] Next, substitute the given values ρ(25 °C) = 2.72 × 10-8 Ω m and αo = 4.29 × 10-3 K-1 to obtain ρ (-40 °C) = (2.72 × 10 -8 Ω m) [1 + (4.29 × 10 -3 K -1 )(-40 K)] [1 + (4.29 × 10 -3 K -1 )(25 K)] ρ (-40 °C) = 2.03 × 10-8 Ω m i.e. b In ρ(T) = ρo[1 + αo(T - To)] we have αo at To where To is the reference temperature, for example, 0° C or 25 °C depending on choice. We will choose To to be first at 0 °C = 273 K and then at -40 °C = 233 K so that ρ(-40 °C) = ρ(0 °C)[1 + αo(233 K - 273 K)] ρ(0 °C) = ρ(-40 °C)[1 + α-40(273 K - 233 K)] and Multiply and simplify the two equations above to obtain [1 + αo(233 K - 273 K)][1 + α-40(273 K - 233 K)] = 1 [1 - 40αo][1 + 40α-40] = 1 or Rearranging, 2.3 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 α-40 = (1 / [1 - 40αo] - 1)(1 / 40) ∴ α-40 = αo / [1 - 40αo] i.e. α -40 = (4.29 × 10-3 K-1) / [1 - (40 K)(4.29 × 10-3 K-1)] = 5.18 × 10-3 K-1 Alternatively, consider, ρ(25 °C) = ρ(-40 °C)[1 + α-40(298 K - 233 K)] so that α-40 = [ρ(25 °C) - ρ(-40 °C)] / [ρ(-40 °C)(65 K)] ∴ α-40 = [2.72 × 10-8 Ω m - 2.03 × 10-8 Ω m] / [(2.03 × 10-8 Ω m)(65 K)] ∴ α -40 = 5.23 × 10-3 K- 1 c We know that 1/ρ = σ = enµ where σ is the electrical conductivity, e is the electron charge, and µ is the electron drift mobility. We also know that µ = eτ/me, where τ is the mean free time between electron collisions and me is the electron mass. Therefore, 1/ρ = e2nτ/me ∴ τ = me/ρe2n (1) Here n is the number of conduction electrons per unit volume. But, from the density d and atomic mass Mat, atomic concentration of Al is nAl = so that 23 3 -1 N A d (6.022 × 10 mol )(2700 kg/m ) = = 6.022 × 10 28 m -3 Mat . kg/mol 0 027 ( ) n = 3nAl = 1.807 × 1029 m-3 assuming that each Al atom contributes 3 "free" conduction electrons to the metal. Substitute into Eqn. (1): τ= ∴ (9.109 × 10 -31 kg) me = ρe 2 n (2.72 × 10 -8 Ω m)(1.602 × 10 -19 C)2 (1.807 × 10 29 m -3 ) τ = 7.22 × 10-15 s (Note: If you do not convert to meters and instead use centimeters you will not get the correct answer because seconds is an SI unit.) The relation between the drift mobility µd and the mean scattering time is given by Equation 2.5 (in textbook), so that µd = ∴ −19 −15 eτ (1.602 × 10 C )(7.22 × 10 s) = me (9.109 × 10 −31 kg) µ d = 1.27 × 10-3 m2 V-1 s -1 = 12.7 cm2 V-1 s - 1 d The mean free path is l = uτ where u is the mean speed. With u ≈ 1.5 × 106 m s-1 we find the mean free path: l = u τ = (1.5 × 106 m s-1)(7.22 × 10-15 s) ≈ 1.08 × 10-8 m ≈ 10.8 nm A thin film of Al must have a much greater thickness than l to show bulk behavior. Otherwise, scattering from the surfaces will increase the resistivity by virtue of Matthiessen's rule. 2.4 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 e Power P = I2R and is proportional to resistivity ρ, assuming the rms current level stays relatively constant. Then we have [P(-40 °C) - P(25 °C)] / P(25 °C) = P(-40 °C) / P(25 °C) - 1 ∴ = ρ(-40 °C) / ρ(25 °C) - 1 ∴ = (2.03 × 10-8 Ω m / 2.72 × 10-8 Ω m) - 1 ∴ = -0.254, or -25.4% (Negative sign means a reduction in the power loss). 2.3 TCR and Matthiessen’s rule Determine the temperature coefficient of resistivity of pure iron and of electrotechnical steel (Fe with 4% C), which are used in various electrical machinery, at two temperatures: 0 °C and 500 °C. Comment on the similarities and differences in the resistivity versus temperature behavior shown in Figure 2Q3-1 for the two materials. Resistivity ( m) 1.5 Fe + 4%C Tangent 1.05 0.96 1 Pure Fe 0.85 Figure 2Q3-1 Resistivity versus temperature for pure iron and 4% C steel. 0.68 0.5 0.4 0.57 0.53 400 C 400 C 0.23 0.11 500 C 0 –400 0 400 800 Temperature ( C) 1200 Solution The temperature coefficient of resistivity αo (TCR) is defined as follows: αo = 1 dρ Slope o = ρo dT To ρo where the slope is dρ/dT at T = To and ρo is the resistivity at T = To. To find the slope, we draw a tangent to the curve at T = To (To = 0 °C and then To = 500 °C) and obtain ∆ρ/∆T ≈ dρ/dT. One convenient way is to define ∆T = 400 °C and find ∆ρ on the tangent line and then calculate ∆ρ/∆T. Iron at 0 ˚C, Slopeo ≈ (0.23 × 10-6 Ω m - 0 Ω m) / (400 °C) = 5.75 × 10-10 Ω m °C -1 2.5 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 Since ρo ≈ 0.11 × 10-6 Ω m, α o = Slopeo/ρo ≈ 0.00523 °C -1 Fe + 4% C at 0 ˚C, Slopeo ≈ (0.57 × 10-6 Ω m - 0.4 × 10-6 Ω m) / (400 °C) = 4.25 × 10-10 Ω m °C -1 Since ρo ≈ 0.53 × 10-6 Ω m, α o = Slopeo/ρo ≈ 0.00802 °C -1 Iron at 500 ˚C, Slopeo ≈ (0.96 × 10-6 Ω m - 0.4 × 10-6 Ω m) / (400 °C) = 1.40 × 10-9 Ω m °C -1 Since ρo ≈ 0.57 × 10-6 Ω m, α o = Slopeo/ρo ≈ 0.00245 °C -1 Fe + 4% C at 500 ˚C, Slopeo ≈ (1.05 × 10-6 Ω m - 0.68 × 10-6 Ω m) / (400 °C) = 9.25 × 10-10 Ω m °C -1 Since ρo ≈ 0.85 × 10-6 Ω m, α o = Slopeo/ρo ≈ 0.00109 °C -1 *2.4 TCR of isomorphous alloys a. Show that for an isomorphous alloy A%-B% (B% solute in A% solvent), the temperature coefficient of resistivity αAB is given by α AB ≈ α AρA ρ AB where ρAB is the resistivity of the alloy (AB) and ρA and αA are the resistivity and TCR of pure A. What are the assumptions behind this equation? b. Estimate the composition of the Cu-Ni alloy that will have a TCR of 4 × 10–4 K–1, that is, a TCR that is an order of magnitude less than that of Cu. Solution a By the Nordheim rule, the resistivity of the alloy is ρalloy = ρo + CX(1–X). We can find the TCR of the alloy from its definition α alloy = 1 dρalloy 1 d = [ ρo + CX (1 ± X )] ρalloy dT ρalloy dT To obtain the desired equation, we must assume that C is temperature independent (i.e. the increase in the resistivity depends on the lattice distortion induced by the impurity) so that d[CX(1 X)]/dT = 0, enabling us to substitute for dρo/dT using the definition of the TCR: αo =(dρo/dT)/ρo. Substituting into the above equation: α alloy = i.e. 1 dρo 1 = α o ρo ρalloy dT ρalloy α alloy ρalloy = α o ρo or α ABρAB = α AρA Remember that all values for the alloy and pure substance must all be taken at the same temperature, or the equation is invalid. b Assume room temperature T = 293 K. Using values for copper from Table 2.1 in Equation 2.17 (both in the textbook), ρCu = 17.1 nΩ m and αCu = 4.0 × 10-3 K-1, and from Table 2.3 (in the textbook) 2.6 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 the Nordheim coefficient of Ni dissolved in Cu is C = 1570 nΩ m. We want to find the composition of the alloy such that αCuNi = 4 × 10-4 K-1. Then, ρalloy = α Cu ρCu (0.0040 K −1 )(17.1 nΩ m ) = = 171.0 nΩ m (0.0004 K −1 ) α alloy Using Nordheim’s rule: ρ alloy = ρ Cu + CX(1 - X) i.e. 171.0 nΩ m = 17.1 nΩ m + (1570 nΩ m)X(1 - X) ∴ X 2 - X + 0.0879 = 0 solving the quadratic, we find X = 0.11. Thus the composition is 89% Cu-11% Ni. However, this value is in atomic percent as the Nordheim coefficient is in atomic percent. Note that as Cu and Ni are very close in the Periodic Table this would also be the weight percentage. Note: the quadratic will produce another value, namely X = 0.86. However, using this number to obtain a composition of 11% Cu-89% Ni is incorrect because the values we used in calculations corresponded to a solution of Ni dissolved in Cu, not vice-versa (i.e. Ni was taken to be the impurity). 2.5 Constantan Constantan has the composition 45% Ni-55% Cu. Cu-Ni alloys show complete solid solubility. As an alloy, constantan is widely used in resistor applications (up to 500 °C), in strain gauges, and as one of the thermocouple metal pairs. Given that the resistivity and TCR of copper at 20 °C are 17 nΩ m and 0.004 K-1, respectively, and the Nordheim coefficient of Ni dissolved in Cu is 1570 nΩ m, calculate the resistivity ρ , TCR (α ), and thermal conductivity κ of constantan and compare the values with the experimental measurements: ρ(20 °C) = 5 × 10-7 Ω m, α (20 °C) = 2 × 10-5 K -1, κ = 21 W m–1 K –1 . What are the reasons for the differences between the calculated and experimental values? Solution Take the given composition to be atomic percentage (approximately true for Cu and Ni in constantan). Otherwise convert 45 wt. % to 47 at. % in calculations. Given ρo = 17 nΩ m, C = 1570 nΩ m and X = 0.45, the alloy resistivity is ρalloy = ρo + CX(1–X) = 17 nΩ m + (1570 nΩ m)(0.45)(1–0.45) ∴ ρ alloy = 405.6 nΩ m (X = 0.47 gives ρalloy = 408.1 nΩ m). The experimental value is 500 nΩ m. The listed value of C in the tables is therefore not very good in predicting the experimental value. In addition, C values are typically determined for very dilute alloys. αalloy = α Cu ρCu (0.004 K −1 )(17 nΩ m ) = 1.68 × 10-4 K- 1 = ρalloy ( 405.6 nΩ m ) This is about an order of magnitude greater than the experimental value. Using ρalloy = 500 nΩ m, we obtain αalloy = 1.36 × 10-4 K-1, only about 20% lower than the theoretical value. Obviously the expression ρalloyαalloy = αAρA does not work well in this case. 2.7 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 The conductivity is σalloy = 1/ ρalloy. The thermal conductivity of the alloy can be found from the Wiedemann-Franz-Lorenz law, κalloy = σalloyTCWFL = (405.6 × 10-9 Ω m)-1(20 + 273 K)(2.44 × 10-8 W Ω K-2) κ alloy = 17.6 W m-1 K- 1 i.e. The experimental value is about 21 W K-1 m-1. Addendum: We know the resistivity to be ρalloy = ρo + CX(1–X). If we were to plot ρalloy -ρo versus X(1 - X), we would obtain a straight line with slope equal to C, the Nordheim coefficient. Table 2Q5-1 lists values taken from Figure 2Q5-1 for resistivity of Ni-Cu alloy in varying compositions, along with corresponding values of X(1 - X): Table 2Q5-1 % of Ni in Ni- at. % of Ni in Resistivity of Change in X(1-X) Cu Ni-Cu alloy (nΩ m) r e s i s t i v i t y ( ρ alloy - ρ o ) 0 0 17 0 0 2 2.162 50 33 0.0211 6 6.464 100 83 0.0605 10 10.74 191 174 0.0958 11 11.80 150 133 0.104 20 21.30 266 249 0.168 22 23.39 300 283 0.179 30 31.69 375 358 0.216 43 44.96 500 483 0.247 45 46.97 500 483 0.249 Using these values, we obtain the following plot: Change in Resistivity (n m) 600 500 400 300 200 100 0 0 0.05 0.1 0.15 0.2 0.25 0.3 -100 X (1 - X) Figure 2Q5-1 Plot of Change in resistivity versus X(1 - X) The plot reveals a straight line with slope equal to 1882.3 nΩ m. This is close to the value of C used before. Using this new value of C, the calculated values would come out closer to the experimental values given. 2.8 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 2.6 Experimental Nordheim coefficient for Pd in Ag Silver and palladium form a complete solid solution and, as alloys, they are widely used in various electrical switches. Pd improves the wear resistance of Ag and the alloy has good fabricability. Table 2Q6-1 shows the resistivity of Ag-Pd alloys for various compositions. Using a suitable plot, obtain the Nordheim coefficient for Pd in Ag. [Note: The atomic masses of Pd and Ag are very close, 106.42 g/mol and 107.87 g/mol, respectively. You can therefore assume that the atomic % = weight %.] Table 2Q6-1 wt.% Pd in Ag Resistivity, nΩ m 0 16.1 1 21.8 3 38.3 10 63.9 30 149.3 Solution Using the data given in Table 2Q6-1, plot ∆ρ versus [X(1 - X)], where ∆ρ is the increase in resistivity due to alloying, namely ∆ρ = ρ - ρo, and X is the atomic fraction of Pd ≈ weight fraction of Pd (ρo = 16.1 nΩ m from Table 2Q6-1). Since the increase in resistivity is ∆ρ = C[X(1 - X)], plotting ∆ρ versus [X(1 - X)] should yield a straight line graph with slope equal to the Nordheim coefficient C. Residual Resistivity, n m 140 120 100 80 60 40 20 0 0 0.05 0.1 0.15 X(1 - X) 0.2 0.25 Figure 2Q6-1 Plot of change in resistivity versus [X(1 - X)]. The slope of the line is the Nordheim coefficient. The equation of the best fit line is ∆ρ = 623.76[X(1 - X)] - 0.5102 Therefore the Nordheim coefficient is C = 623 nΩ m or 62.3 µΩ cm. 2.9 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 2.7 Electrical and thermal conductivity of In Electron drift mobility in indium has been measured to be 6 cm2 V-1 s -1. The room temperature (27 °C) resistivity of In is 8.37 ×10 -8 Ω m, and its atomic mass and density are 114.82 amu or g mol–1 and 7.31 g cm-3, respectively. a. Based on the resistivity value, determine how many free electrons are donated by each In atom in the crystal. How does this compare with the position of In in the Periodic Table (Group IIIB)? b. If the mean speed of conduction electrons in In is 1.74 ×10 8 cm s-1, what is the mean free path? c. Calculate the thermal conductivity of In. How does this compare with the experimental value of 81.6 W m-1 K-1? Solution a From σ = enµd (σ is the conductivity of the metal, e is the electron charge, and µd is the electron drift mobility) we can calculate the concentration of conduction electrons (n): n= i.e. σ (8.37 × 10 −8 Ω m )−1 = eµ d (1.602 × 10 −19 C)(6 × 10 −4 m 2 V -1s-1 ) n = 1.243 × 1029 m-3 Atomic concentration nat is dN A (7.31 × 10 3 kg m -3 )(6.022 × 10 23 mol −1 ) nat = = (114.82 × 10 −3 kg mol −1 ) Mat i.e. nat = 3.834 × 1028 m-3 Effective number of conduction electrons donated per In atom (neff) is: neff = n / nat = (1.243 × 1029 m-3) / (3.834 × 1028 m-3) = 3.24 Conclusion: There are therefore about three electrons per atom donated to the conductionelectron sea in the metal. This is in good agreement with the position of the In element in the Periodic Table (III) and its valency of 3. b If τ is the mean scattering time of the conduction electrons, then from µd = eτ/me (me = electron mass) we have: µ d me (6 × 10 −4 m 2 V -1 s-1 )(9.109 × 10 −31 kg) = 3.412 × 10-15 s τ= = −19 e (1.602 × 10 C) Taking the mean speed u ≈ 1.74 × 106 m s-1, the mean free path (l) is given by l = u τ = (1.74 × 106 m s-1)(3.412 × 10-15 s) = 5.94 × 10-9 m or 5.94 nm c From the Wiedemann-Franz-Lorenz law, thermal conductivity is given as: κ = σTCWFL = (8.37 × 10-8 Ω m)-1(20 ˚C + 273 K)(2.44 × 10-8 W Ω K-2) i.e. κ = 85.4 W m-1 K- 1 This value reasonably agrees with the experimental value. 2.10 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 2.8 Electrical and thermal conductivity of Ag The electron drift mobility in silver has been measured to be 56 cm2 V-1 s-1 at 27 °C. The atomic mass and density of Ag are given as 107.87 amu or g mol–1 and 10.50 g cm-3, respectively. a. Assuming that each Ag atom contributes one conduction electron, calculate the resistivity of Ag at 27 °C. Compare this value with the measured value of 1.6 × 10-8 Ω m at the same temperature and suggest reasons for the difference. b. Calculate the thermal conductivity of silver at 27 °C and at 0 °C. Solution a Atomic concentration nat is nat = dN A (10.50 × 10 3 kg m -3 )(6.022 × 10 23 mol −1 ) = (107.87 × 10 −3 kg mol −1 ) Mat nat = 5.862 × 1028 m-3 i.e. If we assume there is one conduction electron per Ag atom, the concentration of conduction electrons (n) is 5.862 × 1028 m-3, and the conductivity is therefore: σ = enµd = (1.602 × 10-19 C)(5.862 × 1028 m-3)(56 × 10-4 m2 V-1s-1) ∴ σ = 5.259 × 107 Ω-1 m-1 and the resistivity ρ = 1/σ = 19.0 nΩ m The experimental value of ρ is 16 nΩ m. We assumed that exactly 1 "free" electron per Ag atom contributes to conduction - this is not necessarily true. We need to use energy bands to describe conduction more accurately and this is addressed in Chapter 4 (in the textbook). From the Wiedemann-Franz-Lorenz law at 27 °C, b κ = σTCWFL = (5.259 × 107 Ω-1 m-1)(27 + 273 K)(2.44 × 10-8 W Ω K-2) κ = 385 W m-1 K- 1 i.e. For pure metals such as Ag this is nearly independent of temperature (same at 0 °C). 2.9 Mixture rules A 70% Cu - 30% Zn brass electrical component has been made of powdered metal and contains 15 vol. % porosity. Assume that the pores are dispersed randomly. Estimate the effective electrical resistivity of the brass component. Solution: Assume room temperature T = 293 K. From Table 2.1 and Equation 2.17 in the textbook, ρo for Cu is 17 nΩ m, and from Table 2.3 (in the textbook) C = 300 nΩ m. The atomic fraction of the impurity Zn is given as X = 0.3 (we can take the weight fraction given as the atomic fraction because Cu and Zn have very close atomic weights). The brass metal resistivity is then: ρ = ρo + CX(1–X) = 17 nΩ m + (300 nΩ m)(0.3)(1 – 0.3) 2.11 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Chapter 2 ρ = 80.0 nΩ m The component has 15% air pores. Apply the empirical mixture rule in Equation 2.24 (in the textbook). The fraction of volume with air pores is χ = 0.15. Then, ρeff = ρ (1 + 12 χ ) (1 + 12 0.15) = (80.0 nΩ m ) = 101 nΩ m (1 − χ ) (1 − 0.15) 2.10 Thermal conduction a. An 80 at % Cu–20 at % Zn brass disk of 40 mm diameter and 5 mm thickness is used to conduct heat from a heat source to a heat sink. 1. Calculate the thermal resistance of the brass disk. 2. If the disk is conducting heat at a rate of 100 W, calculate the temperature drop along the disk. b. What should be the composition of brass if the temperature drop across the disk is to be halved? Solution a (1) Assume T = 20 ˚C = 293 K. Apply Equation 2.20 (in the textbook) to find the resistivity of the brass in the disk with ρCu = 17.1 nΩ m (Table 2.1 and Equation 2.17, in the textbook) and XZn = 0.20: ρ brass = ρ Cu + C Zn in CuX Zn(1 – X Zn) i.e. ρbrass = 17.1 nΩ m + (300 nΩ m)(0.20)(1 – 0.20) ∴ ρbrass = 65.1 nΩ m We know that the thermal conductivity is given by κ/σbrass = CFWLT where σbrass is the conductivity of the disk, CFWL is the Lorenz number and T is the temperature. This equation can also be written as κρbrass = CFWLT so that κ = CFWLT/ρ. Applying this equation, κ(20 °C) = (2.44 × 10-8 W Ω K-2)(293 K) / (6.51 × 10-8 Ω m) ∴ κ(20 °C) = 109.8 W K-1 m-1 The thermal resistance is θ = L/(κA), where L is the thickness of the disk and A is the crosssectional area of the disk. θ = L/(κA) = (5 × 10-3 m)/[(109.8 W K-1 m-1)(π)(2 × 10-2 m)2] = 0.0362 K W-1 (2) From dQ/dt = Aκ∆T/∆x = ∆T/θ (∆x can be taken to be the same as L), and dQ/dt = P (power conducted), we can substitute to obtain: ∆T = P θ = (100 W)(3.62 × 10-2 K W-1) = 3.62 K or 3.62 °C Note: Change in temperature is the same in either Kelvins or degrees Celsius - i.e. ∆T = T1 - T2 = (T1 + 273) - (T2 + 273). b Since ∆T = Pθ, to get half ∆T, we need half θ or double κ or double σ or half ρ. We thus need /2ρbrass or 1/2(65.1 nΩ m) which can be attained if the brass composition is Xnew so that 1 ρnew = ρCu + C Zn in CuX new(1 – X new) 2.12 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 1 i.e. /2(65.1 nΩ m) = 17 nΩ m + (300 nΩ m)Xnew(1 – Xnew) Solving this quadratic equation we get Xnew = 0.0545, or 5.5% Zn. Thus we need 94.5% Cu5.5% Zn brass. 2.11 Thermal resistance Consider a thin insulating disc made of mica to electrically insulate a semiconductor device from a conducting heat sink. Mica has κ = 0.75 W m-1 K-1. The disk thickness is 0.1 mm, and the diameter is 10 mm. What is the thermal resistance of the disk. What is the temperature drop across the disk if the heat current through it is 25W. Solution The thermal resistance of the mica disk can be calculated directly from Equation 2.37 (in the textbook) 4(1 × 10 −3 m ) L 4L -1 θ= = = 2 = 1.698 K W κ A κ π d 2 π (0.75 W m −1 K −1 )(1 × 10 −2 ) The temperature drop across the disk according to Equation 2.36 (in the textbook) is ∆T = Q′θ = 42.4 °C *2.12 Thermal resistance Consider a coaxial cable operating under steady state conditions when the current flow through the inner conductor generates Joule heat at a rate P = I 2 R. The heat generated per second by the core conductor flows through the dielectric; Q’ = I2R. The inner conductor reaches a temperature Ti whereas the outer conductor is at To. Show that the thermal resistance θ of the hollow cylindrical insulation is b (T − T ) ln a θ= i o = Q© Thermal resistance of hollow cylinder 2πκL where a is the inside (core conductor) radius, b is the outside radius (outer conductor), κ is the thermal conductivity of the insulation, and L is the cable length. Consider a coaxial cable that has a copper core conductor and polyethylene (PE) dielectric with following properties: Core conductor resistivity ρ = 19 nΩ m, core radius, a = 4 mm, dielectric thickness, b -a = 3.5 mm, dielectric thermal conductivity κ = 0.3 W m-1 K-1. The outside temperature To is 25 °C. The cable is carrying a current of 500 A. What is the temperature of the inner conductor? Solution Consider a thin cylindrical shell of thickness dr as shown in Figure 2Q12-1. The temperature difference across dr is dT. The surface area of this shell is 2πrL. Thus, from Fourier’s law, dT dr which we can integrate with respect to r from r = a where T = Ti to r = b where T = To, Q′ = −(2πrL)κ 2.13 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 T b o dr Q′ ∫ = −2πLκ ∫ dT r a Ti i.e. Q′ = (Ti − To ) 2πκL b ln a Thus the thermal resistance of the hollow cylindrical insulation is b ln a (T − T ) θ= i o = Q′ 2πκL L Inner conductor Dielectric Thin shell b To Ti a I2R Q r dr To Outer conductor dT Figure 2Q12-1 Thermal resistance of a hollow cylindrical shell. Consider an infinitesimally thin cylindrical shell of radius r and thickness dr in the dielectric and concentrically around the inner conductor. The surface area is 2πrL. The actual length of the conductor does not affect the calculations as long as the length is sufficiently long that there is no heat transfer along the length; heat flows radially from the inner to the outer conductor. We consider a portion of length L of a very long cable and we set L = 1 m so that calculations are per unit length. The joule heating per unit second (power) generated by the current I through the core conductor is Q′ = I 2 −9 ρL 2 (19 × 10 )(1) ( 500 ) = 94.5 W = πa 2 π ( 4 × 10 −3 )2 The thermal resistance of the insulation is, −3 b ln ( 4 + 3.5) × 10 ln a 4 × 10 −3 θ= = = 0.33 °C / W 2π (0.3)(1) 2πκL Thus, the temperature difference ∆T due to Q′ flowing through θ is, ∆T = Q′θ = (94.5 W)(0.33 °C/W) = 31.2 °C. The inner temperature is therefore, T i = T o + ∆T = 25 + 31.2 = 56.2 °C. 2.14 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 Note that for simplicity we assumed that the inner conductor resistivity ρ and thermal conductivity κ are constant (do not change with temperature). 2.13 The Hall effect Consider a rectangular sample, a metal or an n-type semiconductor, with a length L, width W , and thickness D. A current I is passed along L, perpendicular to the cross-sectional area WD. The face W × L is exposed to a magnetic field density B. A voltmeter is connected across the width, as shown in Figure 2Q13-1, to read the Hall voltage VH. a. Show that the Hall voltage recorded by the voltmeter is IB Hall voltage Den b. Consider a 1-micron-thick strip of gold layer on an insulating substrate that is a candidate for a Hall probe sensor. If the current through the film is maintained at constant 100 mA, what is the magnetic field that can be recorded per µV of Hall voltage? VH = B L W VH D I Figure 2Q13-1 Hall effect in a rectangular material with length L, width W, and thickness D. The voltmeter is across the width W. Solution a The Hall coefficient, RH, is related to the electron concentration, n, by RH = -1 / (en), and is defined by RH = Ey / (JB), where Ey is the electric field in the y-direction, J is the current density and B is the magnetic field. Equating these two equations: 1 Ey = en JB JB Ey = − en This electric field is in the opposite direction of the Hall field (EH) and therefore: − ∴ EH = -Ey = JB en (1) The current density perpendicular (going through) the plane W × D (width by depth) is: I WD I W= JD J= Isolating for W: (2) 2.15 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 The Hall voltage (VH) across W is: VH = WEH If we substitute expressions (1) and (2) into this equation, the following will be obtained: IB Den Note: this expression only depends on the thickness and not on the length of the sample. In general, the Hall voltage will depend on the specimen shape. In the elementary treatment here, the current flow lines were assumed to be nearly parallel from one end to the other end of the sample. In an irregularly shaped sample, one has to consider the current flow lines. However, if the specimen thickness is uniform, it is then possible to carry out meaningful Hall effect measurements using the van der Pauw technique as discussed in advanced textbooks. VH = b We are given the depth of the film D = 1 micron = 1 µm and the current through the film I = 100 mA = 0.1 A. The Hall voltage can be taken to be VH = 1 µV, since we are looking for the magnetic field B per µV of Hall voltage. To be able to use the equation for Hall voltage in part (a), we must find the electron concentration of gold. Appendix B in the textbook contains values for gold’s atomic mass (Mat =196.97 g/mol) and density (d = 19.3 g/cm3 = 19300 kg/m3). And since gold has a valency of 1 electron, the concentration of free electrons is equal to the concentration of Au atoms. Therefore: n= −3 23 -1 dN A (19300 kg m )(6.022 × 10 mol ) = = 5.901 × 10 28 m −3 −3 −1 Mat (196.97 × 10 kg mol ) Now the magnetic field B can be found using the equation for Hall voltage: VH = ∴ ∴ IB Den −6 −6 −19 C)(5.901 × 10 28 m −3 ) VH Den (1 × 10 V)(1 × 10 m )(1.602 × 10 B= = I (0.1 A) B = 0.0945 T As a side note, the power (P) dissipated in the film could be found very easily. Using the value for resistivity of Au at T = 273 K, ρ = 22.8 nΩ m, the resistance of the film is: R= −9 ρL ρL (22.8 × 10 Ω m )(0.001 m ) = = = 0.228 Ω A WD (0.0001 m)(1 × 10 −6 m) The power dissipated is then: P = I2R = (0.1 A)2(0.228 Ω) = 0.00228 W 2.14 The strain gauge A strain gauge is a transducer attached to a body to measure its fractional elongation ∆L/L under an applied load (force) F. The gauge is a grid of many folded runs of a thin, resistive wire glued to a flexible backing, as depicted in Figure 2Q14-1. The gauge is attached to the body under test such that the resistive wire length is parallel to the strain. a. Assume that the elongation does not change the resistivity and show that the change in the resistance ∆R is related to the strain, ε = ∆L/L by 2.16 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∆R ≈ R(1+2υ)ε Chapter 2 Strain gauge equation where υ is the Poisson ratio, which is defined by v=− ε Transverse strain =− t εl Longitudinal strain Position ratio where εl is the strain along the applied load, that is, εl = ∆L/L = ε, and εt is the strain in the transverse direction, that is, εt = ∆D/D, where D is the diameter (thickness) of the wire. b. Explain why a nichrome wire would be a better choice than copper for the strain gauge (consider the TCR). c. How do temperature changes affect the response of the gauge? Consider the effect of temperature on ρ. Also consider the differential expansion of the specimen with respect to the gauge wire such that even if there is no applied load, there is still strain, which is determined by the differential expansion coefficient, λspecimen – λgauge, where λ is the thermal coefficient of linear expansion: L = Lo[1 + λ(T – To)], where To is the reference temperature. d. The gauge factor for a transducer is defined as the fractional change in the measured property ∆R/R per unit input signal (ε). What is the gauge factor for a metal-wire strain gauge, given that for most metals, υ ≈ 1/3? e. Consider a strain gauge that consists of a nichrome wire of resistivity 1 µΩ m, a total length of 1 m, and a diameter of 25 µm. What is ∆R for a strain of 10–3? Assume that υ ≈ 1/3. f. What will ∆R be if constantan wire is used (see Question 2.5)? Gauge Length Solder Tab Adhesive tape Grid of metal wires Figure 2Q14-1 The strain gauge consists of a long, thin wire folded several times along its length to form a grid as shown and embedded in a self-adhesive tape. The ends of the wire are attached to terminals (solder pads) for external connections. The tape is stuck on the component for which the strain is to be measured. Solution a Consider the resistance R of the gauge wire, ρL (1) 2 D π 2 where L and D are the length and diameter of the wire, respectively. Suppose that the applied load changes L and D by δL and δD which change R by δR. The total derivative of a function R of two variables L and D can be found by taking partial differentials (like those used for error calculations in physics labs). Assuming that ρ is constant, R= 2.17 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 ρ ρL ∂R ∂R δR = δL + δD = δL − 2 δD π 3 π 2 ∂L ∂D D D 4 4 so that the fractional change is ρ L ρ π 3 π 2 D D δR 4 δD = δL − 2 4 R ρL ρL π 2 π 2 D D 4 4 i.e. δR δL δD = −2 R L D We can now use the definitions of longitudinal and transverse strain, δL = εl L (2) δD = ε t = −υε l D and in the expression for δR/R in Eqn. (2) to obtain, δR = ε l − 2( −υε l ) = (1 + 2υ )ε R (3) where ε = εl. b The change in R was attributed to changes in L and D due to their extension by the applied load. There are two reasons why a nichrome wire will be a better choice. First is that nichrome has a higher resistivity ρ which means that its resistance R will be higher than that of a similar size copper wire and hence nichrome wire will exhibit a greater change in R (δR is easier to measure). Secondly nichrome has a very small TCR which means that ρ and hence R do not change significantly with temperature. If we were to use a Cu wire, any small change in the temperature will most likely overwhelm any change due to strain. c Consider a change δR in R due to a change δT in the temperature. We can differentiate R with respect to T by considering that ρ, L and D depend on T. It is not difficult to show (See Question 2.15) that if α is the temperature coefficient of resistivity and λ is the linear expansion coefficient , then 1 dR = α − λ R dT (4) Typically λ ≈ 2 × 10-5 K-1, and for pure metals α ≈ 1/273 K-1 or 3.6 × 10-3 K-1. A 1 °C fluctuation in the temperature will result in δR/R = 3.6 × 10-3 which is about the same as δR/R from a strain of ε = 2 × 10-3 at a constant temperature. It is clear that temperature fluctuations would not allow sensible strain measurements if we were to use a pure metal wire. To reduce the temperature fluctuation effects, we need α ≈ λ which means we have to use a metal alloy such as a nichrome wire or a constantan wire. Even if we make α - λ = 0, the temperature change still produces a change in the resistance because the metal wire and specimen expand by different amounts and this creates a strain and hence a 2.18 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 change in the resistance. Suppose that λgauge and λspecimen are the linear expansion coefficients of the gauge wire and the specimen, then the differential expansion will be λspecimen - λspecimen and this can only be zero if λgauge = λspecimen. d The gauge factor is defined as GF = Fractional change in gauge property δR / R (1 + 2υ )ε = = ε ε Input signal GF = 1 + 2υ = 1 + 2(1/3) = 1.67 i.e. which applies to metals inasmuch as υ ≈ 1/3. For nichrome wire, given that ρ = 10-6 Ω m, L = 1 m, and D = 25 × 10-6 m, e we have, R= ρL (10 −6 Ω m)(1 m) = 2 2 = 2037 Ω −6 D m × 25 10 π π 2 2 For a strain of ε = 10–3, δR = R(1 + 2υ)ε = (2037 Ω)[1 + 2(1/3)](10-3) = 3.40 Ω This can be easily measured with an ohm-meter, though instrumentation engineers would normally use the gauge in an electrical bridge circuit. Note that δR/R = (3.4 Ω)/(2037 Ω) = 0.0017, or 0.17 %. For a constantan wire, given that ρ = 5 × 10-7 Ω m, L = 1 m, and D = 25 × 10-6 m, f (5 × 10 −7 Ω m)(1 m) we have, R= so that δR = R(1 + 2υ)ε = (1019 Ω)[1 + 2(1/3)](10-3) = 1.70 Ω 25 × 10 −6 m π 2 2 = 1019 Ω 2.15 Thermal coefficients of expansion and resistivity a. Consider a thin metal wire of length L and diameter D. Its resistance is R = ρ L/A, where A = πD2/4. By considering the temperature dependence of L, A, and ρ individually, show that 1 dR = α o − λo R dT where αo is the temperature coefficient of resistivity (TCR), and λ o is the temperature coefficient of linear expansion (thermal expansion coefficient or expansivity), that is, dL λo = Lo −1 dT T = To or dD λo = Do −1 dT T = To Note: Consider differentiating R = ρL/[(πD2)/4] with respect to T with each parameter, ρ , L, and D, having a temperature dependence. Given that typically, for most pure metals, αo ≈ 1/273 K-1 and λo ≈ 2 × 10–5 K –1 , confirm that the temperature dependence of ρ controls R, rather than the temperature dependence of the geometry. Is it necessary to modify the given equation for a wire with a noncircular cross section? 2.19 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 b. Is it possible to design a resistor from a suitable alloy such that its temperature dependence is almost nil? Consider the TCR of an alloy of two metals A and B, for which αAB ≈ αAρA/ρAB. Solution a Consider the resistance R of the wire, R= ρL 2 D π 2 (1) Consider a change δR in R due to a change δT in the temperature. We can differentiate R with respect to T by considering that ρ, L, and D depend on T, dR d ρL L dρ ρ dL ρL dD −2 + = = π 3 dT dT dT π D2 π D2 dT π D2 dT D 4 4 4 4 divide by R: 1 dR = 1 dρ + 1 dL − 2 1 dD R dT ρ dT L dT D dT (2) At T = To we have, 1 dR = 1 dρ + 1 dL − 2 1 dD R dT ρo dT Lo dT Do dT (3) where the derivatives are at T = To. Recall that the temperature coefficient of resistivity, TCR (αo), and the linear expansion coefficient λo are defined as follows, 1 dρ α0 = ρ0 dT and 1 dL 1 dD = λ0 = L0 dT D0 dT (4) which reduces Eqn. (3) to 1 dR = α − λ 0 0 R dT (5) Typically λo ≈ 2 × 10-5 K-1, and for pure metals αo ≈ 1/273 K-1 or 3.6 × 10-3 K-1. Thus, 1 dR = 3.6 × 10 −3 K −1 − 2 × 10 −5 K −1 ≈ 3.6 × 10 −3 K −1 ≈ α 0 R dT Since αo is much larger than λo, it dominates the change in R. 2.20 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 Rf Af Figure 2Q15-1 Wire with non-circular cross section. There is no need to modify Eqn. (5) for a non-circular cross sectional area. You can derive the same expression for a rectangular or an elliptic cross section, or, indeed, any arbitrary cross section. One can consider the wire to be made up of N thin fibers each of circular cross section Af. Imagine holding a bunch of these in your hand and then sliding them into any cross section you like as in the Figure 2Q15-1. In all cases A = ΣAf. However, since the fibers are in parallel, the total resistance is given by R-1 = ΣRf-1 = NRf-1. Thus R = Rf / N. For each fiber, δRf = Rf(αo - λo)δT as we have derived above. Then, δR = (δRf)/N = [Rf(αo - λo)δT]/N = R(αo - λo)δT [There are a few assumptions such as the resistivity is homogeneous and the cross section does not change along the wire!] b For pure metals, αo > > λo. Alloying metal A with metal B reduces the TCR (temperature coefficient of resistivity) αA of metal A to αAB = αA(ρA/ρAB). The αo - λo can be brought to zero by using a suitable composition alloy for which αo = λo. Since strictly αo depends (however slightly) on T, the condition αo - λo can only be exactly true at one temperature or approximately true in a small region around that temperature. In practice this temperature range may be sufficient to cover a typical application range in which, for most practical purposes, αo - λo is negligible. 2.16 Einstein relation and ionic conductivity In the case of ionic conduction, ions have to jump from one interstice to the neighboring one. This process involves overcoming a potential energy barrier just like atomic diffusion, and drift and diffusion are related. The drift mobility µ of ions is proportional to the diffusion coefficient D because drift is limited by the atomic diffusion process. The Einstein relation relates the two by D kT = e µ Einstein relation The diffusion coefficient of the Na+ ion in sodium silicate (Na2O-SiO2) glasses at 400 °C is 3.4 × 10-9 cm2 s-1. The density of such glasses is 2.4 g cm-3. Calculate the ionic conductivity and resistivity of (17.5 mol% Na2O)(82.5 mol% SiO2) sodium silicate glass at 400 °C and compare your result with the experimental value of about 104 Ω cm for the resistivity. 2.21 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 Solution We can apply the conductivity expression σ = qi ni µi , where qi is the charge of the ion, Na+, so that it is +e, ni is the concentration of the Na+ ions in the glass and µi is their mobility. We can think of the glass as built from [(Na2O)0.175(SiO2)0.825] units. The atomic masses of Na, O and Si are 23, 16 and 28.1 g mole-1 respectively. The atomic mass of one unit is Mat = 0.175[2 × (23) + 16] + 0.825[28.1 + 2 × (16)] = 60.43 g mol-1 of [(Na2O) 0.175(SiO 2)0.825] The number of [(Na2O)0.175(SiO2)0.825] units per unit volume can be found from the density d by −3 23 −1 d N A (2.4 g cm )(6.02 × 10 mol ) n= = = 2.39 × 1022 units cm- 3 −1 Mat (60.43 g mol ) The concentration of Na+ ions is equal to the concentration of Na atoms and then ni = nNa = (atomic fraction of Na in the unit ) × n = 0.175 × (2) 22 −3 21 −3 = (2.39 × 10 cm ) = 2.79 × 10 cm 0 . 175 ( 2 1 ) 0 . 825 ( 1 2 ) + + + The mobility of sodium ions can be calculated from Einstein relation −19 −9 2 −1 eD (1.602 × 10 C)(3.4 × 10 cm s ) = µi = = 6.04 × 10-8 cm2 V-1 s- 1 kT (1.38 × 10 −23 J K −1 )((400 + 273) K) Thus the conductivity of the glass is σ = eni µi = (1.602 × 10 −19 C)(2.79 × 10 21 cm −3 )(6.04 × 10 -8 cm 2 V −1 s −1 ) = 2.7 × 10-5 Ω -1 cm-1 and its resistivity is ρ= 1 1 = = 3.8 × 104 Ω cm −5 −1 −1 σ (2.7 × 10 Ω cm ) 2.17 Skin effect a. What is the skin depth for a copper wire carrying a current at 60 Hz? The resistivity of copper at 27 °C is 17 nΩ m. Its relative permeability is µr ≈ 1. Is there any sense in using a conductor for power transmission with a diameter of more than 2 cm? b. What is the skin depth for an iron wire carrying a current at 60 Hz? The resistivity of iron at 27 °C is 97 nΩ m. Assume that its relative permeability is µr ≈ 700. How does this compare with the copper wire? Discuss why copper is preferred over iron for power transmission even though the iron is nearly 100 times cheaper than copper. Solution a The conductivity is 1/ρ. The relative permeability (µr) for copper is 1, thus µCu = µo. The angular frequency is ω = 2πf = 2π(60 Hz). Using these values in the equation for skin depth (δ): 2.22 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) 1 δ= ∴ 1 1 ω o µCu 2 ρ Chapter 2 1 = 4π × 10 −7 H m −1 ) 1 -1 ( (2π 60 s ) (17 × 10 −9 Ω m) 2 δ = 0.00847 m or 8.47 mm This is the depth of current flow. If the radius of wire is 10 mm or more, no current flows through the core region and it is wasted. There is no point in using wire much thicker than a radius of 10 mm (diameter of 20 mm). b The conductivity is 1/ρ. The relative permeability (µr) for Iron is 700, thus µFe = 700µo. The angular frequency is ω = 2πf = 2π(60 Hz). Using these values in the equation for skin depth (δ): 1 δ Fe = ∴ 1 1 ω o µ Fe 2 ρ = 1 (700)(4π × 10 −7 H m −1 ) 1 -1 2 60 π s ( ) (97 × 10 −9 Ω m) 2 δ = 0.000765 m or 0.765 mm Thus the skin depth is 0.765 mm, about 11 times less than that for copper. To calculate the resistance we need the cross sectional area for conduction. The material cross sectional area is πr2 where r is the radius of the wire. But the current flow is within depth δ. We deduct the area of the core, π (r - δ)2, from the overall area, πr2, to obtain the cross sectional area for conduction. Comparison of Cu and Fe based on solid core wires The resistance per unit length of the solid core Fe wire (RFe) is: ρFe ρFe ρFe = 2 = 2 AFe πrFe − π (rFe − δ Fe ) 2πrFeδ Fe − πδ Fe 2 RFe = The resistance per unit length of solid core Cu wire is: RCu = ρCu ρCu ρCu = 2 = 2 ACu πrCu − π (rCu − δ Cu ) 2πrCuδ Cu − πδ Cu 2 If we equate these two resistances, we can make a comparison between Fe and Cu: RFe = RCu ∴ ρFe ρCu = 2 2πrFeδ Fe − πδ Fe 2πrCuδ Cu − πδ Cu 2 ∴ 2πρCu rFeδ Fe = ρFe (2πrCuδ Cu − πδ Cu 2 ) (Neglect the δFe2 term which is small) ∴ rFe = ρFe (2πrCuδ Cu − πδ Cu 2 ) 2πρCuδ Fe We can assume a value for rCu for calculation purposes, rCu = 10 mm. The resistivity ρCu is given as 17 nΩ m and the skin depth of Cu is known to be δCu = 8.47 mm. The resistivity of Fe is given as ρFe = 97 nΩ m and its skin depth was just calculated to be δFe = 0.765 mm. We can substitute these values into the above equation to determine the radius of Fe wire that would be equivalent to 10 mm diameter Cu wire. 2.23 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) rFe ∴ (97 × 10 = −9 [ Ω m ) 2π (0.010 m )(0.00847 m ) − π (0.00847 m ) 2π (17 × 10 −9 Ω m )(0.000765 m ) 2 Chapter 2 ] r Fe = 0.364 m Now compare the volume (V) of Fe per unit length to the volume of Cu per unit length: VFe πrFe 2 (1 m ) (0.364 m ) = = 2 = 1325 VCu πrCu 2 (1 m ) (0.010 m ) 2 Even though Fe costs 100 times less than Cu, we need about 1300 times the volume of Cu if Fe is used. The cost disadvantage is 13 times in addition to weight disadvantage. ADDENDUM JANUARY 2001 Submitted by George Belev Comparison of Cu and Fe wires: any shape and any number To determine if it is worthwhile to use iron rather than copper, we must compare the amount of iron needed to perform the equivalent task of some amount of copper (i.e. have the same resistance). Let us first assume that by choosing a proper shape for the conductors we can eliminate the influence of the skin effect on conduction. The resistance per unit length of the Fe wire (RFe) is: RFe = ρFe AFe The resistance per unit length of Cu wire is RCu = ρCu ACu If we equate these two resistances, we can make a direct comparison between Fe and Cu: RFe = RCu ∴ AFe = ρ Fe ACu ρCu and the volumes of iron and copper per unit length will be in the same ratio VFe = ρ Fe VCu ρCu Let us compare the masses of Fe and Cu needed per unit length MFe VFe DFe ρ Fe DFe 97 nΩ m 7.86 g cm −3 = = = ⋅ ≈5 MCu VCu DCu ρCu DCu 17 nΩ m 8.96 g cm −3 Since Fe costs 100 times less than Cu, if we use iron conductors, we will reduce the cost for wire by 100/5 = 20 times. So it seems that the use of Fe will have great economic advantage if we can find a reasonable way to eliminate the influence of the skin effect on conduction. There is no sense in making the conductor with a radius bigger than the skin depth, so let us consider a single copper conductor with radius δCu, and using N iron conductors each with radius δCu as shown bellow: 2.24 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 2 δFe 2 δ Cu N As we have calculated above both conductors will have equal resistance per unit length if 2 AFe N πδ Fe ρ = = Fe 2 ACu πδ Cu ρCu and we can calculate the number N of Fe wires which should run in parallel N= 2 ρ Fe δ Cu ≈ 700 2 ρCu δ Fe Thus, copper as a single conductor has 700 times better performance than a single iron conductor. It is not necessary to manufacture 700 Fe wires and run them in parallel. The iron conductor can be produced more conveniently in the shapes shown bellow and it will be cheaper than the Cu conductor. δFe = 0.763 mm 2 δ Fe = 1.53 mm 4 N δ = 682 mm Fe π 2 N δ Fe = 1071 mm But it will be of impractical size, it will have poor mechanical properties and will be 5 times heavier compared with the single Cu wire. A power grid based on Cu conductors will be much cheaper and much smaller than the one based on Fe conductors. 2.18 Temperature of a light bulb a. Consider a 100 W, 120 V incandescent bulb (lamp). The tungsten filament has a length of 0.579 m and a diameter of 63.5 µm. Its resistivity at room temperature is 56 nΩ m. Given that the resistivity of the filament can be represented as T ρ = ρ0 T0 n Resistivity of W where T is the temperature in K, ρo is the resistance of the filament at To K, and n = 1.2, estimate the temperature of the bulb when it is operated at the rated voltage, that is, directly from the mains outlet. Note that the bulb dissipates 100 W at 120 V. b. Suppose that the electrical power dissipated in the tungsten wire is totally radiated from the surface of the filament. The radiated power at the absolute temperature T can be described by Stefan's Law Pradiated =∈σsA(T4 – To4) Radiated power where σs is Stefan's constant (5.67 × 10–8 W m–2 K –4), ∈ is the emissivity of the surface (0.35 for tungsten), A is the surface area of the tungsten filament, and T o is the room temperature (293 K). Obviously, for T > To, Pradiated = ∈σsAT4. 2.25 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 2 Assuming that all of the electrical power is radiated from the surface, estimate the temperature of the filament and compare it with your answer in part (a). c. If the melting temperature of W is 3407 °C, what is the voltage that guarantees that the light bulb will blow? Solution a ∴ First, find the current through the bulb at 100 W and 120 V. P = VI I = P/V = (100 W)/(120 V) = 0.8333 A From Ohm’s law the resistance of the bulb can be found: R = V/I = (120 V)/(0.8333 A) = 144.0 Ω The values for length of the filament (L = 0.579 m) and diameter of the filament (D = 63.5 µm) at operating temperature are given. Using these values we can find the resistivity of the filament when the bulb is on (ρ1). ρ1 L π 2 D 4 2 π π R D2 (144.0 Ω) (63.5 × 10 −6 m ) 4 ρ1 = 4 = 7.876 × 10 −7 Ω m = L (0.579 m) R= ∴ Now the bulb’s operating temperature (T1) can be found using our obtained values in the equation for resistivity of W (assuming room temperature To = 293 K and given n = 1.2): T ρ = ρ0 T0 n 1 1 ρ n 7.876 × 10 −7 Ω m 1.2 T1 = T0 1 = (293 K ) isolate: -9 = 2652 K 56 × 10 Ω m ρ0 b First we need the surface area A of the Tungsten filament. Since it is cylindrical in shape: A = L(πD) = (0.579 m)(π)(63.5 × 10-6 m) = 0.0001155 m2 Now the temperature of the filament T1 can be found by isolating it in Stefan’s equation and substituting in the given values for emissivity (∈ = 0.35), Stefan’s constant (σs = 5.67 × 10-8 W m-2 K-4) and room temperature (To = 293 K). P =∈σ s A(T14 − T0 4 ) 1 ∴ P 4 T1 = + T0 4 ∈σ s A ∴ 4 100 W 4 293 K T1 = + ( ) −8 −2 −1 2 (0.35)(5.67 × 10 W m K )(0.0001155 m ) 1 2.26 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Chapter 2 T 1 = 2570 K Note: We can even ignore To to get the same temperature since To << T1: P =∈σ s AT14 1 1 4 ∴ 4 P 100 W = T1 = −8 −2 −1 2 ∈σ s A (0.35)(5.67 × 10 W m K )(0.0001155 m ) ∴ T 1 = 2570 K These values are fairly close to the answer obtained in part (a). c Let V be the voltage and R be the resistance when the filament is at temperature Tm. We are given the temperature Tm = 3407 °C + 273 = 3680 K. Since we know the following: R= L π 2 D 4 T ρ = ρ0 m T0 ρ n We can make a substitution for ρ and use the values given for the light bulb filament to find the resistance of the filament at temperature Tm. n 1.2 T (0.579 m) 3680 K −9 R= × ρ0 m = 56 10 Ω m ( ) 293 K 2 π 2 T0 π 63.5 × 10 −6 m ) D ( 4 4 L ∴ R = 213.3 Ω Assuming that all electrical power is radiated from the surface of the bulb, we can use Stefan’s law and substitute in V2/R for power P: V2 =∈σ s A(T14 − T0 4 ) R ∴ ∴ ∴ V 2 = R ∈σ s A(Tm 4 − T0 4 ) V 2 = (213.3 Ω)(0.35)(5.67 × 10 −8 W m −2 K −4 )(0.0001155 m ) [(3680 K ) − (293 K ) 4 4 ] V = 299 V "I think there is a world market for maybe five computers" Thomas Watson (Chairman of IBM, 1943) 2.27 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 Second Edition ( 2001 McGraw-Hill) Chapter 3 3.1 Photons and photon flux a. Consider a 1 kW AM radio transmitter at 700 kHz. Calculate the number of photons emitted from the antenna per second. b. The average intensity of sunlight on Earth's surface is about 1 kW m-2. The maximum intensity is at a wavelength around 800 nm. Assuming that all the photons have an 800 nm wavelength, calculate the number of photons arriving on Earth's surface per unit time per unit area. What is the magnitude of the electric field in the sunlight? c. Suppose that a solar cell device can convert each sunlight photon into an electron, which can then give rise to an external current. What is the maximum current that can be supplied per unit area (m2) of this solar cell device? Solution a Given: power P = 1000 W and frequency υ = 700 × 103 s-1. Then the photon energy is Eph = hυ = (6.626 × 10-34 J s)(700 × 103 s-1) ∴ Eph = 4.638 × 10-28 J/photon or 2.895 × 10-9 eV/photon The number of photons emitted from the antenna per unit time (Nph) is therefore: N ph = 1000 W P = = 2.16 × 1030 photons per second -28 E ph 4.638 × 10 J b Average intensity Iaverage = 1000 W/m2 and maximum wavelength λmax = 800 nm. The photon energy at λmax is, Eph = hc/λmax = (6.626 × 10-34 J s)(3.0 × 108 m s-1)/(800 × 10-9 m) ∴ Eph = 2.485 × 10-19 J or 1.551 eV The photon flux Γph is the number of photons arriving per unit time per unit area, Γ ph Iaverage 1000 W m −2 = 4.02 × 1021 photons m-2 s- 1 = = −19 E ph 2.485 × 10 J For the electric field we use classical physics. Iaverage = (1/2)cεoE2, so that E= c 2Iaverage = cε o (3.0 × 10 8 2(1000 W m −2 ) ms −1 )(8.854 × 10 −12 If each photon gives rise to one electron then the current density J is: J = eΓph = (1.602 × 10-19 C)(4.02 × 1021 m-2 s-1) ∴ Fm J = 644 A m- 2 3.1 −1 ) = 868 V m-1 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 *3.2 Human eye Photons passing through the pupil are focused by the lens onto the retina and are detected by two types of photosensitive cells, called rods and cones. Rods are highly sensitive photoreceptors with a peak response at the wavelength 500 nm. They do not register color, but they are responsible for our vision under dimmed light conditions, which is termed scotopic vision. Cones are color sensitive and are responsible for our daytime vision, called photopic vision. There are three types of cone photoreceptors, which are sensitive to the blue, green, and red wavelengths 430 nm, 535 nm and 575 nm, respectively. All three cones have an overall peak response of 555 nm. a. Calculate the photon energy (in eV) for the peak responsivity of each photoreceptor in the eye. b. The fovea is a retina region on the visual axis; images are focused onto this region. The density of the cones in the fovea is on the order of 150,000/mm2. Below a light intensity of the order of 100 µW m-2, cones are not functional and rods take over the vision. (Due to marked variation from person to person, these values are very approximate.) 1. Estimate the minimum photon flux for color vision assuming that photopic vision occurs roughly at 555 nm. 2. If a visual sensation persists for a time (1/5)th of a second, how many photons does the eye need per cone for a visual color sensation? 3. If the eye is 10 percent efficient overall, due to photon reflections, etc., how many photons are actually absorbed per cone to generate a colored visual sensation? Solution Minimum light intensity Imin is given to be 100 × 10-6 W m-2. a The photon energy for peak responsivity can be found as follows (firstly for blue light): Eph = hc/λ = (6.626 × 10-34 J s)(3.0 × 108 m/s)/(430 × 10-9 m) E ph = 4.623 × 10-19 J or 2.886 eV ∴ b(1) The photon flux can then be found (blue light): Γ = Imin/Eph = (100 × 10-6 W m-2)/(4.623 × 10-19 J) = 2.163 × 1014 m-2 s-1 The values for the other wavelengths are listed in Table 3Q2-1: Table 3Q2-1 Summarized values of photon energy and photon flux for different wavelengths of light. Wavelength, λ (nm) a) Photon energy (× 10-19) Eph = hc/λ Red Peak response 430 535 575 555 4.623 3.716 3.457 3.582 2.886 2.319 2.158 2.236 2.163 2.691 2.893 2.792 (eV) b) (1) Photon flux needed (× 1014) Γ = Imin/Eph Green (J) Photon energy Eph × 1/q Blue (m-2 s-1) 3.2 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 The minimum photon flux Γ min needed for visual color sensation is 2.792 × 1014 m-2 s-1. (2) The number of photons needed for a visual color sensation is (where Dcone is the density of the cones in the fovea): Nph − cone Γ t = min = Dcone 1 m −2 s −1 ) s 5 = 288 photons 1.5 × 1011 m −2 (2.163 × 10 14 (3) As the eye is only 10% efficient because most of the photons are reflected, the actual number of absorbed photons that give rise to a visual signal is 29. 3.3 Photoelectric effect A photoelectric experiment indicates that violet light of wavelength 420 nm is the longest wavelength radiation that can cause photoemission of electrons from a particular multialkali photocathode surface. a. What is the work function in eV? b. If a UV radiation of wavelength 300 nm is incident upon the photocathode, what will be the maximum kinetic energy of the photoemitted electrons, in eV? c. Given that the UV light of wavelength 300 nm has an intensity of 20 mW/cm2, if the emitted electrons are collected by applying a positive bias to the opposite electrode, what will be the photoelectric current density in mA cm-2 ? Solution a We are given λmax = 420 nm. The work function is then: Φ = hυo = hc/λmax = (6.626 × 10-34 J s)(3.0 × 108 m s-1)/(420 × 10-9 m) ∴ b Φ = 4.733 × 10-19 J or 2.96 eV Given λ = 300 nm, the photon energy is then: Eph = hυ = hc/λ = (6.626 × 10-34 J s)(3.0 × 108 m s-1)/(300 × 10-9 m) Eph = 6.626 × 10-19 J = 4.14 eV ∴ The kinetic energy KE of the emitted electron can then be found: KE = E ph - Φ = 4.14 eV - 2.96 eV = 1.18 eV c The photon flux Γph is the number of photons arriving per unit time per unit area. If Ilight is the light intensity (light energy flowing through unit area per unit time) then, Γph = I light E ph Suppose that each photon creates a single electron, then J = Charge flowing per unit area per unit time = Charge × Photon Flux ∴ J = eΓph = eI light (1.602 × 10 −19 C)(200 W m −2 ) = = 48.36 A m-2 = 4.836 mA cm- 2 −19 (6.626 × 10 J ) E ph 3.3 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) 3.4 Chapter 3 Photoelectric effect and quantum efficiency. Cesium metal is to be used as the photocathode material in a photoemissive electron tube because electrons are relatively easily removed from a cesium surface. The work function of a clean cesium surface is 1.9 eV. a. What is the longest wavelength of radiation which can result in photoemission? b. If blue radiation of wavelength 450 nm is incident onto the Cs photocathode, what will be the kinetic energy of the photoemitted electrons in eV? What should be the voltage required on the opposite electrode to extinguish the external photocurrent? c. Quantum efficiency (QE) of a photocathode is defined by, Quantum efficiency = Number of photoemitted electrons Number of incident photons QE is 100% if each incident photon ejects one electron. Suppose that blue light of wavelength 450 nm with an intensity of 30 mW cm-2 is incident on a Cs photocathode that is a circular disk of diameter 6 mm. If the emitted electrons are collected by applying a positive bias voltage to the anode, and the photocathode has a QE of 25%, what will be the photocurrent? Solution a The longest wavelength will be Φ = hυo = hc/λmax λ max = b −34 J s)(3 × 108 m s −1 ) h c (6.626 × 10 = = 653.9 × 10-7 m = 653.9 nm −19 Φ (1.9 × 1.602 × 10 J) The energy of the photon at 450 nm is −34 J s)(3 × 108 m s −1 ) h c (6.626 × 10 E ph = = = 4.417 × 10-19 J = 2.76 eV −9 λ (450 × 10 m) The excess energy over Φ goes to kinetic energy of the photoelectron. Thus the electron kinetic energy is KE = E ph − Φ = (2.76 eV) − (1.9 eV) = 0.86 eV and the stopping voltage for this wavelength will be -0.86 V. c. The number of photons arriving per unit area per unit time at the photocathode is 30 × 10 −3 × 10 4 J s −1 m 2 ) ( I = Γph = = 6.792 × 1020 s-1 m-2 E ph (4.417 × 10 −19 J) The current density is then simply J = e Γph QE = (1.602 × 10 −19 C)(6.792 × 10 20 s −1 m −2 )(0.25) = 27.2 A m-2 The photoelectric current is then π (6 × 10 πd 2 I = AJ = J= 4 4 −3 m) 2 (27.2 3.4 A m −2 ) = 7.691 × 10-4 A = 0.769 mA Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 3.5 Diffraction by X-rays and an electron beam Diffraction studies on a polycrystalline Al sample using X-rays gives the smallest diffraction angle (2θ) of 29.5° corresponding to diffraction from the (111) planes. The lattice parameter a of Al (FCC), is 0.405 nm. If we wish to obtain the same diffraction pattern (same angle) using an electron beam, what should be the voltage needed to accelerate the electron beam? Note that the interplanar separation d for planes (h,k,l) and the lattice parameter a for cubic crystals are related by d = a / [h2+k2+l2]1/2. Solution The plane indices are given as h = 1, k = 1 and l = 1, and the lattice parameter is given as a = 0.405 nm. Therefore, a d= h +k +l 2 2 = 2 0.405 nm 12 + 12 + 12 = 0.2338 nm From the Bragg condition with θ = 29.5° / 2 = 14.75° we have, λ = 2dsin(θ) =2(0.2338 nm)sin(14.75°) = 0.1191 nm The voltage V required to accelerate the electron is (see Example 3.4 in text), 2 2 1.226 nm 1.226 nm V= = 106 V = λ 0.1191 nm 3.6 Heisenberg's uncertainty principle Show that if the uncertainty in the position of a particle is on the order of its de Broglie wavelength, then the uncertainty in its momentum is about the same as the momentum value itself. Solution The de Broglie wavelength is λ= h p where p is the momentum. From Heisenberg’s uncertainty principle we have, ∆x∆p ≈ h If we take the uncertainty in the position to be of the order of the wavelength, ∆x ~ λ, then h 1 h 1 ≈ p~ p = ∆x 2π λ 2π so that the uncertainty in the momentum will be of the same order as the momentum itself. ∆p ≈ 3.7 Heisenberg's uncertainty principle An excited electron in an Na atom emits radiation at a wavelength 589 nm and returns to the ground state. If the mean time for the transition is about 20 ns, calculate the inherent width in the emission line. What is the length of the photon emitted? 3.5 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 Solution The emitted wavelength is λ= 589 nm and the transition time is ∆t = 20 ns. This time corresponds to an uncertainty in the energy that is ∆E. Using the uncertainty principle we can calculate ∆E, ∆t∆E ≈ h 1 6.626 × 10 −34 J s) ( h ∆E ≈ = 2π = 5.273 × 10-27 J or 3.29 × 10-8 eV ∆t 20 × 10 −9 s ∴ The corresponding uncertainty in the emitted frequency ∆υ is (5.273 × 10 −27 J ) ∆E = 7.958 × 106 s-1 ∆υ = = −34 h (6.626 × 10 J s) To find the corresponding spread in wavelength and hence the line width ∆λ, we can differentiate λ = c/υ. dλ c λ2 =− 2 =− dυ υ c λ2 (589 × 10 −9 m )2 (7.958 × 10 6 s-1 ) = 9.20 × 10-15 m or 9.20 fm Thus, ∆λ = ∆υ = 8 c 3.0 × 10 m/s The length of the photon l is l = (light velocity) × (emission duration) = (3.0 × 108 m/s)(20 × 10-9 s) = 6.0 m 3.8 Tunneling a. Consider the phenomenon of tunneling through a potential energy barrier of height V o and width a, as shown in Figure 3Q8-1. What is the probability that the electron will be reflected? Given the transmission coefficient T, can you find the reflection coefficient R? What happens to R as a or V o or both become very large? b. For a wide barrier (αa >> 1), show that To can at most be 4 and that To = 4 when E = (1/2)Vo. 3.6 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 D Start here from rest E C A B V(x) Vo E < Vo (x) (x) (x) Incident A1 A2 Reflected I Transmitted III II x=0 x=a x Figure 3Q8-1 (a) The roller coaster released from A can at most make it to C, but not to E. Its PE at A is less than the PE at D. When the car is at the bottom, its energy is totally KE. CD is the energy barrier that prevents the car from making it to E. In quantum theory, on the other hand, there is a chance that the car could tunnel (leak) through the potential energy barrier between C and E and emerge on the other side of the hill at E. (b) The wavefunction for the electron incident on a potential energy barrier (Vo). The incident and reflected waves interfere to give ψI(x). There is no reflected wave in region III. In region II, the wavefunction decays with x because E < Vo. Solution a The relative reflection probability or reflection coefficient R is given as the ratio of the square of the amplitude of the reflected wave to that of the incident wave, that is: R= A22 A12 Also, R can be found from the transmission coefficient T by the equation R = 1 - T, as stated in Equation 3.28 (in the textbook). From Equation 3.24 (in the textbook), T is given as: T= 1 2 1 + D(sinh[aα ]) where a is the width of the potential energy barrier, α is the rate of decay, and D is given by: Vo2 D= 4 E(Vo − E ) To determine the behavior of R as a or Vo or both become very large, we can use the equation R = 1 - T to express R in terms of a and D (remember D is a function of Vo). 3.7 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) R =1− T =1− ∴ R= Chapter 3 1 2 1 + D(sinh[aα ]) D(sinh[aα ]) 2 1 + D(sinh[aα ]) 2 We know that sinh(∞) = ∞, and also that 1 / ∞ = 0. Therefore, as Vo becomes large, so does D, which leads to T = 0 and R = 1, meaning total reflection occurs. If a becomes large then sinh(∞) = ∞ and T = 0, making R = 1 for total reflection. b We need to find the maximum value of To. Since To depends on the energy E, we can differentiate it with respect to E, set the result to 0 and isolate E. To = ∴ 16 E(Vo − E ) Vo2 (Eqn. 3.27 in the textbook) −2 E + Vo δTo = 16 =0 δE Vo2 1 Vo 2 Thus To is maximum when E = Vo / 2. If this expression for energy is substituted back into the equation for To to find its maximum value (To′): ∴ E= 1 1 16 Vo Vo − Vo 16 E(Vo − E ) 2 2 To ′ = = 2 2 Vo Vo ∴ To ′ = 16Vo2 =4 4Vo2 3.9 Electron impact excitation a. A projectile electron of kinetic energy 12.2 eV collides with a hydrogen atom in a gas discharge tube. Find the n-th energy level to which the electron in the hydrogen atom gets excited. b. Calculate the possible wavelengths of radiation (in nm) that will be emitted from the excited H atom in part (a) as the electron returns to its ground state. Which one of these wavelengths will be in the visible spectrum? c. In neon street lighting tubes, gaseous discharge in the Ne tube involves electrons accelerated by the electric field impacting Ne atoms and exciting some of them to the 2p53p1 states, as shown in Figure 3Q9-1. What is the wavelength of emission? Can the Ne atom fall from the 2p53p1 state to the ground state by spontaneous emission? 3.8 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) He (1s12s1) Chapter 3 Ne Collisions 5 1 (2p 5s ) 20.66 eV 20.61 eV 632.8 nm Lasing emission (2p53p1) Electron impact Fast spontaneous decay ~600 nm (2p53s1) Collisions with tube walls 0 (2p6) (1s2) Ground states Figure 3Q9-1 The principle of operation of the HeNe laser. Important HeNe laser energy levels (for 632.8 nm emission). Solution a The energy of the electron at the nth level is given by the equation: −13.6 eV 2 Z n2 where Z is the atomic number, in this case 1 for hydrogen. Also, the kinetic energy present in the electron will be equal to the original energy state of the electron (in this case the ground energy), subtracted from the energy of the nth level at which the electron resides. That is: KE = En - Eground En = The ground energy of the Hydrogen electron is known to be -13.6 eV. We can then substitute this value and the equation for En given above into the expression for KE and isolate for n. KE = ∴ −13.6 eV + 13.6 eV n2 13.6 eV 13.6 eV − KE Substituting in the given value for KE of 12.2 eV: n= 13.6 eV = 3.12 13.6 eV − 12.2 eV Therefore the electron is at the 3rd energy level in the hydrogen atom (since n must be an integer). The amount of energy absorbed by the electron in the excitation is: n= ∆E = (-13.6 eV) / 32 - Eground = 12.09 eV and the projectile electron that hit it originally leaves the collision site with a new KE that is: KE′ = KE - ∆E = 12.2 eV - 12.09 eV = 0.11 eV b The electron can return down to the ground level either from n = 3 to n = 1, or from n = 3 to n =2 then to n = 1. For the first transition (n = 3 to n = 1): 3.9 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 −13.6 eV −13.6 eV − = 12.09 eV 32 12 This value can be put into the following equation to find the emitted wavelength: ∆E31 = ∆E31 = hυ = h c λ31 c 3.0 × 108 m/s = (6.626 × 10 −34 J s) ∆E31 (12.09 eV)(1.602 × 10 −19 J/eV) ∴ λ31 = h ∴ λ 31 = 1.03 × 10-7 m or 103 nm This wavelength is not in the visible spectrum. For the n = 3 to n = 2 to n = 1 transition, two different wavelengths of light will be released, one in the first transition from 3 to 2 and the other in the second. ∆E32 = −13.6 eV −13.6 eV − = 1.889 eV 32 22 ∴ λ32 = h c 3.0 × 108 m/s = (6.626 × 10 −34 J s) ∆E32 (1.889 eV)(1.602 × 10 −19 J/eV) ∴ λ 32 = 6.57 × 10-7 m or 657 nm This wavelength is visible (red). For the 2 to 1 transition: ∆E21 = −13.6 eV −13.6 eV − = 10.2 eV 22 12 ∴ λ21 = h c 3.0 × 108 m/s = (6.626 × 10 −34 J s) ∆E21 (10.2 eV)(1.602 × 10 −19 J/eV) ∴ λ 21 = 1.22 × 10-7 m or 122 nm This wavelength is not visible. c The wavelength is given in Figure 3Q9-1, as 600 nm approximately which is in the red. The Ne atom must change its orbital quantum number l in a transition involving a photon (emission or absorption). The transition (2p53p1) to (2p6) does not change l and hence it is NOT allowed in a photonemission transition. 3.10 Line spectra of hydrogenic atoms Spectra of hydrogen-like atoms are classified in terms of electron transitions to a common lower energy level. a. All transitions from energy levels n = 2, 3, ... to n = 1 (the K shell) are labeled K lines and constitute the Lyman series. The spectral line corresponding to the smallest energy difference (n = 2 to n = 1) is labeled the Kα line, next is labeled Kβ , and so on. The transition from n = ∞ to n = 1 has the largest energy difference and defines the greatest photon energy (shortest wavelength) in the K series; hence it is called the absorption edge Kαe. What is the range of wavelengths for the K lines? What is Kαe? Where are these lines with respect to the visible spectrum? 3.10 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 b. All transitions from energy levels n = 3, 4, ... to n = 2 (L shell) are labeled L lines and constitute the Balmer series. What is the range of wavelengths for the L lines (i.e., Lα and Lαe)? Are these in the visible range? c. All transitions from energy levels n = 4, 5, … to n = 3 (M shell) are labeled M lines and constitute the Paschen series. What is the range of wavelengths for the M lines? Are these in the visible range? d. How would you expect the spectral lines to depend on the atomic number Z? Solution Consider the expression for the energy change for a transition from n = n2 to n = n1, 1 1 ∆E = − Z 2 EI 2 − 2 n2 n1 (1) where EI = 13.6 eV is the ionization energy from the ground state (n = 1) and Z is the atomic number; for hydrogen Z = 1. The absorption wavelength is given by λ = hc / ∆E. a For the Lyman series (K-shell), n1 = 1. For Kα, n2 = 2, which gives ∆E = 10.2 eV from Eqn. (1) and λ( Kα ) = −34 8 −1 hc (6.626 × 10 J s)(3.0 × 10 m s ) = = 122 nm ∆E (10.2 eV)(1.602 × 10 −19 J/eV) For Kαe, n2 = ∞ and ∆E = 13.6 eV and −34 8 −1 hc (6.626 × 10 J s)(3.0 × 10 m s ) λ( Kαe ) = = = 91.2 nm ∆E (13.6 eV)(1.602 × 10 −19 J/eV) The wavelengths range from 91.2 nm to 122 nm; much shorter than the visible spectrum. b For the Balmer series (L-shell), n1 = 2. For La, n2 = 3, which gives ∆E = 1.889 eV and −34 8 −1 hc (6.626 × 10 J s)(3.0 × 10 m s ) λ( Lα ) = = = 657 nm ∆E (1.889 eV)(1.602 × 10 −19 J/eV) For Lαe, n2 = ∞ and ∆E = 3.4 eV and λ( Lαe ) = −34 8 −1 hc (6.626 × 10 J s)(3.0 × 10 m s ) = = 365 nm ∆E (3.4 eV)(1.602 × 10 −19 J/eV) The wavelengths range from 365 nm to 657 nm. Part of this is in the visible spectrum (blue and green). c For the Paschen series (M-shell), n1 = 3. For Ma, n2 = 4, which gives ∆E = 0.6611 eV and λ( Mα ) = −34 8 −1 hc (6.626 × 10 J s)(3.0 × 10 m s ) = = 1877 nm ∆E (0.6611 eV)(1.602 × 10 −19 J/eV) 3.11 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 For Mαe, n2 = ∞ and ∆E = 1.511 eV and −34 8 −1 hc (6.626 × 10 J s)(3.0 × 10 m s ) λ( Mαe ) = = = 821 nm ∆E (1.511 eV)(1.602 × 10 −19 J/eV) The wavelengths range from 821 nm to 1877 nm which is in the infrared region. d The transition energy depends on Z2, and therefore the emitted photon wavelength of the spectral lines depends inversely on Z2. *3.11 X-rays and the Moseley relation X-rays are photons with wavelengths in the range 0.01 nm-10 nm, with typical energies in the range 100 eV to 100 keV. When an electron transition occurs in an atom from the L to the K shell, the emitted radiation is generally in the X-ray spectrum. For all atoms with atomic number Z > 2, the K shell is full. Suppose that one of the electrons in the K shell has been knocked out by an energetic projectile electron impacting the atom (the projectile electron would have been accelerated by a large voltage difference). The resulting vacancy in the K shell can then be filled by an electron in the L shell transiting down and emitting a photon. The emission resulting from the L to K shell transition is labeled the Kα line. The table shows the Kα line data obtained for various materials. Table 3Q11-1 Values for atomic number Z and wavelength of Kα emissions. Material Mg Al S Ca Cr Fe Cu Rb W Z 12 13 16 20 24 26 29 37 74 Kα line (nm) 0.987 0.834 0.537 0.335 0.229 0.194 0.154 0.093 0.021 a. If υ is the frequency of emission, plot υ1/2 against the atomic number Z of the element. b. H.G. Moseley, while still a graduate student of E. Rutherford in 1913, found the empirical relationship υ 1/2 = B (Z – C) Moseley relation where B and C are constants. What are B and C from the plot? Can you give a simple explanation as to why Kα absorption should follow this relationship? Solution a If λ(Kα) is the wavelength of the Kα emission, then the corresponding frequency is υ = c / λ(Kα) For example, for Mg, λ(Kα) = 0.987 nm and υ = c / λ(Kα) = (3.0 × 108 m s-1)/(0.987 × 10-9 m) = 3.04 × 1017 s-1 We can then calculate υ for each metal and plot below. The result is a straight line indicating that (υ ) vs. Z (atomic number) as in the figure (υ)1/2 = (5.210 × 107 s-1/2)Z - 9.626 × 107 s-1/2 3.12 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 √(Frequency) (s-1/2) 4.5E+9 3.5E+9 2.5E+9 1.5E+9 0.5E+9 0 20 40 60 80 Atomic number, Z Figure 3Q11-1 Plot of square root of frequency versus atomic number. Table 3Q11-2 Summarized values for frequency of Kα emissions. Metal Mg Al S Ca Cr Fe Cu Rb W Z 12 13 16 20 24 26 29 37 74 υ (Kα) × 1017 s-1 3.04 3.60 5.59 8.96 13.1 15.5 19.5 32.3 143 n=1 n=1 n=2 0 0 0 0.2 r (nm) R2,1 0 0.4 1s 2p 0 0.2 0.4 0.6 r (nm) r2|R2,12| R1,0 2s 1s 2s r2|R1,02| R2,0 r2|R2,02| n=2 0 0.8 0 0.2 0.4 0 2p 0 r (nm) (a) 0.2 0.4 0.6 r (nm) 0.8 (b) Figure 3Q11-2 (a) Radial wavefunctions of the electron in a hydrogenic atom for various n and l values. (b) r 2 Rn2,l gives the radial probability density. Vertical axis scales are linear in arbitrary units. 3.13 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 b The first energy level, given by Equation 3.37a (in the textbook), depends on Z2. This is for a hydrogenic atom i.e., only 1 electron in the atom but a nuclear charge of +Ze. In the present case, the transiting electron in the L shell sees an effective charge of +Ze – 1e because there is 1 electron in the Kshell shielding the nucleus. Therefore the photon energy should be proportional to (Z – 1)2. However, some of the L-shell electrons also spend time near the nucleus (see Figure 3Q11-2) and therefore provide some additional shielding. This means the photon energy should be proportional to (Z – C)2 where C > 1 due to the additional shielding effect. It seems that the additional shielding is not negligible. The best fit line is (υ)1/2 = (5.210 × 107 s-1/2)Z - 9.626 × 107 s-1/2 ∴ B = 5.210 × 107 s- 1 / 2 ∴ C = (9.626 × 107 s-1/2)/B = 1.85 3.12 The He atom Suppose that for the He atom, zero energy is taken to be the two electrons stationary at infinity (and infinitely apart) from the nucleus (He++). Estimate the energy (in eV) of the electrons in the He atom by neglecting the electron-electron repulsion, that is, neglecting the potential energy due to the mutual Coulombic repulsion between the electrons. How does this compare with the experimental value of -79 eV? How strong is the electron-electron repulsion energy? Solution 1s orbital +2e 1 -e -e Z=2 2 a Figure 3Q12-1 Basic diagram of the He atom. In He there are 2 electrons and the nucleus has a positive charge of +2e. Both electrons occupy the same orbital and hence have the same spatial distribution (same n, l, ml). The two electrons, however, have opposite spins, i.e. different spin quantum numbers (Pauli exclusion principle). Due to their Coulombic repulsion, we envisage the two electrons as doing their best to avoid each other as shown. Let a be the distance to the maximum of the probability distribution. Then, a = ao / Z where ao is the Bohr radius. In the hydrogen atom (Z = 1), a is simply the Bohr radius (ao). Energy in eV of an electron in some energy state of a hydrogenic atom is given by: 13.6 eV 2 Z n2 where n = 1 corresponds to the ground energy state. This energy is with respect to the electron and nucleus infinitely separated. It also assumes that there is also no other electron so that there is no electron-electron repulsion energy. En = − 3.14 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 Electron 1 sees a net charge of +2e in the nucleus so that Z = 2. Obviously the electron is in the ground state (1s), thus n = 1. 13.6 eV 2 (2) = −54.4 eV 12 Similarly in the absence of electron 1, electron 2 will have the energy: E1 = − 13.6 eV 2 (2) = −54.4 eV 12 Thus neglecting electron-electron interactions, the total energy of the electrons in the He atom is: Etotal = E1 + E2 = -108.8 eV E2 = − As we would expect, this value is negative, corresponding to an attractive force (between a positive and negative charge). However, when this value is compared to the experimentally obtained value of -79 eV, it is apparent that ignoring the electron-electron repulsion gives a substantially different number. The experimental value is different because it includes the interaction energy between the two electrons. Using the value for Etotal and the experimental, the interaction energy can be found: E interaction = Eexperiment - Etotal = 29.8 eV As expected this is a positive quantity corresponding to repulsion (two negative charges). As a footnote to demonstrate the power of intuition, let us estimate this repulsion energy differently (“back of an envelope” type calculation). Both electrons occupy the same orbital and thus have the time averaged same spatial distribution except that due to electron-electron repulsion they will be avoiding each other. They will correlate their motions to avoid each other as follows: At one instant when one is at the extreme right the other will be at the extreme left and vice versa later on. The maximum probability radius for the 1s state for Z = 2 is given as: a = ao / Z = (5.29 × 10-11 m) / (2) = 2.645 × 10-11 m And the potential energy is then: 1 e2 PE = 4πε o (2 a) q ∴ (1.602 × 10 C) PE = 4π (8.854 × 10 F/m )(2 × 2.645 × 10 ∴ P E = 27.2 eV −19 2 −12 −11 1 −19 J/eV m ) 1.602 × 10 This is very close to what we found above (29.8 eV). 3.13 Hund's rule For each of the following atoms and ions, sketch the electronic structure, using a box for an orbital wavefunction and an arrow (up or down) for an electron: a. Manganese, [Ar]3d54s2 b. Cobalt, [Ar]3d74s2 c. Iron ion, Fe+2, given that Fe is [Ar]3d64s2 d. Neodymium ion, Nd+3, given that Nd is [Xe]4f46s2. 3.15 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 Solution Mn Cobalt Fe + 2 4s 3d 4s 3d 4s 3d [A r]3 d5 4s2 [A r]3 d7 4s2 [A r]3 d4 4s2 4f 6s Nd+3 [Xe]4 f 16s2 Figure 3Q13-1 Electronic structures of various atoms. 3.14 The HeNe Laser A particular HeNe laser operating at 632.8 nm has a tube that is 40 cm long. The operating gas temperature is about 130 °C. a. Calculate the Doppler-broadened linewidth ∆λ in the output spectrum. b. What are the n values that satisfy the resonant cavity condition? How many modes are therefore allowed? c. Calculate the frequency separation and the wavelength separation of the laser modes. How do these change as the tube warms up during operation? Taking the linear expansion coefficient to be 10-6 K1 , estimate the change in the mode frequency separation. Solution Operating wavelength = λo = 632.8 × 10-9 m Length of the tube = L = 0.4 m Operating temperature = T = 403 K Room temperature = 293 K (20 °C) a Mat = 20.2 × 10-3 kg mol-1. The mass of the Ne atom is Mat/NA. The effective velocity vx of the Ne atoms along the x-direction can be found from (1/2)Mvx2 = (1/2)kT, vx = kT = M kT (1.381 × 10 −23 J K −1 )( 403 K ) = Mat / N A (20.18 × 10 −3 kg mol −1 ) /(6.022 × 10 23 mol −1 ) 3.16 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Chapter 3 vx = 407.5 m s-1 The wavelength λo = 632.8 nm corresponds to a frequency υo: υo = c (3.0 × 108 m s −1 ) = = 4.741 × 1014 s-1 λo (632.8 × 10 −9 m ) The width in the frequency spectrum, ∆υ, is given by (see Example 3.21, in the textbook): 2υ o vx 2( 4.741 × 1014 s −1 )( 407.5 m s −1 ) = 1.288 × 109 s-1 = 8 −1 c (3.0 × 10 m s ) ∆υ = From Example 3.21 (in the textbook), (632.8 × 10 −9 m ) λo (1.288 × 10 9 s −1 ) ∆υ = ( 4.741 × 1014 s −1 ) υo ∆λ = ∆ λ = 1.72 × 10-12 m or 0.00172 nm ∴ The velocity vx we used is the root mean square (rms) velocity along one dimension (the laser tube axis) as given by the kinetic theory, i.e. (1/2)Mvx2 = (1/2)kT. Intuitively we would therefore expect the linewidth ∆υ or ∆λ to correspond to the width between the rms points in the Gaussian output spectrum. b Let n be the mode number corresponding to λo, then n= ( 40 × 10 −2 m ) L = 1264222.5; a very large number. = −9 1 1 m) 2 (632.8 × 10 2 λo However n must be an integer so 1264222 and 126223 would be two possible modes that are within the spectrum. The n1 and n2 values corresponding to the shortest λ1 and longest λ2 wavelengths are L ( 40 × 10 −2 m ) = 1 n1 = 1 = 1264224.2 −9 1 m ± 12 1.7195 × 10 −12 m ) 2 (632.8 × 10 2 ( λ o − 2 ∆λ ) n2 = L ( 40 × 10 −2 m ) = = 1264220.8 −9 1 1 1 m + 12 1.7195 × 10 −12 m ) 2 (632.8 × 10 2 ( λ o + 2 ∆λ ) Thus n values are: 1264221, 1264222, 1264223, 1264224. four modes. There are about Alternatively we can find the number of modes as follows. Changes in n and λ are related by the above equation; n = 2L /λ. We can relate the change in n to the change in λ by differentiating n = 2L /λ. Then over the width ∆λ of the spectrum, n changes by ∆n : dn 1 = −2 L 2 dλ λ substitute: ∆n = 2 ∴ ∆n = nλ 1 ∆λ 2 λ2 (1.264 × 10 6 ) n (1.72 × 10 −12 m ) = 3.44 ∆λ = (632.8 × 10 −9 m ) λ 3.17 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 Since ∆n must be an integer ∆n =3. The number of modes, however, is ∆n + 1 or 4 (there are 4 numbers between 13 and 16 inclusively although 16 - 13 = 3). Note: These 4 modes are within the central region, i.e. between the rms points, of the Gaussian output spectrum, that is within ∆λ. If we wished to find all the modes, we can estimate this by considering the full “extent” of the spectrum. Suppose that the full extent of the spectrum is roughly 2∆λ. Then there would be ∼8 modes. The situation is more complicated by the fact that the exact number of modes depends on how the emission spectrum and the cavity modes coincide. Further, we need to find the net optical gain, that is the optical gain minus cavity losses not just the optical gain curve. All these fine points are far beyond the scope of this text (see S. O. Kasap, Applied Optoelectronics: Principles and Practices, Prentice Hall, 1999). Lasing emission spectrum Cavity modes Optical gain curve (a) 3 modes (b) 4 modes Figure 3Q14-1 Number of laser modes depends on how the cavity modes intersect the optical gain curve (lasing emission spectrum). In (a) there are 3 modes and in (b) there are 4 modes. The width of the emission spectrum here is roughly between rms (root mean square points of the intensity vs. wavelength behavior. c The separation ∆υ of two neighboring modes can be found by finding the change in υ corresponding to a change of 1 in n, that is ∆n = 1. Substitute λ = c/υ in n(λ/2) = L to find υ = cn/(2L). Differentiate υ with respect to n to find ∆υ = c (3.0 × 108 m s −1 ) ∆n = (1) = 3.75 × 108 s-1 or 375 MHz 2L 2(0.40 m ) When the tube warms up, L expands and ∆ υ decreases. The new length L′ of the tube at 130 °C is given by L′ = L[1 + α(T – To)] = (0.4 m)[1 + (10-6 °C-1)(130 - 20) °C] = 0.400044 m where α is the linear expansion coefficient, To is room temperature assumed to be 20 °C. The new frequency separation is c (3.0 × 108 m s −1 ) ∆υ′ = ∆n = (1) = 3.74959 × 108 s- 1 2 L′ 2(0.400044 m ) The change in the frequency separation is ∆ υ ′ – ∆ υ = -4.10 × 104 s-1 or 41.0 kHz (decrease) 3.18 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 3 "It was not unusual in theoretical physics to spend a lot of time in speculative notion that turns out to be wrong. I do it all the time. Having a lot of crazy ideas is the secret of my success. Some of them turned out to be right!" Sheldon L. Glashow (Higgins Professor of Physics at Harvard University; Nobel Laureate, 1979) 3.19 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 Second Edition ( 2001 McGraw-Hill) Chapter 4 4.1 Phase of an atomic orbital a. What is the functional form of a 1s wavefunction, ψ(r)? wavefunction ψ1s(r) as a function of distance from the nucleus. Sketch schematically the atomic b. What is the total wavefunction Ψ1s(r,t)? c. What is meant by two wavefunctions Ψ1s(A) and Ψ1s(B) that are out of phase? d. Sketch schematically the two wavefunctions Ψ1s(A) and Ψ1s(B) at one instant. Solution a ψ1s(r) decays exponentially as exp(–r/ao), where ao is the Bohr radius (Table 3.2 in the textbook). ψ1s(r) r Figure 4Q1-1 Atomic wavefunction as a function of distance from the nucleus. b Ψ1s(r,t) = ψ1s(r) × exp(–jEt/h) = ψ1s(r) × exp(–jωt) where ω = E/h is an angular frequency (Section 3.2.2 of the textbook). The total wavefunction is harmonic in time. Recall that exp(jθ) = cosθ + jsinθ so that exp(jωt) = cosωt + jsinωt. c Two sine waves of the same frequency will have a certain phase difference which represents the time delay between the time oscillations of the two waves. Two waves will be in phase if their maxima coincide and out of phase if the maximum of one coincides with the minimum of the other. Two 1s wavefunctions, Ψ1s(A) and Ψ1s(B) will have the same frequency. Like two sine waves of the same frequency, the waves can be in phase or out of phase. When they are out of phase, if Ψ1s(A) = 1 then Ψ1s(B) = –1 and vice versa. 4.1 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 d 1s(A) 1s(B) r B r A Figure 4Q1-2 Two 1s wavefunctions which are out of phase (sketches at the same instant) 4.2 Molecular orbitals and atomic orbitals Consider a linear chain of five identical atoms representing a hypothetical molecule. Suppose that each atomic wavefunction is 1s wavefunction. This system of identical atoms has a center of symmetry C with respect to the center of molecule (third atom from the end) and all molecular wavefunctions must be either symmetric or antisymmetric about C. a. Using LCAO principle, sketch the possible molecular orbitals. b. Sketch the probability distribution ψ 2 c. If more nodes in the wavefunction lead to greater energies, order the energies of the molecular orbitals. Solution 4.2 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) POSSIBLE MOLECULAR ORBITALS IN INCREACING ENERGY ORDER Chapter 4 PROBABILITY DISTRIBUTIONS A B C D E A B C D E A B C D E A B C D E C D E A B C D E A B A B C D E A B C D E A B C D E A B C D E 4.3 Diamond and Tin Germanium, silicon, and diamond have the same crystal structure, that of diamond. Bonding in each case involves sp3 hybridization. The bonding energy decreases as we go from C to Si to Ge, as noted in Table 4Q3-1. Table 4Q3-1 Property Diamond Silicon Germanium Tin Melting temperature, 0C 3800 1417 937 232 Covalent radius, nm 0.077 0.117 0.122 0.146 Bond energy, eV 3.60 1.84 1.7 1.2 First ionization energy, eV 11.26 8.15 7.88 7.33 Bandgap, eV ? 1.12 0.67 ? a. What would you expect for the band gap of diamond? How does it compare with the experimental value of 5.5 eV? b. Tin has a tetragonal crystal structure, which makes it different than its group members, diamond, silicon, and germanium. 1. Is it a metal or a semiconductor? 2. What experiments do you think would expose its semiconductor properties? 4.3 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 Solution Given the properties in Table 4Q3-1, we have the following plots: Energy Gap, eV 8 Energy Gap, eV 6 Diamond 7 6 5 5 4 Diamond 4 3 3 2 Ge 1 2 Si 1 Tin 0 0 200 Si Ge -2 (a) Energy gap versus melting temperature. From the plot, it seems that diamond has an Eg = 3.4 eV and Sn has Eg Tin -1 1200 2200 3200 Melting Temperature C 2 3 Bond Energy, eV 1 4 (b) Energy gap versus bond energy. From the plot, it seems that diamond has an Eg = 6.8 eV 0.9 or is a metal and Sn has Eg 0 or is a metal Energy Gap, eV 7 Energy Gap, eV 6 Diamond 6 5 Diamond 4 5 3 4 2 3 Si Ge 1 2 0 Si Ge 1 -1 Tin 0 -2 0.05 0.1 Covalent Radius, nm 0.15 Tin -1 (c) Energy gap versus covalent radius. From the plot, it seems that diamond has an Eg = 4.7 eV and Sn has Eg .5 eV or is a metal. 6 12 (d) Energy gap versus first ionization energy. From the plot, it seems that diamond has an Eg = 6.3 eV and Sn has Eg 0.3 eV or is a metal. Figure 4Q3-1 4.4 7 8 9 10 11 First Ionization Energy, eV Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 Each is a plot of the band gap Eg (or energy gap) vs. some property. The straight line in each is drawn to pass through Si and Ge. Diamond and tin points are then located on this straight line at the intersections with the vertical lines representing the corresponding properties of diamond and tin. a Diamond has an Eg greater than Si and Ge. Averaging the four Eg for diamond we find 5.3 eV which is close to the experimental value of 5.5 eV. b (1) All the four properties indicate that tin has Eg ≤ 0 or is a metal. (2) Tin’s semiconductivity can be tested by examining its electrical conductivity and optical absorption (see Chapter 5 in the textbook). For example, for metals the conductivity should NOT be thermally activated over a wide temperature range, whereas for semiconductors there will be an Arrhenius temperature dependence over at least some temperature range. Further, semiconductors have an absorption edge that corresponds to hυ > Eg (Chapter 5 in the textbook). 4.4 Compound III-V semiconductors Indium as an element is a metal. It has a valency of III. Sb as an element is a metal and has a valency of V. InSb is a semiconductor, with each atom bonding to four neighbors, just like in silicon. Explain how this is possible and why InSb is a semiconductor and not a metal alloy. (Consider the electronic structure and sp3 hybridization for each atom.) Solution The one s and three p orbitals hybridize to form 4 ψhyb orbitals. In Sb there are 5 valence electrons. One ψhyb has two paired electrons and 3 ψhyb have 1 electron each as shown in Figure 4Q4-1. In In there are 3 electrons so one ψhyb is empty. This empty ψhyb of In can overlap the full ψhyb of Sb. The overlapped orbital, the bonding orbital, then has two paired electrons. This is a bond between In and Sb even though the electrons come from Sb (this type of bonding is called dative bonding). It is a bond because the electrons in the overlapped orbital are shared by both Sb and In. The other 3 ψhyb of Sb can overlap 3 ψhyb of neighboring In to form "normal bonds". Repeating this in three dimensions generates the InSb crystal where each atom bonds to four neighboring atoms as shown. As all the bonding orbitals are full, the valence band formed from these orbitals is also full. The crystal structure is reminiscent of that of Si, as all the valence electrons are in bonds. Since it is similar to Si, InSb is a semiconductor. In Sb In atom (Valency III) Sb atom (Valency V) hyb orbitals Valence electron Sb ion core (+5e) hyb orbitals Valence electron In ion core (+3e) Figure 4Q4-1 Bonding structure of InSb. 4.5 In Sb In Sb Sb In Sb In In Sb In Sb Sb In Sb In Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 4.5 Compound II-VI semiconductors CdTe is a semiconductor, with each atom bonding to four neighbors, just like in silicon. In terms of covalent bonding and the positions of Cd and Te in the Periodic Table, explain how this is possible. Would you expect the bonding in CdTe to have more ionic character than that in III-V semiconductors? Solution Cd Te Te atom (Valency VI) hyb orbitals Valence electron Te ion core (+6e) hyb Cd atom (Valency II) orbitals Valence electron Cd ion core (+2e) Cd Te Cd Te Te Cd Te Cd Cd Te Cd Te Te Cd Te Cd Dative bonding Figure 4Q5-1 Bonding structure of CdTe. In CdTe one would expect a mixture of covalent and ionic bonding. Transferring 2 Cd electrons to Te would generate Te2– and Cd2+ which then bond ionically. In covalent bonding we expect hybridization of s and p orbitals. The one s and three p orbitals hybridize to form 4 ψhyb orbitals. In Te there are 6 valence electrons. Two ψhyb have two paired electrons each and two ψhyb have 1 electron each as shown. In Cd there are 2 electrons so two ψhyb are empty. An empty ψhyb of Cd can overlap a full ψhyb of Te. The overlapped orbital, the bonding orbital, then has two paired electrons. This is a bond between Cd and Te even though the electrons come from Te (this type of bonding is called dative bonding). It is a bond because the electrons in the overlapped orbital are shared by both Te and Cd. The other full ψhyb of Te can similarly overlap another empty ψhyb of a different neighboring Cd to form another dative bond. The half occupied orbitals (two on Te and two on Cd) overlap and form "normal bonds". Repeating two dative bonds and two normal bonds in three dimensions generates the CdTe crystal where each atom bonds to four neighboring atoms as shown. Since all the bonding orbitals are full, the valence band formed from these orbitals is also full. Strictly the bonding is neither fully covalent nor fully ionic but a mixture. *4.6 Density of states for a two-dimensional electron gas Consider a two-dimensional electron gas in which the electrons are restricted to move freely within a square area a2 in the xy plane. Following the procedure in Section 4.5 (in the textbook), show that the density of states g(E) is constant (independent of energy). 4.6 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 Solution For a two dimensional electron gas confined within a square region of sides a we have: E= h2 (n12 + n2 2 ) 8me a 2 Only positive n1 and n2 are allowed. Each n1 and n2 combination is an orbital state. Define a new variable n as: n 2 = n 12 + n 22 substitute: E= h2 n2 2 8me a Let us consider how many states there are with energies less than E′. E′ corresponds to n ≤ n′. ∴ E′ = h2 n′ 2 8me a 2 n′ = 8a 2 me E ′ h2 n2 n12 + n22 = n'2 5 4 3 n1 = 1 n2 = 3 2 1 –n1 0 1 2 3 4 –n2 n1 = 2, n2 = 2 5 6 n1 Figure 4Q6-1 Each state, or electron wavefunction in the crystal, can be represented by a box at n1, n2. Consider Figure 4Q6-1. All states within the quarter arc defined by n′ have E < E′. The area of this quarter arc is the total number of orbital states. The total number of states, S, including spin is twice as many, 1 8a 2 m E ′ 2 1 2 e S = 2 πn′ = 2 π 2 4 4 h ∴ 4πa 2 me E ′ S= h2 The density of states g is defined as the number of states per unit area per unit energy. Therefore, 4.7 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 1 dS 1 4πa 2 me 4πme g= 2 = = 2 a dE ′ a 2 h 2 h Thus, for a two dimensional gas, the density of states is constant. 4.7 Fermi energy of Cu The Fermi energy of electrons in copper at room temperature is 7.0 eV. The electron drift mobility in copper, from Hall effect measurements, is 33 cm2 V–1 s–1. a. What is the speed vF of conduction electrons with energies around EF in copper? By how many times is this larger than the average thermal speed vthermal of electrons, if they behaved like an ideal gas (Maxwell-Boltzmann statistics)? Why is vF much larger than vthermal? b. What is the De Broglie wavelength of these electrons? Will the electrons get diffracted by the lattice planes in copper, given that interplanar separation in Cu = 2.09 Å? (Solution guide: Diffraction of waves occurs when 2dsinθ = λ, which is the Bragg condition. Find the relationship between λ and d that results in sinθ > 1 and hence no diffraction.) c. Calculate the mean free path of electrons at EF and comment. Solution a The Fermi speed vF is given by: EF = 1 me vF 2 2 ∴ (7 eV)(1.602 × 10 −19 J/eV) EF vF = 2 = 2 me (9.109 × 10 −31 kg) ∴ v F = 1.57 × 106 m/s Maxwell - Boltzmann statistics predicts an effective velocity (rms velocity) which is called “thermal velocity” given by (assume room temperature T = 20 ˚C = 293 K): 1 3 me vthermal 2 = kT 2 2 3(293 K )(1.381 × 10 −23 J/K ) 3Tk = = me (9.109 × 10 −31 kg) ∴ vthermal ∴ v thermal = 1.15 × 105 m/s Comparing the two values: Ratio = v F/v thermal = 13.7 vF is about 14 times greater than vthermal. This is because vthermal assumes that electrons do not interact and obey Maxwell-Boltzmann statistics (Eav = 3/2kT). However, in a metal there are many conduction electrons. They interact with the metal ions and obey the Pauli exclusion principle, i.e. Fermi-Dirac statistics. They extend to higher energies to avoid each other and thereby fulfill the Pauli exclusion principle. b The De Broglie wavelength is λ = h/p where p = mevF is the momentum of the electrons. 4.8 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 h 6.626 × 10 −34 J s λ= = me vF (9.109 × 10 −31 kg)(1.57 × 10 6 m/s) λ = 4.63 × 10-10 m or 4.63 Å ∴ The interplanar separation, d, is given as 2.09 Å. The diffraction condition is: λ = 2dsinθ sin θ = ∴ ( ( ) ) 1 λ 1 4.63 Å = = 1.11 2 d 2 2.09 Å Since this is greater than 1, and sinθ cannot be greater than 1, the electrons will not be diffracted. The drift mobility is related to the mean scattering time τ by: c µ= eτ me ∴ τ= −4 2 −1 −1 −31 kg) µme (33 × 10 m V s )(9.109 × 10 = -19 e 1.602 × 10 C ∴ τ = 1.876 × 10-14 s The mean free path, lF, of electrons with speed, vF is: lF = vFτ = (1.57 × 106 m/s)(1.876 × 10-14 s) = 2.95 × 10-8 m or 295 Å The mean free path of those electrons with effective speeds ve (close to mean speed) can be found as follows (EF has little change with temperature, therefore EF ≈ EFO): 1 3 3 me ve 2 = EFO = EF 2 5 5 6 EF = 5 me −19 J/eV) 6 (7.0 eV)(1.602 × 10 = 1.215 × 10 6 m/s −19 5 (9.109 × 10 kg) ∴ ve = ∴ le = veτ = (1.215 × 106 m/s)(1.876 × 10-14 s) = 2.28 × 10-8 m or 228 Å 4.8 Fermi energy and electron concentration Consider the metals in Table 4Q8-1 from groups I, II and III in the Periodic Table. Calculate the Fermi energies at absolute zero, and compare the values with the experimental values. What is your conclusion? Table Q4.8-1 Metal Group Cu Zn Al I II III M at (g/mol) 63.55 65.38 27 Density (g cm-3 ) 8.96 7.14 2.70 4.9 E F (eV) [Calculated] - E F (eV) [Experiment] 6.5 11.0 11.8 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 Solution Since Cu is in group I, its valency (G) is also 1. The electron concentration n is then the atomic concentration multiplied by the group number, or: 6.022 × 10 23 mol −1 )(8.96 × 10 3 kg/m 3 ) ( NA D n=G = (1) = 8.490 × 10 28 m −3 Mat 63.55 × 10 −3 kg/mol Using Equation 4.22 in the text: 2 EFO h 2 3n 3 1 = 8me π q ∴ EFO (6.626 × 10 = 8(9.109 × 10 ∴ E F O = 7.04 eV 2 J s) 3(8.490 × 10 28 m −3 ) 3 1 −19 −31 π J/eV kg) 1.602 × 10 −34 2 Comparing with the experimental value: % difference = 7.04 eV − 6.5 eV × 100% = 8.31% 6.5 eV EFO can be calculated for Zn and Al in the same way (remember to take into account the different valencies). The values are summarized in the following table and it can be seen that calculated values are close to experimental values: Table Q4.8-2 Summarized values for Fermi energy at absolute zero temperature. Metal n (m-3) (× 1028) E FO (eV) (calculated) EFO (eV) (experimental) % Difference Cu Zn Al 8.490 13.15 18.07 7.04 9.43 11.7 6.5 11.0 11.8 8.31 14.3 0.847 4.9 Temperature dependence of the Fermi energy a . Given that the Fermi energy for Cu is 7.0 eV at absolute zero, calculate the EF at 300 K. What is the percentage change in EF and what is your conclusion? b. Given the Fermi energy for Cu at absolute zero, calculate the average energy and mean speed per conduction electron at absolute zero and 300 K, and comment. Solution a The Fermi energy in eV at 0 K is given as 7.0 eV. The temperature dependence of EF is given by Equation 4.23 in the textbook. Remember that EFO is given in eV. π 2 kT 2 EF = EFO 1 − 12 EFO 4.10 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 ∴ π 2 (1.381 × 10 −23 J/K )(300 K ) 2 EF = (7.0 eV)1 − = 6.999921 eV 12 (7.0 eV)(1.602 × 10 −19 J/eV) ∴ % difference = b 6.999921 eV − 7.0 eV × 100% = 0.00129% 7.0 eV This is a very small change. The Fermi energy appears to be almost unaffected by temperature. The average energy per electron at 0 K is: E av(0 K) = 3/5 (E FO) = 4.2 eV The average energy at 300 K can be calculated from Equation 4.26 (in the textbook): 5π 2 3 Eav (T ) = EFO 1 + 5 12 2 kT EFO q ∴ 5π 2 3 Eav (300 K ) = (7.0 eV)1 + 5 12 ∴ E av (300 K) = 4.200236 eV (1.381 × 10 −23 J/K )(300 K ) −19 J/eV) (7.0 eV)(1.602 × 10 2 This is a very small change. Assume that the mean speed will be close to the effective speed ve. Effective speed at absolute zero is denoted as veo, and is given by: Eav (0 K ) × q = ∴ 1 me veo 2 2 1.602 × 10 −19 J/eV)( 4.2 eV) ( qEav (0 K ) veo = 2 = 2 = 1215446 m/s me (9.109 × 10-31 kg) At 300 K, the effective speed is ve: ve = 2 (1.602 × 10 −19 J/eV)(4.200236 eV) =1215480 m/s qEav (300 K ) = 2 me (9.109 × 10-31 kg) Comparing the values: % difference = 1215480 m/s − 1215446 m/s × 100% = 0.002797% 1215446 m/s The mean speed has increased by a negligible amount (0.003%) from 0 K to 300 K. Note: For thermal conduction this tiny increase in the velocity is sufficient to transport energy from hot regions to cold regions. This very small increase in the velocity also allows the electrons to diffuse from hot to cold regions giving rise to the Seebeck effect. 4.10 X-ray emission spectrum from sodium Structure of Na atom is [Ne]3s1. Figure 4Q10-1a shows formation of the 3s and 3p energy bands in Na as a function of internuclear separation. Figure 4Q10-1b shows the x-ray emission spectrum (called the L-band) from crystalline sodium in the soft x-ray range as explained in Example 4.6 (in the textbook). 4.11 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 a. From Figure 4Q10-1, estimate the nearest neighbor equilibrium separation between Na atoms in the crystal if some electrons in the 3s band spill over into the states in the 3p band. b. Explain the origin of X-ray emission band in Figure 4Q10-1b and the reason for calling it the Lband. c. What is the Fermi energy of the electrons in Na from Figure 4Q10-1b ? d. Taking the valency of Na to be I, what is the expected Fermi energy and how does it compare with that in (c)? (a) Energy (eV) 0 3p –5 3s – 10 0 0.5 1 Internuclear distance (nm) 1.5 Intensity of emitted radiation Solution (b) 25 26 27 28 29 30 Photon energy in eV 31 Figure 4Q10-1 (a) Energy band formation in sodium. (b) L-emission band of X-rays from sodium. SOURCE: (b) Data extracted from W. M. Cadt and D. H. Tomboulian, Phys. Rev., 59, 1941, p.381). a As represented in Figure 4Q10-1a, the estimated nearest interatomic separation is about 0.36 ÷ 0.37 nm. b When an electron, for some reason, is leaving the closed inner L-shell of an atom, an empty state is created there. An electron from the energy band of the metal drops into the L-shell to fill the vacancy and emits a soft X-ray photon in this process. The spectrum of this X-ray emission from metal involves a range of energies, corresponding to transitions from the bottom of the band and from the Fermi level to the L-shell. So all the X-ray photons emitted from the electrons during their transitions to the L-shell will have energies laying in that range (band) and because all of them are emitted due to a transition to the L-shell, this X-ray band is called L-band. c As explained in b, the width of the X-ray band corresponds to the distance from the bottom of the energy band to the Fermi level. As shown in Figure 4Q10-1b, the position of the Fermi level with respect to the bottom of the energy band is approximately 3.2 eV. d Theoretically, the position of the Fermi level is given by Equation 4.23 (in the textbook). Since the temperature dependence of the Fermi level is really very weak, we can neglect it. Then EF(T) = EF0 and using Equation 4.22 (in the textbook), we receive 2 h 2 3n 3 EF = 8me π The electron concentration in Na can be calculated, assuming that each Na atom donates exactly one electron to the crystal. Taking from Appendix B (in the textbook) the density of Na d (0.97 g cm-3) and its atomic mass Mat (22.99 g mol-1), we receive 4.12 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) n= Chapter 4 −3 23 −1 d N A (0.97 g cm )(6.022 × 10 mol ) = = 2.54 × 1022 cm-3 = 2.54 × 1028 m- 3 −1 Mat (22.99 g mol ) Thus for the Fermi level is −34 J s) h 2 3n 3 (6.626 × 10 EF = = 31 − 8me π kg) 8 (9.1 × 10 2 2 2 3(2.54 × 10 28 m 3 ) 3 -19 = 5.05 × 10 J π Or E F = 3.15 eV, which is very close to the value obtained from the X-ray spectrum. 4.11 Thermoelectric effects and E F Consider a thermocouple pair that consists of gold and aluminum. One junction is at 100 °C and the other is at 0 °C. A voltmeter (with a very large input resistance) is inserted into the aluminum wire. Use the properties of Au and Al in Table 4Q11-1 below to estimate the emf registered by the voltmeter and identify the positive end. Table 4Q11-1 Atomic Mass, Mat, g/mol Density, g cm-3 Conduction electrons per atom x (Mott-Jones index) Au 197 19.3 1 Al 27.0 2.7 3 -1.48 2.78 Solution Au Hot 0 °C 100 °C Cold 0 Al µV Al Figure 4Q11-1 The Al-Au thermocouple. The cold end is maintained at 0 °C which is the reference temperature. The other junction is used to sense the temperature. In this example it is heated to 100 °C. We essentially have the arrangement shown above. For each metal there will be a voltage across it given by integrating the Seebeck coefficient. From the Mott-Jones equation: T T 0 0 xπ 2 k 2 T xπ 2 k 2 2 ∆V = ∫ SdT = ∫ − (T − T02 ) dT = − 3eE FO 6eE FO T T so that The emf (VAB) available is the difference in ∆V for the two metals labeled A (= Al) and B (= Au) VAB = ∆VA − ∆VB 4.13 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 where in this example, T = 373 K and T0 = 273 K. We can calculate EFAO and EFBO for each metal as in Example 4.9 (in the textbook) by using EFO = h 2 3n 8me π 2/3 where n is the electron concentration: n = atomic concentration (nat) × number of conduction electrons per atom From the density d and atomic mass Mat, the atomic concentration of Al is: nAl = 23 3 -1 N A d (6.022 × 10 mol )(2700 kg/m ) = = 6.022 × 10 28 m -3 Mat . kg/mol 0 027 ( ) n = 3nAl = 1.807 × 1029 m-3 so that which leads to: EFAO h 2 3n = 8me π 2/3 (6.626 × 10 ) 3(1.807 × 10 ) = π 8(9.109 × 10 ) −34 2 29 2/3 −31 EFAO = 1.867 × 10-18 J or 11.66 eV Similarly for Au, we find EFBO = 5.527 eV. Substituting x and EF values for A (Al) and B (Au) we find, ∆VA = − and π 2k 2 xA 2 (T − T02 ) = -188.4 µV 6eEFAO π 2k 2 xB 2 ∆VB = − T − T02 ) = 211.3 µV ( 6eEFBO so that the magnitude of the voltage difference is |VAB | = |-188.3 µV - 211.3 µV| = 399.7 µV Au 157 µV Cold Hot I Al 188 µV Meter Figure 4Q11-2 To find which end is positive, we put in the resistance of the voltmeter and replace each metal by its emf and determine the direction of current flow as in the figure. For the particular circuit shown, the cold connected side of the voltmeter is positive. 4.14 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 4 . 1 2 The thermocouple equation Although inputting the measured emf for V in the thermocouple equation V = a∆T + b(∆T)2 leads to a quadratic equation, which in principle can be solved for ∆T, in general ∆T is related to the measured emf via ∆T = a1V + a2V2 + a3V3 + ... with the coefficients a1, a2 etc., determined for each pair of TCs. By carrying out a Taylor's expansion of TC equation, find the first two coefficients a1 and a2. Using an emf table for the K-type thermocouple or Figure 4Q12-1, evaluate a1 and a2. EMF (mV) 80 E-Type 70 60 50 J-Type 40 K-Type 30 20 S-Type T-Type 10 0 0 200 400 600 800 1000 Temperature (°C) Figure 4Q12-1 Output emf versus temperature (°C) for various thermocouples between 0 to 1000 °C. Solution From Example 4.11 (in the textbook), the emf voltage (V) can be expressed: V= π 2k 2 1 1 2 2 − (T − To ) 4e EFAO EFBO Since we know that ∆T = T - To, and therefore T = ∆T + To, we can make the following substitution: V= π 2k 2 1 1 2 2 − [ ∆T + To ] − To 4e EFAO EFBO expanding, V= π 2 k 2 To ∆T π 2 k 2 To ∆T π 2 k 2 ( ∆T ) π 2 k 2 ( ∆T ) − + − 2eEFAO 2eEFBO 4eEFAO 4eEFBO factoring, V= π 2 k 2 To 1 π 2k 2 1 1 1 2 − T + − ∆ ( ∆T ) 2e EFAO EFBO 4e EFAO EFBO ( 2 4.15 ) 2 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 Upon inspection, it can be seen that this equation is in the form of the thermocouple equation, V = a∆T + b∆T2, and therefore we know that the coefficients a and b are equal to: π 2 k 2 To 1 1 a= − 2e EFAO EFBO π 2k 2 1 1 b= − 4e EFAO EFBO Continuing with the thermocouple equation, we can rearrange it as follows to obtain a quadratic equation: -b(∆T)2 - a∆T + V = 0 This is a quadratic equation in ∆T. The solution is ∆T = a − a 2 + 4bV a a 1 + 4bV / a 2 =− + 2b 2b −2b ∴ ∆T = − 1 a a + (1 + 4bV / a 2 ) 2 2b 2b ∴ ∆T = − ∞ −1 / 2 a a 4bV + + 1 ∑ a2 2b 2b n = 0 n Taylor expansion: 2 3 3 1 1 a a 1 4bV ( 12 )( − 12 ) 4bV ( 2 )( − 2 )( − 2 ) 4 bV ∆T = − + + + ... + 1+ 2 2 2 a 2b 2b 2 a 2! a 3! ∴ ∆T = n V bV 2 b 2V 3 − 3 + 2 5 + ... a a a The positive root is not used because V = 0 must give ∆T = 0, and with the positive root the a/2b terms will not cancel out. This equation is of the form given in the question, ∆T = a1V + a2V 2 +a3V 3 + ... such that ∴ and 1 −b and a2 = 3 a a 1 1 2eE E a1 = = 2 2 = 2 2 FAO FBO a π k To 1 1 π k To ( EFBO − EFAO ) − 2e EFAO EFBO a1 = a2 = − π 2k 2 1 1 − 4e EFAO EFBO b 2e 2 EFAO 2 EFBO 2 = − = − 3 2 a3 π 4 k 4 To 3 ( EFBO − EFAO ) π 2 k 2 To 1 1 − 2e EFAO EFBO From Figure 4Q12-1, at ∆T = 200 ˚C, V = 8 mV, and at ∆T = 800 ˚C, V = 33 mV. Using these values and neglecting the effect of a3, we have two simultaneous equations we can solve: ∆T = a1V + a2V2 ∆T = 200 ˚C: 200 ˚C = a1(8 mV) + a2(8 mV)2 ∴ a1 = (-(64 mV2)a2 + 200 ˚C) / (8 mV) 4.16 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∆T = 800 ˚C: 800 ˚C = a1(33 mV) + a2(33 mV)2 substitute for a1: isolate a2: 800 ˚C = [(-(64 mV2)a2 + 200 ˚C) / (8 mV)](33 mV) + a2(33 mV)2 a 2 = -0.03030 ˚C/mV2 ∴ 200 ˚C = a1(8 mV) + (-0.03030 ˚C/mV2)(8 mV)2 ∴ a 1 = 25.24 ˚C/mV Chapter 4 We can check our calculation by calculating ∆T when V = 20 mV and comparing the value we obtain from examining Figure 4Q12-1. ∆T = (25.24 ˚C/mV)(20 mV) + (-0.03030 ˚C/mV2)(20 mV)2 = 492.7 ˚C This is close to that shown in Figure 4Q12-1. Obviously to get a more accurate determination, we need a3 as well but then we must solve a cubic equation to find a1, a2 and a3 or use numerical techniques. 4.13 Thermionic emission A vacuum tube is required to have a cathode operating at 800 °C and providing an emission (saturation) current of 10 A. What should be the surface area of the cathode for the two materials in Table 4Q13-1? What should be the operating temperature for the Th on W cathode, if it is to have the same surface area as the oxide-coated cathode? Table 4Q13-1 B e (A m–2 K–2 ) Th on W Oxide coating 3 × 10 100 4 Φ (eV) 2.6 1 Solution Operating temperature T is given as 800 ˚C = 1073 K and emission current I is given as 10 A. The temperature and current of the tube are related to its area by Equation 4.44 (in the textbook): J= ∴ I A= ) ( ) − Φ − βs E Be T 2 exp kT Assuming there is no assisting field emission, the area needed for Th on W is: A= ∴ ( − Φ − βs E I = Be T 2 exp A kT 10 A −(2.6 eV)(1.602 × 10 −19 J/eV) 2 4 −2 −2 3 10 1073 exp × A m K K ) ( )( −23 J K −1 )(1073 K ) (1.381 × 10 A = 467 m2 (large tube) For the oxide coating: 4.17 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) A= ∴ Chapter 4 10 A −(1 eV)(1.602 × 10 −19 J/eV) 2 −2 −2 (100 A m K )(1073 K) exp (1.381 × 10 −23 J K −1 )(1073 K) A = 0.00431 m2 (small and practical tube) To find the temperature that the Th on W cathode would have to work at to have the same surface area as the oxide coated cathode, the area of the oxide cathode can be used in the current equation and the temperature can be solved for. It is a more difficult equation, but can be solved through graphical methods. −Φ I = ABe T 2 exp kT The plot of thermionic emission current I versus temperature T is shown below, with A = 0.00431 m2, Be = 3 × 10-4 A m–2 K–2, and Φ = (2.6 eV)(1.602 × 10-19 J/eV). 15 10 I (A) 5 1.6x10 3 1.7x10 3 1.8x10 3 T (K) Figure 4Q13-1 Behavior of current versus temperature for the Th on W cathode. From the graph, it appears that at 10 A of current the cathode will be operating at a temperature of T = 1725 K or 1452 ˚C. 4.14 Field-assisted emission in MOS device Metal-oxide-semiconductor (MOS) transistors in microelectronics have metal gate on SiO2 insulating layer on the surface of doped Si crystal. Consider this as a parallel plate capacitor. Suppose the gate is an Al electrode of area 50 µm × 50 µm and has a voltage of 10 V with respect of the Si crystal. Consider two thicknesses for the SiO2, (a) 100 Å and (b) 40 Å, where (1 Å = 10-10 m). The workfunction of Al is 4.2 eV, but this refers to electron emission in vacuum, whereas in this case, the electron is emitted into the oxide. Given that the potential energy barrier ΦB between Al and SiO2 is about 3.1 eV, and the field emission current density in Equation 4.47 (in the textbook) in full is 4.18 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) J field − emission e3 2 Ec = E exp − ; E 8π h Φ B Chapter 4 8π (2 me Φ 3B ) 1/ 2 Ec = 3eh calculate the field emission current for the two cases. For simplicity take me to be the electron mass in free space. What is your conclusion? Solution We can begin the calculation of field emission current finding the values of the field independent e3 constants Ec, B = and the area A of the Al electrode. 8π h Φ B Thus Ec = 8π (2 me Φ 3eh ) 3 1/ 2 B = [ 8π 2(9.1 × 10 −31 3 (1.602 × 10 kg)(3.1 × 1.602 × 10 −19 C)(6.626 × 10 −34 −19 J) ] 1 3 2 J s) = 3.726 × 1010 V m- 1 (1.602 × 10 −19 C) e3 B= = = 4.971 × 10-7 A V- 2 −34 −19 8π h Φ B 8π (6.626 × 10 J s)(3.1 × 1.602 × 10 J) 3 A = (50 × 10 −6 m ) × (50 × 10 −6 m ) = 2.5 × 10-9 m2 When the thickness of SiO2 layer d is 100 Å, the field in the MOS device is 10 V = 1 × 109 V m-1 100 × 10 −10 m and the field emission current is E= E I = AJ field − emissiom = ABE 2 exp − c = E = (2.5 × 10 −9 m 2 )(4.971 × 10 −7 A V −2 )(1 × 10 9 V m ) −1 2 (3.726 × 1010 V m −1 ) exp − (1 × 109 V m −1 ) = 8.18 × 10-14 A. In the second case the SiO2 layer is 2.5 times thinner ( 40 Å ) and the field in the device is 2.5 times stronger. 10 V = 2.5 × 109 V m- 1 40 × 10 −10 m The current in this case is E= E I = AJ field − emissiom = ABE 2 exp − c = E (3.726 × 1010 V m −1 ) 2 = (2.5 × 10 −9 m 2 )( 4.971 × 10 −7 A V −2 )(2.5 × 10 9 V m −1 ) exp − 9 −1 (2.5 × 10 V m ) = 2.62 × 10-3 A. So as predicted by equation 4.47 (in the textbook), the field-assisted emission current is a very strong function of the electric field. 4.19 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 4.15 Lattice waves and heat capacity a. Consider an aluminum sample. The nearest separation 2R (2 × atomic radius) between the Al-Al atoms in the crystal is 0.286 nm. Taking a to be 2R, and given the sound velocity in Al as 5100 m s-1, calculate the force constant β in Equation 4.66 (in the textbook). Use the group velocity νg from the actual dispersion relation, Equation 4.55 (in the textbook), to calculate the “sound velocity” at wavelengths of Λ = 1 mm, 1 µm and 1 nm. What is your conclusion? b. Aluminum has a Debye temperature of 394 K. Calculate its specific heat capacity in the summer at 25 °C and in winter at -10 °C. c. Calculate the specific heat capacity of a Ge crystal at 25 °C and compare it with the experimental value. Solution a The group velocity of lattice waves is given by Equation 4.55 (in the textbook). For sufficiently small K, or long wavelengths, such that 1/2Ka →0, the expression for the group velocity can be simplified like in Equation 4.66 (in the textbook) to νg = a β M From here we cam calculate the force constant β ν β = M g a 2 The mass of one Al atom is M= Mat NA and finally for the force constant we receive 2 27 × 10 −3 kg mol −1 ) 5100 m s −1 ( Mat ν g -2 = = 14.26 kg s −9 23 −1 NA a (6.022 × 10 mol ) 0.286 × 10 m 2 β= Now considering the dispersion relation K = 2π and Equation 4.55 (in the textbook) we receive Λ 1 β NA 2 π a ν g (Λ) = a cos Λ Mat Performing the calculations for the given wavelengths, we receive the following results: ν g (10 −3 m) = 5100 m s-1 ν g (10 −6 m) = 5099.998 m s- 1 ν g (10 −9 m) = 3176.22 m s- 1 4.20 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 It is evident that for the first two wavelengths, 1/2Ka →0 and we can use the approximation in Equation 4.66 (in the textbook). For the third wavelength, this is not true and we have to use the exact dispersion relation when calculating the group velocity. b In summer, the temperature is given to be T = 25 °C = 298 K and T/TD is 298/394 = 0.756. From Figure 4Q15-1, the molar heat capacity of Al during the summer is C m = 0.92 × (3R) = 22.95 J K-1 mol-1 The corresponding specific heat capacity is −1 −1 Cm (22.95 J K mol ) cs = = = 0.85 J K-1 g- 1 −1 Mat (27 g mol ) In similar way during the winter we have −1 −1 Cm (22.40 J K mol ) C m = 0.898 × (3R) = 22.40 J K mol and cs = = = 0.83 J K-1 g- 1 −1 Mat 27 g mol ( ) -1 -1 25 1 Cm = 3R 0.9 0.8 20 0.7 Al - winter Al - summer 0.6 Cm/(3R) 0.5 0.4 0.3 0.2 15 Cm J mole-1 10 5 0.1 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 T / TD Figure 4Q15-1 Heat capacity of Al at -10 °C and at 25 °C . c We can find the heat capacity of Ge in the way described in b. Alternatively, we can find Cm performing the integration in Equation 4.64 (in the textbook) numerically 3 T x e 298 Cm = 9 R ∫ x dx = 3 R 3 2 360 TD 0 (e − 1) TD 3 T 4 x 360 298 ∫ 0 -1 mol-1 2 dx = 3 R( 0.931) = 23.22 J K x (e − 1) x4 ex Thus the specific heat capacity is: cs = (23.22 J K −1 mol −1 ) = 319.9 J K-1 kg-1 Cm = Mat (72.59 × 10 −3 kg mol −1 ) The difference between the calculated value and the experimental one is less than 1%. 4.21 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 4 4.16 Thermal conductivity a. Given that silicon has Young’s modulus of about 110 GPa density of 2.3 g cm-3, calculate the mean free path of phonons in Si at room temperature. b. Diamond has the same crystal structure as Si but has a very large thermal conductivity, about 1000 W m-1 K-1 at room temperature. Given that diamond has a specific heat capacity cs of 0.50 J K-1 g-1, Young’s modulus of 830 GPa, and density ρ of 0.35 g cm-3, calculate the mean free path of phonons in diamond. c. GaAs has a thermal conductivity of 200 W m-1 K-1 at 100 K and 80 W m-1 K-1 at 200 K. Calculate its thermal conductivity at 25 °C and compare with the experimental value of 44 W m-1 K-1. (Hint: Consider Figure 4Q16-1.) 100000 (W m-1 K-1) 10000 Sapphire 1000 100 MgO 10 1 1 10 100 1000 Temperature (K) Figure 4Q16-1 Thermal conductivity of sapphire and MgO as a function of temperature. Solution a Assume room temperature of 25 °C (298 K). For this temperature from Table 4.5 (in the textbook), we can find the thermal conductivity κ (κ = 148 W m-1 K-1) for silicon and its specific heat capacity Cs (Cs = 0.703 J K-1 g-1). We can calculate the phonon mean free path at this temperature from Equation 4.68 (in the textbook): l ph = 3κ CVυ ph where CV is the heat capacity per unit volume. CV can be found from the specific heat capacity CV = ρCs and the phonon velocity can be obtained from Equation 4.67 (in the textbook) - υ ph ≈ Y . ρ Thus the phonon mean free path in Si at 25 °C is l ph = = 3κ ρ Cs ρ 3κ = = Y Cs ρ Y (0.703 × 10 3 (148 W m −1 K −1 ) 3 J K −1 kg −1 ) (2.3 × 10 3 kg m 4.22 −3 ) (110 × 10 9 Pa ) = 3.971 × 10-8 m Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) b Chapter 4 The mean free path of phonons in diamond is l ph 3 (1000 W m −1 K −1 ) 3κ = 1.113 × 10-7 m = = 3 − 1 − 1 3 − 3 9 Cs ρY (0.5 × 10 J K kg ) (3.5 × 10 kg m )(830 × 10 Pa ) c The temperatures at which the thermal conductivity κ is given can be considered as relatively high. For this temperature range, we can assume that CV is almost constant and since the phonon velocity is approximately independent from temperature according to Equation 4.68 (in the textbook) the thermal conductivity is proportional to the mean free path of phonons l ph . Since the phonon 1 concentration increases with temperature, nph ∝ T, the mean free path decreases as l ph ∝ . Thus, κ T decreases in the same manner with temperature as in Figure 4Q16-1. We can assume that the temperature dependence of the thermal conductivity is given by: A +B T Then we have two equations and two unknowns κ (T ) = A +B 100 B 80 = +B 200 200 = and for the coefficients A and B we receive: A = 2.4 × 104 W m-1 and B = -40 W m-1 K-1 The thermal conductivity at 25 °C (298 K) is 2.4 × 10 4 W m −1 − 40 W m −1 K −1 = 40.5 W m-1 K-1 298 K which is close to the experimental value. κ= *4.17 Overlapping bands Consider Cu and Ni with their density of states as schematically sketched in Figure 4Q17-1. Both have overlapping 3d and 4s bands, but the 3d band is very narrow compared to the 4s band. In the case of Cu the band is full, whereas in Ni, it is only partially filled. a. In Cu, do the electrons in the 3d band contribute to electrical conduction? Explain. b. In Ni, do electrons in both bands contribute to conduction? Explain. c. Do electrons have the same effective mass in the two bands? Explain. d. Can an electron in the 4s band with energy around EF become scattered into the 3d band as a result of a scattering process? Consider both metals. e. Scattering of electrons from the 4s band to the 3d band and vice versa can be viewed as an additional scattering process. How would you expect the resistivity of Ni to compare with that of Cu, even though Ni has 2 valence electrons and nearly the same density as Cu? In which case would you expect a stronger temperature dependence for the resistivity? 4.23 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) g(E) 3d g(E) 3d Chapter 4 Cu Ni 4s 4s E E EF EF Figure 4Q17-1 Density of states and electron filling in Cu and Ni. Solution a In Cu the 3d band is full so the electrons in this band do not contribute to conduction. b In Ni both the 3d and 4s bands are partially filled so electrons in both bands can gain energy from the field and move to higher energy levels. Thus both contribute to electrical conductivity. c No, because the effective mass depends on how easily the electron can gain energy from the field and accelerate or move to higher energy levels. The energy distributions in the two bands are different. In the 4s band, the concentration of states is increasing with energy whereas in the 3d band, it is decreasing with energy. One would therefore expect different inertial resistances to acceleration, different effective mass and hence different drift mobility for electrons in these bands. d Not in copper because the 3d band is full and cannot take electrons. In Ni the electrons can indeed be scattered from one band to the other, e.g. an electron in the 4s band can be scattered into the 3d band. Its mobility will then change. Electrons in the 3d band are very sluggish (low drift mobility) and contribute less to the conductivity. e Ni should be more resistive because of the additional scattering mechanism from the 4s to the 3d band (Matthiessen's rule). This scattering is called s-d scattering. One may at first think that this s-d scattering de-emphasizes the importance of scattering from lattice vibrations and hence, overall, the resistivity should be less temperature dependent. In reality, electrons in Ni also get scattered by magnetic interactions with Ni ion magnetic moments (Nickel is ferromagnetic; Ch. 8 in the textbook) which has a stronger temperature dependence than ρ ∼ T. "After a year's research, one realises that it could have been done in a week." Sir William Henry Bragg 4.24 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 Second Edition ( 2001 McGraw-Hill) Chapter 5 5.1 Bandgap and photodetection a. Determine the maximum value of the energy gap that a semiconductor, used as a photoconductor, can have if it is to be sensitive to yellow light (600 nm). b. A photodetector whose area is 5 × 10-2 cm2 is irradiated with yellow light whose intensity is 2 mW cm–2. Assuming that each photon generates one electron-hole pair, calculate the number of pairs generated per second. c. From the known energy gap of the semiconductor GaAs (Eg = 1.42 eV), calculate the primary wavelength of photons emitted from this crystal as a result of electron-hole recombination. d. Is the above wavelength visible? e. Will a silicon photodetector be sensitive to the radiation from a GaAs laser? Why? Solution a We are given the wavelength λ = 600 nm, therefore we need Eph = hυ = Eg so that, Eg = hc/λ = (6.626 × 10-34 J s)(3.0 × 108 m s-1) / (600 × 10-9 m) ∴ b E g = 3.31 × 10-19 J or 2.07 eV Area A = 5 × 10-2 cm2 and light intensity Ilight = 2 × 10-3 W/cm2. The received power is: P = AIlight = (5 × 10-2 cm2)(2 × 10-3 W/cm2) = 1.0 × 10-4 W Nph = number of photons arriving per second = P/Eph ∴ Nph = (1.0 × 10-4 W) / (3.31 × 10-19 J) ∴ Nph = 3.02 × 1014 Photons s-1 Since the each photon contributes one electron-hole pair (EHP), the number of EHPs is then: N E H P = 3.02 × 1014 EHP s- 1 c For GaAs, Eg = 1.42 eV and the corresponding wavelength is λ = hc/Eg = (6.626 × 10-34 J s)(3.0 × 108 m s-1) / (1.42 eV × 1.602 × 10-19 J/eV) ∴ λ = 8.74 × 10-7 m or 874 nm The wavelength of emitted radiation due to electron-hole pair (EHP) recombination is therefore 874 nm. d It is not in the visible region (it is in the infrared). e From Table 5.1 (in the textbook), for Si, Eg = 1.10 eV and the corresponding cut-off wavelength is, λg = hc/Eg = (6.626 × 10-34 J s)(3.0 × 108 m s-1) / (1.1 eV × 1.602 × 10-19 J/eV) 5.1 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 λ g = 1.13 × 10-6 m or 1130 nm ∴ Since the 874 nm wavelength of the GaAs laser is shorter than the cut-off wavelength of 1130 nm, the Si photodetector can detect the 874 nm radiation (Put differently, the photon energy corresponding to 874 nm, 1.42 eV, is larger than the Eg, 1.10 eV, of Si which means that the Si photodetector can indeed detect the 874 nm radiation). 5.2 Minimum conductivity a. Consider the conductivity of a semiconductor, σ = enµe + epµh. Will doping always increase the conductivity? b. Show that the minimum conductivity for Si is obtained when it is p-type doped such that the hole concentration is pm = ni µe µh and the corresponding minimum conductivity (maximum resistivity) is σ min = 2eni µe µh c. Calculate pm and σmin for Si and compare with intrinsic values. Solution a Doping does not always increase the conductivity. Suppose that we have an intrinsic sample with n = p but the hole drift mobility is smaller. If we dope the material very slightly with p-type then p > n. However, this would decrease the conductivity because it would create more holes with lower mobility at the expense of electrons with higher mobility. Obviously with further doping p increases sufficiently to result in the conductivity increasing with the extent of doping. b To find the minimum conductivity, first consider the mass action law: np = ni2 isolate n: n = ni2/p Now substitute for n in the equation for conductivity: σ = enµe + epµh ∴ eni 2 µe σ= + µh ep p To find the value of p that gives minimum conductivity (pm), differentiate the above equation with respect to p and set it equal to zero: dσ en 2 µ = − i 2 e + µhe dp p ∴ − eni 2 µe + µhe = 0 pm 2 Isolate pm and simplify: pm = ni µe µh 5.2 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 Substituting this expression back into the equation for conductivity will give the minimum conductivity: σ min = eni 2 µe eni 2 µe µe + µh epm = + µh eni pm ni µe µh µh µh + eni µe µh = eni µe µh + eni µe µh µe ∴ σ min = eni µe ∴ σ min = 2eni µe µh c From Table 5.1, for Si: µe = 1350 cm2 V-1 s-1, µh = 450 cm2 V-1 s-1 and ni = 1.45 × 1010 cm-3. Substituting into the equations for pm and σmin: pm = ni ∴ µe 1350 cm 2 V −1 s −1 = (1.45 × 1010 cm −3 ) µh 450 cm 2 V −1 s −1 p m = 2.51 × 1010 cm- 3 σ min = 2eni µe µh ∴ σ min = 2(1.602 × 10 −19 C)(1.45 × 1010 cm −3 ) (1350 cm 2 V −1 s −1 )( 450 cm 2 V −1 s −1 ) ∴ σ min = 3.62 × 10-6 Ω -1 cm- 1 The corresponding maximum resistivity is: ρmax = 1 / σmin = 2.76 × 105 Ω cm The intrinsic value corresponding to pm is simply ni (= 1.45 × 1010 cm-3). Comparing it to pm: pm 2.51 × 1010 cm −3 = = 1.73 ni 1.45 × 1010 cm −3 The intrinsic conductivity is: σint = eni(µe + µh) ∴ σint = (1.602 × 10-19 C)(1.45 × 1010 cm-3)(1350 cm2 V-1 s-1 + 450 cm2 V-1 s-1) ∴ σint = 4.18 × 10-6 Ω-1 cm-1 Comparing this value to the minimum conductivity: σ int 3.62 × 10 −6 W −1 cm −1 = = 0.866 σ min 4.18 × 10 −6 W -1 cm -1 Sufficient p-type doping that increases the hole concentration by 73% decreases the conductivity by 15% to its minimum value. 5.3 Compensation doping in Si a. A Si wafer has been doped n-type with 1017 As atoms cm–3. 1. Calculate the conductivity of the sample at 27 °C. 5.3 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 2. Where is the Fermi level in this sample at 27 °C with respect to the Fermi level (EFi) in intrinsic Si? 3. Calculate the conductivity of the sample at 127 °C. b. The above n-type Si sample is further doped with 9 × 1016 boron atoms (p-type dopant) per centimeter cubed. 1. Calculate the conductivity of the sample at 27 °C. 2. Where is the Fermi level in this sample with respect to the Fermi level in the sample in (a) at 27 °C? Is this an n-type or p-type Si? Solution a Given temperature T = 27 ˚C = 300 K, concentration of donors Nd = 1017 cm-3, and drift mobility µe ≈ 800 cm2 V-1 s-1 (from Figure 5Q3-1). At room temperature the electron concentration n = Nd >> p (hole concentration). Drift Mobility(cm2 V-1s-1) 2000 1000 Holes Electrons 100 50 1015 1016 1017 1018 Dopant Concentration, 1019 1020 cm-3 Figure 5Q3-1 The variation of the drift mobility with dopant concentration in Si for electrons and holes at 300 K. (1) The conductivity of the sample is: σ = eNdµe ≈ (1.602 × 10-19 C)(1017 cm-3)(800 cm2 V-1 s-1) = 12.8 Ω-1 cm-1 (2) In intrinsic Si, EF = EFi, ni = Ncexp[-(Ec - EFi)/kT] In doped Si, n = Nd, EF = EFn, n = Nd = Ncexp[-(Ec - EFn)/kT] Eqn. (2) divided by Eqn. (1) gives, Nd E − EFi = exp Fn kT ni ∴ (1) (2) (3) N E − EFi ln d = Fn kT ni 5.4 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Chapter 5 ∆EF = EFn - EFi = kT ln(Nd/ni) (4) Substituting we find (ni = 1.45 × 1010 cm-3 from Table 5.1 in the textbook), ∆EF = (8.617 × 10-5 eV/K)(300 K)ln[(1017 cm-3)/ (1.45 × 1010 cm-3)] ∴ ∆E F = 0.407 eV above E fi Electron Drift Mobility(cm2 V-1s-1) 50000 L Ge Nd =1014 10000 T–1.5 Nd =1013 Nd =1016 Nd =1017 1000 Nd =1018 100 Nd =1019 Si T1.5 10 70 100 Temperature (K) 800 Figure 5Q3-2 Log-log plot for drift mobility versus temperature for n-type Ge and n-type Si samples. Various donor concentrations for Si are shown, Nd are in cm-3. The upper right insert is the simple theory for lattice limited mobility whereas the lower left inset is the simple theory for impurity scattering limited mobility. (3) At Ti = 127 ˚C = 400 K, µe ≈ 450 cm2 V-1 s-1 (from Figure 5Q3-2). The semiconductor is still ntype (check that Nd >> ni at 400 K), then σ = eNdµe ≈ (1.602 × 10-19 C)(1017 cm-3)(450 cm2 V-1 s-1) = 7.21 Ω-1 cm-1 b The sample is further doped with Na = 9 × 1016 cm-3 = 0.9 × 1017 cm-3 acceptors. Due to compensation, the net effect is still an n-type semiconductor but with an electron concentration given by, n = Nd - Na = 1017 cm-3 - 0.9 × 1017 cm-3 = 1 × 1016 cm-3 (>> ni) We note that the electron scattering now occurs from Na + Nd (1.9 × 1017 cm-3) number of ionized centers so that µe ≈ 700 cm2 V-1 s-1 (Figure 5Q3-1). (1) (2) σ = eNdµe ≈ (1.602 × 10-19 C)(1016 cm-3)(700 cm2 V-1 s-1) = 1.12 Ω-1 cm-1 Using Eqn. (3) with n = Nd - Na we have Nd − Na E ′ − EFi = exp Fn kT ni so that ∆E′Φ = EFn′ - EFi = (0.02586 eV)ln[(1016 cm-3) / (1.45 × 1010 cm-3)] ∴ ∆E′ Φ = 0.348 eV above E F i The Fermi level from (a) and (b) has shifted “down” by an amount 0.059 eV. Since the energy is still above the Fermi level, this an n-type Si. 5.5 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 5.4 Temperature dependence of conductivity An n-type Si sample has been doped with 1015 phosphorus atoms cm–3. The donor energy level for P in Si is 0.045 eV below the conduction band edge energy. a. Calculate the room temperature conductivity of the sample. b. Estimate the temperature above which the sample behaves as if intrinsic. c. Estimate to within 20% the lowest temperature above which all the donors are ionized. d. Sketch schematically the dependence of the electron concentration in the conduction band on the temperature as log(n) versus 1/T, and mark the various important regions and critical temperatures. For each region draw an energy band diagram that clearly shows from where the electrons are excited into the conduction band. e. Sketch schematically the dependence of the conductivity on the temperature as log(σ) versus 1/T and mark the various critical temperatures and other relevant information. Solution 600°C 400°C 200°C 27°C 0°C Intrinsic Concentration (cm-3) 1018 2.4 1013 cm-3 1015 Ge 1012 1.45 1010 cm-3 109 Si 106 2.1 106 cm-3 GaAs 103 1 1.5 2 2.5 3 1000/T (1/K) 3.5 4 Figure 5Q4-1 The temperature dependence of the intrinsic concentration. a The conductivity at room temperature T = 300 K is (µe = 1350 × 10-4 m2 V-1 s-1 can be found in Table 5.1 in the textbook): σ = eNdµe ∴ σ = (1.602 × 10-19 C)(1 × 1021 m-3)(1350 × 10-4 m2 V-1 s-1) = 21.6 Ω-1 m-1 b At T = Ti, the intrinsic concentration ni = Nd = 1 × 1015 cm-3. From Figure 5Q4-1, the graph of ni(T) vs. 1/T, we have: 1000 / Ti = 1.9 K-1 5.6 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Chapter 5 T i = 1000 / (1.9 K-1) = 526 K or 253 ˚C c The ionization region ends at T = Ts when all donors have been ionized, i.e. when n = Nd. From Example 5.7, at T = Ts: 1 − ∆E 2 1 n = Nd = Nc Nd exp 2 2 kTs ∴ ∴ Ts = Ts = − ∆E 2 k ln 1 2 Nd Nc Nd = − ∆E 2 Nd 2 k ln Nc ∆E N k ln c 2 Nd Take Nc = 2.8 × 1019 cm-3 at 300 K from Table 5.1 (in the textbook), and the difference between the donor energy level and the conduction band energy is ∆E = 0.045 eV. Therefore our first approximation to Ts is: (0.045 eV)(1.602 × 10 −19 J/eV) ∆E Ts = = = 54.68 K Nc (2.8 × 1019 cm −3 ) − 23 k ln J/K ) ln (1.381 × 10 15 −3 2 Nd 2(10 cm ) Find the new Nc at this temperature, Nc′: 3 3 T 2 54.68 K 2 Nc′ = Nc s = (2.8 × 1019 cm −3 ) = 2.179 × 1018 cm-3 300 300 K Find a better approximation for Ts by using this new Nc′: Ts′ = (0.045 eV)(1.602 × 10 −19 J/eV) ∆E = 74.64 K = 18 −3 Nc′ × . cm 2 179 10 ( ) −23 k ln J/K ) ln (1.381 × 10 2 Nd 2(1015 cm −3 ) 3 ∴ 3 74.64 K 2 T′ 2 Nc′′= Nc s = 3.475 × 1018 cm-3 = (2.8 × 1019 cm −3 ) 300 K 300 A better approximation to Ts is: (0.045 eV)(1.602 × 10 −19 J/eV) ∆E Ts′′= = 69.97 K = (3.475 × 1018 cm −3 ) Nc′′ − 23 k ln J/K ) ln (1.381 × 10 2 Nd 2(1015 cm −3 ) 3 ∴ 3 69.97 K 2 T ′′ 2 Nc′′′= Nc s = 3.154 × 1018 cm-3 = (2.8 × 1019 cm −3 ) 300 K 300 5.7 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Ts′′′= Chapter 5 (0.045 eV)(1.602 × 10 −19 J/eV) ∆E = 70.89 K = 18 −3 Nc′′′ × cm . 3 154 10 ( ) −23 k ln J/K ) ln (1.381 × 10 2 Nd 2(1015 cm −3 ) We can see that the change in Ts is very small, and for all practical purposes we can consider the calculation as converged. Therefore T s = 70.9 K = -202.1 ˚C. d and e See Figures 5Q4-2 and 5Q4-3. ln(n) Intrinsic slope = –Eg/2k Extrinsic ln(Nd) Ts Ionization slope = – E/2k Ti ni(T) 1/T Figure 5Q4-2 The temperature dependence of the electron concentration in an n-type semiconductor. log( ) Resistivity LOGARITHMIC SCALE log(n) INTRINSIC Semiconductor Metal T EXTRINSIC Lattice scattering IONIZATION T log( ) –3/2 T 3/2 Impurity scattering High Temperature 1/T Low Temperature Figure 5Q4-3 Schematic illustration of the temperature dependence of electrical conductivity for a doped (n-type) semiconductor. 5.5 GaAs Ga has a valency of III and As has V. When Ga and As atoms are brought together to form the GaAs crystal, as depicted in Figure 5Q5-1, the 3 valence electrons in each Ga and the 5 valence electrons in each As are all shared to form four covalent bonds per atom. In the GaAs crystal with some 1023 or so equal numbers of Ga and As atoms, we have an average of four valence electrons per atom, whether Ga or As, so we would expect the bonding to be similar to that in the Si crystal: four bonds per atom. The crystal structure, however, is not that of diamond but rather that of zinc blende (Chapter 1 of the textbook). a. What is the average number of valence electrons per atom for a pair of Ga and As atoms and in the GaAs crystal? 5.8 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 b. What will happen if Se or Te, from Group VI, are substituted for an As atom in the GaAs crystal? c. What will happen if Zn or Cd, from Group II, are substituted for a Ga atom in the GaAs crystal? d. What will happen if Si, from Group IV, is substituted for an As atom in the GaAs crystal? e. What will happen if Si, from Group IV, is substituted for a Ga atom in the GaAs crystal? What do you think amphoteric dopant means? f. Based on the above discussion ,what do you think the crystal structures of the III-V compound semiconductors AlAs, GaP, InAs, InP, and InSb will be? Ga As Ga atom (Valency III) As atom (Valency V) Ga As Ga As As Ga As Ga Ga As Ga As As Ga As Ga Figure 5Q5-1 The GaAs crystal structure in two dimensions. Average number of valence electrons per atom is four. Each Ga atom covalently bonds with four neighboring As atoms and vice versa. Solution As As atom (Valency V) ψhyb orbitals Valence electron As ion core (+5e) Ga Ga atom (Valency III) ψhyb orbitals Valence electron Ga As Ga As As Ga As Ga Ga As Ga As As Ga As Ga Ga ion core (+3e) Explanation of bonding in GaAs: The one s and three p orbitals hybridize to form 4 ψhyb orbitals. In As there are 5 valence electrons. One ψhyb has two paired electrons and 3 ψhyb have 1 electron each as shown. In Ga there are 3 electrons so one ψhyb is empty. This empty ψhyb of Ga can overlap the full ψhyb of As. The overlapped orbital, the bonding orbital, then has two paired electrons. This is a bond between Ga and As even though the electrons come from As (this type of bonding is called dative bonding). It is a bond because the electrons in the overlapped orbital are shared by both As and Ga. The other 3 ψhyb of As can overlap 3 ψhyb of neighboring Ga to form "normal bonds". Repeating this in three dimensions generates the GaAs crystal where each atom bonds to four neighboring atoms as shown. Because all the bonding orbitals are full, the valence band formed from these orbitals is also full. The crystal structure is reminiscent of that of Si. GaAs is a semiconductor. a The average number of valence electrons is 4 electrons per atom. 5.9 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 b Se or Te replacing As will have one additional electron that cannot be involved in any of the four bonds. Hence Se and Te will act as a donor. c Zn or Cd replacing Ga will have one less electron than the substituted Ga atom. This creates a hole in a bond. Zn and Cd will act as acceptors. d The Si atom has 1 less electron than the As atom and when it substitutes for an As atom in GaAs there is a "hole" in one of the four bonds. This creates a hole, or the Si atom acts as an acceptor. e The Si atom has 1 more electron than the Ga atom and when it substitutes for a Ga atom in GaAs there is an additional electron that cannot enter any of the four bonds and is therefore donated into the CB (given sufficiently large temperature). Si substituting for Ga therefore acts as a donor. f All these compounds (AlAs, GaP, InAs, InP, InSb) are compounds of III elements and V elements so they will follow the example of GaAs. 5.6 Doped GaAs Consider the GaAs crystal at 300 K. a. Calculate the intrinsic conductivity and resistivity. b. In a sample containing only 1015 cm–3 ionized donors, where is the Fermi level? What is the conductivity of the sample? c. In a sample containing 1015 cm–3 ionized donors and 9 × 1014 cm–3 ionized acceptors, what is the free hole concentration? Solution a Given temperature, T = 300 K, and intrinsic GaAs. From Table 5.1 (in the textbook), ni = 1.8 × 106 cm-3, µe ≈ 8500 cm2 V-1 s-1 and µh ≈ 400 cm2 V-1 s-1. Thus, σ = eni(µe + µh) ∴ σ = (1.602 × 10-19 C)(1.8 × 106 cm-3)(8500 cm2 V-1 s-1 + 400 cm2 V-1 s-1) ∴ σ = 2.57 × 10-9 Ω -1 cm- 1 ∴ ρ = 1/σ = 3.89 × 108 Ω cm b Donors are now introduced. At room temperature, n = Nd = 1015 cm-3 >> ni >> p. σn = eNdµe ≈ (1.602 × 10-19 C)(1015 cm-3)(8500 cm2 V-1 s-1) = 1.36 Ω-1 cm-1 ∴ ρ n = 1/σ n = 0.735 Ω cm In the intrinsic sample, EF = EFi, ni = Ncexp[-(Ec - EFi)/kT] In the doped sample, n = Nd, EF = EFn, n = Nd = Ncexp[-(Ec - EFn)/kT] Eqn. (2) divided by Eqn. (1) gives, ∴ (1) (2) Nd E − EFi = exp Fn ni kT (3) ∆EF = EFn - EFi = kT ln(Nd/ni) (4) 5.10 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 Substituting we find, ∆EF = (8.617 × 10-5 eV/K)(300 K)ln[(1015 cm-3)/(1.8 × 106 cm-3)] ∴ ∆E F = 0.521 eV above E Fi (intrinsic Fermi level) c The sample is further doped with Na = 9 × 1014 cm-3 = 0.9 × 1015 cm-3 acceptors. Due to compensation, the net effect is still an n-type semiconductor but with an electron concentration given by, n = Nd - Na = 1015 cm-3 - 0.9 × 1015 cm-3 = 1 × 1014 cm-3 (>> ni) The sample is still n-type though there are less electrons than before due to the compensation effect. From the mass action law, the hole concentration is: p = ni2 / n = (1.8 × 106 cm-3)2 / (1 × 1014 cm-3) = 0.0324 cm-3 On average there are virtually no holes in 1 cm3 of sample. We can also calculate the new conductivity. We note that electron scattering now occurs from Na + Nd number of ionized centers though we will assume that µe ≈ 8500 cm2 V-1 s-1. σ = enµe ≈ (1.602 × 10-19 C)(1014 cm-3)(8500 cm2 V-1 s-1) = 0.136 Ω-1 cm-1 5.7 Degenerate semiconductor Consider the general exponential expression for the concentration of electrons in the CB, ( E − EF ) n = Nc exp − c kT and the mass action law, np = ni2. What happens when the doping level is such that n approaches N c and exceeds it? Can you still use the above expressions for n and p? Consider an n-type Si that has been heavily doped and the electron concentration in the CB is 1020 cm–3. Where is the Fermi level? Can you use np = ni2 to find the hole concentration? What is its resistivity? How does this compare with a typical metal? What use is such a semiconductor? Solution Consider n = Ncexp[-(Ec - EF)/kT] (1) 2 and np = ni (2) These expressions have been derived using the Boltzmann tail (E > EF + a few kT) to the Fermi - Dirac (FD) function f(E) as in Section 5.1.4 (in the textbook). Therefore the expressions are NOT valid when the Fermi level is within a few kT of Ec. In these cases, we need to consider the behavior of the FD function f(E) rather than its tail and the expressions for n and p are complicated. It is helpful to put the 1020 cm-3 doping level into perspective by considering the number of atoms per unit volume (atomic concentration, nSi) in the Si crystal: nat = i.e. see that (Density) N A (2.33 × 10 3 kg m -3 )(6.022 × 10 23 mol −1 ) = (28.09 × 10 −3 kg mol −1 ) Mat nat = 4.995 × 1028 m-3 or 4.995 × 1022 cm-3 Given that the electron concentration n = 1020 cm-3 (not necessarily the donor concentration!), we 5.11 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 n/nat = (1020 cm-3) / (4.995 × 1022 cm-3) = 0.00200 which means that if all donors could be ionized we would need 1 in 500 doping or 0.2% donor doping in the semiconductor (n is not exactly Nd for degenerate semiconductors). We cannot use Equation (1) to find the position of EF. The Fermi level will be in the conduction band. The semiconductor is degenerate (see Figure 5Q7-1). E CB Impurities forming a band CB EF n Ec g(E) Ec Ev EFp Ev VB (a) (b) Figure 5Q7-1 (a) Degenerate n-type semiconductor. Large number of donors form a band that overlaps the CB. (b) Degenerate p-type semiconductor. Drift Mobility(cm2 V-1s-1) 2000 1000 Holes Electrons 100 50 1015 1016 1017 1018 Dopant Concentration, 1019 1020 cm-3 Figure 5Q7-2 The variation of the drift mobility with dopant concentration in Si for electrons and holes at 300 K. Take T = 300 K, and µe ≈ 900 cm2 V-1 s-1 from Figure 5Q7-2. The resistivity is ρ = 1/(enµe) = 1/[(1.602 × 10-19 C)(1020 cm-3)(900 cm2 V-1 s-1)] ∴ ρ = 6.94 × 10-5 Ω cm or 694 × 10-7 Ω m Compare this with a metal alloy such as nichrome which has ρ = 1000 nΩ m = 10 × 10-7 Ω m. The difference is only about a factor of 70. This degenerate semiconductor behaves almost like a “metal”. Heavily doped degenerate semiconductors are used in various MOS (metal- oxide- semiconductor) devices where they serve as the gate electrode (substituting for a metal) or interconnect lines. 5.12 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 5.8 Photoconductivity and speed Consider two p-type Si samples both doped with 1015 B atoms cm–3. Both have identical dimensions of length L (1 mm), width W (1 mm), and depth (thickness) D (0.1 mm). One sample, labeled A, has an electron lifetime of 1 µs whereas the other, labeled B, has an electron lifetime of 5 µs. a. At time t = 0, a laser light of wavelength 750 nm is switched on to illuminate the surface (L × W) of both the samples. The incident laser light intensity on both samples is 10 mW cm–2. At time t = 50 µs, the laser is switched off. Sketch the time evolution of the minority carrier concentration for both samples on the same axes. b. What is the photocurrent (current due to illumination alone) if each sample is connected to a 1 V battery? Solution a Gph Time Laser on t=0 Laser off t = 10 s Log[excess carrier concentration] t t BGph B B A AGph A Time t=0 A B t =0 Figure 5Q8-1 Schematic sketch of the excess carrier concentration in the samples A and B as a function of time from the laser switch-on to beyond laser switch-off. b From the mobility vs. dopant graph (Figure 5Q8-2), µh = 450 × 10-4 m2 V-1 s-1 and µe = 1300 × 10-4 m2 V-1 s-1. Given are the wavelength of illumination λ = 750 × 10-9 m, light intensity I = 100 W/m2, length L = 1 mm, width W = 1 mm, and depth (thickness) D = 0.1 mm. 5.13 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 Drift Mobility(cm2 V-1s-1) 2000 1000 Electrons Holes 100 50 1015 1016 1017 1018 Dopant Concentration, 1019 1020 cm-3 Figure 5Q8-2 The variation of the drift mobility with dopant concentration in Si for electrons and holes at 300 K. The photoconductivity is given by (see Example 5.11 in the textbook): ∆σ = eηIλτ ( µe + µh ) hcD where η = 1 is the quantum efficiency. Assume all light intensity is absorbed (correct assumption as the absorption coefficient at this wavelength is large). The photocurrent density and hence the photo current is given by: ∆J = ∆I/A = E∆σ Substitute: V eηIλτ ( µe + µh ) ∆I = (W × D) L hcD ∴ ∆I = WVeηIλτ ( µe + µh ) Lhc The photocurrent will travel perpendicular to the W × D direction, while the electric field E will be directed along L. Substituting the given values for sample A (electron lifetime τA = 10-6 s, voltage V = 1 V): ∆I A = ∴ (0.001 m)(1 V)(1.602 × 10 −19 C)(1)(100 W/m 2 )(750 × 10 −9 m)(10 −6 s) 0.13 (0.001 m)(6.626 × 10 −34 J s)(3.0 × 108 m/s) m2 m2 + 0.045 Vs Vs ∆ I A = 1.06 × 10-5 A The photocurrent in sample B can be calculated with the same equation, using the given value of τB = 5 × 10-6 s. After calculation: ∆ I B = 5.29 × 10-5 A 5.14 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 *5.9 Hall Effect in Semiconductors The Hall effect in a semiconductor sample involves not only the electron and hole concentrations n and p, respectively, but also the electron and hole drift mobilities, µe and µh. The hall coefficient of a semiconductor is (see Chapter 2 of the textbook), RH = where b = p − nb 2 2 e( p + nb) Hall coefficient of a semiconductor µe . µh a. Given the mass action law, pn = ni2, find n for maximum RH (negative and positive RH ). Assume that the drift mobilities remain relatively unaffected as n changes (due to doping). Given the electron and hole drift mobilities, µe = 1350 cm2 V-1 s-1, µh = 450 cm2 V-1 s-1 for silicon determine n for maximum RH in terms of ni. b. Taking b = 3, plot RH as a function of electron concentration n/ni from 0.1 to 10. Solution a Substituting the mass action law (p = ni2/n) into the given equation, we get ni2 − nb 2 p − nb 2 u RH = = n2 2 = 2 e( p + nb) v n e i + nb n where u and v represent the numerator and denominator as a function of n. Then dRH u′v − uv ′ = =0 dn v2 where primes are derivatives with respect to n. This means that u′v - u v′ = 0, so that, 2 ni2 ni2 ni2 n2 n2 2 u′v − uv ′ = − 2 − b e + nb − − nb 2 2e i + nb − i2 + b = 0 n n n n n We can multiply through by n3 and then combine terms and factor to obtain, b 3n 4 − [3ni2 b(1 + b)]n 2 + ni4 = 0 or, b3x2 + [-3b(1 + b)]x + 1 = 0 where x = (n/ni)2. This is a quadratic equation in x. Its solution is, x= n 2 3b(1 + b) ± 9b 2 (1 + b)2 − 4b 3 = 2b3 ni2 For Si, b = 3, and we have two solutions corresponding to, and n/ni = 0.169, or p/ni = 1/0.169 = 5.92 n/ni = 1.14 b Substituting the mass action law p = ni2/n into the given equation and using a normalized electron concentration x = n/ni, we get, 5.15 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 1 ni2 − nb 2 − xb 2 p − nb 2 x RH = = n2 2 = 2 e( p + nb)2 ni 1 + xb en e + nb i x n or 1 − xb 2 RH x y= = 2 (1 / eni ) 1 + xb x Normalized Hall coefficient (eni)RH vs. normalized concentration n/ni obviously follows y vs. x which is shown in Figure 5Q9-1 for b = 3. (eni)RH 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 0.01 0.17 1/3 1.14 0.1 n/ni 1 10 Normalized Hall coefficient vs. normalized electron concentration. Values 0.17, 1.14 and 0.33 shown are n/n values when the magnitude of RH reaches i maxima and zero respectively. Figure 5Q9-1 *5.10 Compound semiconductor devices Silicon and germanium crystalline semiconductors are what are called elemental group IV semiconductors. It is possible to have compound semiconductors from atoms in groups III and V. For example, GaAs is a compound semiconductor that has Ga from group III and As from group V, so that in the crystalline structure we have an "effective" or "mean" valency of IV per atom and the solid behaves like a semiconductor. Similarly GaSb (gallium antimonide) would be a III-V type semiconductor. Provided we have a stochiometric compound, the semiconductor will be ideally intrinsic. If, however, there is an excess of Sb atoms in the solid GaSb, then we will have nonstochiometry and the semiconductor will be extrinsic. In this case, excess Sb atoms will act as donors in the GaSb structure. There are many useful compound semiconductors, the most important of which is GaAs. Some can be doped both n- and p-type, but many are one type only. For example, ZnO is a II-VI compound semiconductor with a direct bandgap of 3.2 eV, but unfortunately, due to the presence of excess Zn, it is naturally n-type and cannot be doped to p-type. a. GaSb (gallium antimonide) is an interesting direct bandgap semiconductor with an energy bandgap, Eg = 0.67 eV, almost equal to that of germanium. It can be used as an LED (light-emitting diode) or laser diode material. What would be the wavelength of emission from a GaSb LED? Will this be visible? 5.16 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 b. Calculate the intrinsic conductivity of GaSb at 300 K taking Nc = 2.3 × 1019 cm–3, N υ = 6.1 × 1019 cm–3, µe = 5000 cm2 V–1 s–1, and µh = 1000 cm2 V–1 s –1 . Compare with the intrinsic conductivity of Ge. c. Excess Sb atoms will make gallium antimonide nonstoichiometric, that is, GaSb1+δ, which will result in an extrinsic semiconductor. Given that the density of GaSb is 5.4 g cm–3, calculate δ (excess Sb) that will result in GaSb having a conductivity of 100 Ω-1 cm-1. Will this be an n- or ptype semiconductor? You may assume that the drift mobilities are relatively unaffected by the doping. Solution a Given Eg = 0.67 eV, the corresponding wavelength is: λ = hc / Eg = (6.626 × 10-34 J s)(3.0 × 108 m s-1) / (0.67 eV × 1.602 × 10-19 J/eV) ∴ b λ = 1.85 × 10-6 m or 1850 nm, not in the visible. Assume temperature, T = 300 K. 600°C 400°C 200°C 27°C 0°C Intrinsic Concentration (cm-3) 1018 2.4 1013 cm-3 1015 Ge 1012 1.45 1010 cm-3 109 Si 106 2.1 106 cm-3 GaAs 103 1 1.5 2 2.5 3 1000/T (1/K) 3.5 4 Figure 5Q10-1 The temperature dependence of the intrinsic concentration. For GaSb, we are given Nc = 2.3 × 1025 m-3; Nυ = 6.1 × 1025 m-3; µe = 5000 × 10-4 m2 V-1 s-1; µh = 1000 × 10-4 m2 V-1 s-1, therefore the intrinsic concentration is: ni = (NcNυ)1/2exp[-Eg/2kT] ∴ ∴ ni = [(2.3 × 1025 m-3)(6.1 × 1025 m-3)]1/2exp[-(0.67 eV)/(2×8.617 × 10-5 eV/K × 300 K)] ni = 8.822 × 1019 m-3 The intrinsic conductivity is therefore: σ = eni(µe + µh) 5.17 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Chapter 5 σ = (1.602 × 10-19 C)(8.822 × 1019 m-3)(0.5 m2 V-1 s-1 + 0.1 m2 V-1 s-1) = 8.48 Ω-1 m-1 For Ge, from Figure 5Q10-1 ni = 2.4 × 1019 m-3 and from Table 5.1 (in the textbook) we have µe = 0.39 m2 V-1 s-1 and µh = 0.19 m2 V-1 s-1. The intrinsic conductivity is then: σ = eni(µe + µh) ∴ σ = (1.602 × 10-19 C)(2.4 × 1019 m-3)(0.39 m2 V-1 s-1 + 0.19 m2 V-1 s-1) = 2.23 Ω-1 m-1 Intrinsic Ge and GaSb have comparable conductivities (same order of magnitude). (Typically, intrinsic conductivities of semiconductors are largely determined by the bandgap). c Excess antimony will make the sample n-type. We will assume that the electron mobility is roughly the same. In the extrinsic sample, n = Nd >> p. Given that σn = 100 Ω-1 m-1, and σn ≈ eNdµe: Nd = σn / eµe = (100 Ω-1 m-1)/[(1.602 × 10-19 C)(0.5 m2 V-1 s-1)] ∴ Nd = 1.248 × 1021 m-3 (>> ni) Now consider the atomic concentration. GaSb has the GaAs crystal structure so that the unit cell (cubic) has 4 Ga atoms and 4 Sb atoms. The cube side is a. The density ρ is therefore given by, 1 ( 4 MGa + 4 MSb ) ρ = 3 a NA which means that the Sb atom concentration is nSb = ρN A 4 = 3 a ( MGa + MSb ) or nSb = (5.4 × 10 3 kg m −3 )(6.022 × 10 23 mol −1 ) (10 −3 )(69.72 g mol −1 + 121.75 g mol −1 ) ∴ nSb = 1.698 × 1028 m-3 Thus, δ = N d/nSb = (1.248 × 1021 m-3) / (1.698 × 1028 m-3) = 7.35 × 10-8 5.11 Semiconductor Strain Gauge Suppose that an extrinsic semiconductor of conductivity σ = e Nd µe is strained in a certain direction by the application of uniaxial stress. Then there will be a change in the conductivity due to a change in the mobility as a result of the applied stress and the resulting strain ε , the effective mass me* in the direction of strain changes which affects the mobility (why?) in that direction. The effect is strongest in the [100] direction but almost negligible in the [111] direction. The Gauge factor G of a strain gauge is defined as the fractional change in the resistance per unit strain. Show that for an extrinsic ntype semiconductor, G is given by 5.18 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∆R 1 ∂µ G = R = − e µe ∂ε T ∆L L Chapter 5 Semiconductor strain gauge ∂µ where e is the mobility-strain coefficient that describes the effect of strain on the drift mobility ∂ε T at a constant temperature, due to a change in the effective mass with strain. Calculate the approximate ∂µ gauge factor for a particular n-type Si that has e = 10 5 cm 2 V −1 s −1 and compare this with a ∂ε T typical metal strain gauge factor of about 10 (see Question 2.14 in the textbook). Which would you recommend? Solution Differentiate R = l /σA, to get ∆R/R = –∆σ /σ. But σ = eNdµe where µe is strain dependent, i.e., µe = µe (e). Thus, ∂σ/∂ε = eNd (∂µe/∂ε) = (σ /µe) (∂µe /∂ε). Clearly an incremental strain δε will result in δσ = (∂σ /∂ε)δε = (σ /µe) (∂µe /∂ε)δε, so that G = (dR/R)/(dL/L) = – (δσ /σ)/(δε) = – (∂µe /∂ε)T /µe. From above, G = – (∂µe /∂ε)T /µe = (105 cm2 V–1 s–1)/(1350 cm2 V-1 s-1) ≈ 74, which is much higher than those values for metals. 5.12 Excess minority carrier concentration Consider an n-type semiconductor and weak injection conditions. Assume that the minority carrier recombination time, τh, is constant (independent of injection - hence the weak injection assumption). The rate of change of the instantaneous hole concentration, ∂pn/∂t, due to recombination is given by ∂pn p =− n ∂t τh Recombination rate [1] The net rate of increase (change) in pn is the sum of the total generation rate G and the rate of change due to recombination, that is, dpn p =G− n dt τh [2] By separating the generation term G into thermal generation Go and photogeneration Gph and considering the dark condition as one possible solution, show that d∆pn ∆p = Gph − n dt τh Excess carriers under uniform photogeneration and recombination How does your derivation compare with Equation 5.27 (in the textbook)? What are the assumptions inherent in Equation 3? 5.19 [3] Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 Solution We are given G as total generation rate or thermal generation (Go) + photogeneration (Gph), i.e. G = Go + Gph. Also, we know that ∆pn = pn - pno and therefore pn = ∆pn + pno. From Equation 2: dpn p =G− n dt τ dpn p Substitute for G: = Go + Gph − n (4) dt τ In the dark and in equilibrium we have no photogeneration (Gph = 0), an equilibrium concentration of holes (pn = pno) and no net increase in the hole concentration (dpn/dt = 0). Therefore: 0 = Go − ∴ pno τ pno τ Substitute this expression for Go into Eqn. (4). Go = dpn pno p = + Gph − n dt τ τ Substitute for pn in terms of excess hole concentration ∆pn. d ( ∆pn + pno ) pno ∆p + pno = + Gph − n dt τ τ d∆pn ∆p = Gph − n ∴ dt τ This is Eqn. 3 which is the same as Eqn. 5.27 (in the textbook). The derivation above is more rigorous whereas that in Section 5.4.2 (in the textbook) is intuitive. 5.13 Schottky junction a. Consider a Schottky junction diode between Au and n-Si, doped with 1016 donors cm–3. The crosssectional area is 1 mm2. Given the work function of Au as 5.1 eV, what is the theoretical barrier height, ΦB, from the metal to the semiconductor? b. Given that the experimental barrier height ΦB is about 0.8 eV, what is the reverse saturation current and the current when there is a forward bias of 0.3 V across the diode? (Use Equation 4.37 in the textbook.) Solution a The number of donors (Nd = 1022 m-3) is related to the energy difference from the Fermi level (∆E = Ec - EF) by: ∆E Nd = Nc exp − kT 5.20 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Chapter 5 N ∆E = − kT ln d Nc From Table 5.1 (in the textbook), Nc = 2.8 × 1025 m-3, and we are given and Nd = 1022 m-3. Assuming temperature T = 300 K: 10 22 m −3 1 ∆E = −(1.381 × 10 −23 J/K )(300 K ) ln 25 −3 −19 2.8 × 10 m 1.602 × 10 J/eV ∴ ∆E = 0.2053 eV The work function of n-Si (ΦnSi) must be less than that of Au (ΦAu = 5.1 eV) in order to have a Schottky junction. ΦnSi is given as ΦnSi = ∆E + χ, where χ = 4.01 eV (see Example 5.18 in the textbook) is the electron affinity of Si. Therefore: ΦnSi = 0.2053 eV + 4.01 eV = 4.215 eV This is indeed less than ΦAu and therefore we have a Schottky junction. The effective barrier height ΦB is then: Φ B = Φ Au - χ = 5.1 eV - 4.01 eV = 1.09 eV b The experimental potential barrier is given as ΦB = 0.8 eV. Assume temperature T = 300 K. The reverse saturation current (Io) is given by (see Example 5.18 in the textbook) (where Be = 1.2 × 106 A K-2 m-2 is the effective Richardson-Dushman constant): Φ Io = ABe T 2 exp − B kT ∴ ∴ Io = (10 −6 (0.8 eV)(1.602 × 10 −19 J/eV) m )(1.2 × 10 A K m )(300 K ) exp − −23 (1.381 × 10 J/K )(300 K ) 2 6 −2 −2 2 I o = 3.97 × 10-9 A The forward bias voltage Vf is given as 0.3 V. The forward current If is then: (1.602 × 10 −19 C)(0.3 V) eVf −9 I f = Io exp − 1 = (3.97 × 10 A )exp − 1 −23 (1.381 × 10 J/K )(300 K ) kT ∴ I f = 0.000433 A 5.14 Schottky and ohmic contacts Consider an n-type Si sample doped with 1016 donors cm–3. The length L is 100 µm; the crosssectional area A is 10 µm × 10 µm. The two ends of the sample are labeled as B and C. The electron affinity (χ) of Si is 4.01 eV and the work functions, Φ, of four potential metals for contacts at B and C are listed in Table 5Q14-1. a. Ideally, which metals will result in a Schottky contact? b. Ideally, which metals will result in an Ohmic contact? 5.21 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 c. Sketch the I-V characteristics when both B and C are ohmic contacts. What is the relationship between I and V? d. Sketch the I-V characteristics when B is ohmic and C is a Schottky junction. What is the relationship between I and V? e. Sketch the I-V characteristics when both B and C are Schottky contacts. What is the relationship between I and V? Table 5Q14-1 Work functions in eV Cs Li Al Au 1.8 2.5 4.25 5.1 Solution We are given the concentration of donors, Nd = 1022 m-3. From Table 5.1 (in the textbook), the electron affinity of Si is 4.01 eV and the effective density of states at the conduction edges is 2.8 × 1025 m-3. Assuming temperature T = 300 K, ∆E Nd = Nc exp − kT ∴ N ∆E = − kT ln d Nc ∴ 10 22 m −3 1 ∆E = −(1.381 × 10 −23 J/K )(300 K ) ln 25 −3 −19 2.8 × 10 m 1.602 × 10 J/eV ∴ ∆E = 0.2053 eV The energy required to move an electron from the Si semiconductor is then: ΦnSi = ∆E + χ = 0.2053 eV + 4.01 eV = 4.215 eV a For a Schottky contact you need Φm > ΦnSi so Au will result in a Schottky junction. Note however that for Al, Φm - ΦnSi is 0.04 eV, of the order of the thermal energy so Al / Si “should not” be a Schottky junction. This is not, however, necessarily the case as the junction depends very much on the surface conditions (surface states) as well as the crystal plane on to which the contact is made (Φ depends on the crystal surface). b For an Ohmic contact you need Φm < ΦnSi so Cs and Li will result in Ohmic contacts. c This is a straight line with slope equal to the conductance, or the inverse of the resistance (∆I /∆V = 1/R). The resistance can be found using the drift mobility of the electrons µe = 1350 × 10-4 m2 V-1 s-1 (from Table 5.1 in the textbook) and the equation for conductivity, σ = eNdµe: R= ∴ 100 × 10 −6 m L L = = 2 σA eNd µe A (1.602 × 10 −19 C)(10 22 m −3 )(0.135 m 2 V −1 s −1 )(10 −5 m ) R = 4620 Ω 5.22 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 Slope = Conductance = 1/R = A/L I B negative V B positive B C Ohmic Ohmic Figure 5Q14-1 I-V characteristic with B and C as Ohmic contacts. d When C is reverse biased (B is positive), current is limited by thermionic emission over ΦB between the metal and semiconductor at C. When C is forward biased (B negative), current is primarily limited by the resistance of the semiconductor, unless this is very small indeed in which case the I-V is the forward biased Schottky junction. I V B C Ohmic Schottky Figure 5Q14-2 I-V characteristic with B as Ohmic and C as Schottky. We can then find the maximum saturation current (Io) by first finding the barrier height, ΦB = ΦAu - χ = 5.1 eV - 4.01 eV = 1.09 eV, and then assuming the maximum effective Richardson-Dushman constant, Be = 1.2 × 106 A m-2 K-2. Using Equation 4.38 (in the textbook): Φ Io = ABe T 2 exp − B kT ∴ ∴ (1.09 eV)(1.602 × 10 −19 J/eV) 2 2 Io = (10 × 10 −6 m ) (1.2 × 10 6 A K −2 m −2 )(300 K ) exp − −23 (1.381 × 10 J/K )(300 K ) I o = 5.36 × 10-18 A This is the maximum saturation current as we assumed maximum Be. e There is reverse saturation current in both directions. The current is saturated for even small voltages (above a few kT/e) and is the thermionic emission current over ΦB. The calculation for reverse saturated current can be found as in part d. 5.23 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 I V B C Schottky Schottky Figure 5Q14-3 I-V characteristic with B and C as Schottky contacts. 5.15 Peltier effect and electrical contacts Consider the Schottky junction and the ohmic contact shown in Figures 5Q15-1 and 5Q15-2 between a metal and n-type semiconductor. a. Is the Peltier effect similar in both contacts? b. Is the sign in Q′ = ±ΠI the same for both contacts? c. Which junction would you choose for a thermoelectric cooler? Give reasons. Vo o Vacuum level Metal n-Type Semiconductor CB Metal n m Ec EFm EFm Depletion region Neutral semiconductor region W Ev VB m - n m - n CB BEFORE CONTACT EFm B Ec EFm Ev VB AFTER CONTACT Figure 5Q15-1 Formation of a Schottky junction between a metal and an n-type semiconductor when Φm > Φn. 5.24 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Accumulation Region Ohmic Contact Chapter 5 Bulk Semiconductor CB CB m n EFm Ec EFn EFm Ec EFn Ev Ev VB VB n-type Semiconductor Metal Metal BEFORE CONTACT n-type Semiconductor AFTER CONTACT Figure 5Q15-2 When a metal with a smaller work function than an n-type semiconductor is put into contact with the n-type semiconductor, the resulting junction is an ohmic contact in the sense that it does not limit the current flow. Solution a, b Peltier effect is based on change in the energy of an electron in passing through a junction of two dissimilar materials. It will appear whether the junction is ohmic or Schottky if there is a change in the electron energy in going through the junction. Vr V I Metal n-Type Semiconductor – m –eV n CB Ec – m B +eVr n CB Ec Ev 1 A V Ev VB VB (a) Forward biased Schottky junction. Electrons in the CB of the semiconductor can eadily overcome the small PE barrier to enter the metal. 1 mA (b) Reverse biased Schottky junction. Electrons in the metal can not easily overcome the PE barrier B to enter the semiconductor. Figure 5Q15-3 The Schottky junction. 5.25 0.2 V 10 A (c) I–V Characteristics of a Schottky junction exhibits rectifying properties (negative current axis is in microamps) Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) (a) (b) I I CB Q CB Q Ec EFn EFm Ec EFn EFm VB VB Ohmic Contact Region Metal Chapter 5 Ohmic Contact Region n-Type Semiconductor Metal n-Type Semiconductor Figure 5Q15-4 (a) Current from an n-type semiconductor to the metal results in heat absorption at the junction. (b) Current from the metal to an n-type semiconductor results in heat release at the junction. Figure 5Q15-4 (a) is an ohmic contact between a metal and an n-type SC (semiconductor). Current from SC to metal in Figure 5Q15-4 (a) absorbs heat at the junction because the electron at the metal EF has to absorb energy (heat) when entering the semiconductor Ec which is higher. In the Schottky junction in Figure 5Q15-3 (b) the current is from SC to metal and the electron from the metal to the semiconductor absorbs heat as in the ohmic case. The current from metal to SC in Figure 5Q15-4 (b) releases heat because the electron at the semiconductor Ec has to lose energy when entering the metal EF. In Figure 4Q15-3 (a) the current is from metal to SC and the electron from the SC releases heat at the junction because Ec has to lose energy in entering the metal EF. The sign in QP′ = ± ∏I is therefore the same. c One can only pass a small current though a semiconductor with Schottky junctions at both ends (M/SC/M where M = metal, SC = semiconductor) which is the reverse saturation current. This is because one Schottky junction is always reverse biased. If ohmic contacts are used then the contacts do not limit the current (which is determined by the bulk semiconductor) and Q′ can be substantial and can be used practically as in a thermoelectric cooler. Note that it is possible to have a device with one junction ohmic and the other Schottky if the current is not limited by the Schottky contact but rather limited by the bulk resistance. *5.16 Peltier coolers and figure of merit (FOM) Consider the thermoelectric effect shown in Figure 5Q16-1 in which a semiconductor has two contacts at its ends and is conducting an electric current I. We assume that the cold junction is at a temperature Tc and the hot junction is at T h and that there is a temperature difference of ∆T = T h – Tc between the two ends of the semiconductor. The current I flowing through the cold junction absorbs Peltier heat at a rate Q'P, given by QP′= ΠI [1] 5.26 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 where Π is the Peltier coefficient for the junction between the metal and semiconductor. The current I flowing through the semiconductor generates heat due to the Joule heating of the semiconductor. The rate of Joule heat generated through the bulk of the semiconductor is L QJ′ = I 2 σA [2] We assume that half of this heat flows to the cold junction. In addition there is heat flow from the hot to the cold junction through the semiconductor, given by the thermal conduction equation Aκ ∆T QTC ′ = L The net rate of heat absorption (cooling rate) at the cold junction is then Q′net cool = Q′P – 1/2Q′J – Q′TC [3] [4] By substituting from Equations 1 to 3 into Equation 4, obtain the net cooling rate in terms of the current I. Then by differentiating Q′net cool with respect to current, show that maximum cooling is obtained when the current is A Im = Πσ L [5] and the maximum cooling is Qmax ′ cool = A 1 2 Π σ − κ∆T L 2 [6] Under steady state operating conditions, the temperature difference, ∆T, reaches a steady-state value and the net cooling rate at the junction is then zero (∆T is constant). From Equation 6 show that the maximum temperature difference achievable is ∆Tmax = 1 Π 2σ 2 κ Maximum temperature difference [7] The quantity Π2σ/k is defined as the figure of merit (FOM) for the semiconductor as it determines the maximum ∆T achievable. The same expression also applies to metals, though we will not derive it here. Use Table 5Q16-1 to determine the FOM for various materials listed therein and discuss the significance of your calculations. Would you recommend a thermoelectric cooler based on a metal-tometal junction? Table 5Q16-1 Material FOM ∏ ρ κ –1 –1 V Ωm W m K –5 -2 n-Bi2Te3 10 1.70 6.0 × 10 –5 p-Bi2Te3 10 1.45 7.0 × 10-2 390 Cu 1.7 × 10–8 5.5 × 10–4 167 W 3.3 × 10-4 5.5 × 10–8 5.27 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Metal Metal I Chapter 5 Q Cold Junction I Q Cold Junction h+ eDC SUPPLY n-Type Semiconductor I Q Hot Junction p-Type Semiconductor I Q Hot Junction Metal Metal Figure 5Q16-1 When a dc current is passed through a semiconductor to which metal contacts have been made, one junction absorbs heat and cools (the cold junction) and the other releases heat and warms (the hot junction). Solution The net rate of heat absorption or cooling, Q′, is given by Equation 4: Q′net cool = Q′P – 1/2Q′J – Q′TC Substituting for Qp′, QJ′, and QTC′ as given by equations 1 to 3: LI 2 Aκ∆T − L 2σA Differentiating with respect to current: Qnet ′ cool = ΠI − dQnet LI ′ cool =Π− dI σA This rate is maximum at the maximum current (Im), when dQ′net cool/dI = 0. 0=Π− ∴ LIm σA A Im = Πσ L giving Equation 5. This expression can be then substituted back into the equation for Q′net cool to find the maximum cooling: Qmax ′ cool = ΠIm − LIm 2 Aκ∆T − L 2σA ∴ Qmax ′ cool = AσΠ 2 AσΠ 2 Aκ∆T AσΠ 2 Aκ∆T − − = − L L L 2L 2L ∴ Qmax ′ cool = A σΠ 2 − κ∆T L 2 which is the desired equation. In the steady state, the net cooling rate will be zero and the maximum temperature difference will be ∆Tmax: A σΠ 2 − κ∆Tmax = 0 L 2 5.28 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 σΠ 2 ∴ ∆Tmax = 2κ The figure of merit (FOM) of the materials listed can now be found. From Table 5Q16-1, for nBi2Te3, Π = 0.06 V, σ = 1/ρ = 1/(10-5 Ω m) = 105 Ω-1 m-1, and κ = 1.7 W m-1 K-1. Substituting: −1 −1 5 σΠ 2 (10 Ω m )(0.06 V) FOM = = = 212 K κ (1.7 W m −1 K −1 ) 2 The values for all materials are listed in Table 5Q16-2: Table 5Q16-2 Summarized values for FOM. Material FOM K n-Bi2Te3 212 p-Bi2Te3 338 Cu 0.0456 W 0.0119 Obviously metals have the worst FOM and semiconductors have the best FOM. Metal - metal junctions would obviously not make practical Peltier coolers! *5.17 Seebeck coefficient of semiconductors and thermal drift in semiconductor devices Consider an n-type semiconductor that has a temperature gradient across it. The right end is hot and the left end is cold, as depicted in Figure 5Q17-1. There are more energetic electrons in the hot region than in the cold region. Consequently, electron diffusion occurs from hot to cold regions, which immediately exposes negatively charged donors in the hot region and therefore builds up an internal field and a built-in voltage, as shown in the Figure 5Q17-1. Eventually an equilibrium is reached when the diffusion of electrons is balanced by their drift driven by the built-in field. The net current must be zero. The Seebeck coefficient (or thermoelectric power) S measures this effect in terms of the voltage developed as a result of an applied temperature gradient as dV [1] dT a. How is the Seebeck effect in a p-type semiconductor different than that for an n-type semiconductor when both are placed in the same temperature gradient in Figure 5Q17-1? Recall that the sign of the Seebeck coefficient is the polarity of the voltage at the cold end with respect to the hot end (see Section 4.8.2 in the textbook) b. Given that for an n-type semiconductor, S= Seebeck coefficient for k ( E − EF ) Sn = − 2 + c [2] n − type semiconductor e kT what are typical magnitudes for S n in Si doped with 1014 and 1016 donors cm–3? What is the significance of Sn at the semiconductor device level? c. Consider a pn junction Si device that has the p-side doped with 1018 acceptors cm–3 and the n-side doped 1014 donors cm–3. Suppose that this pn junction forms the input stage of an op amp with a large gain, say 100. What will be the output signal if a small thermal fluctuation gives rise to a 1 ˚C temperature difference across the pn junction? 5.29 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 Electron diffusion Electron drift COLD HOT T dV dx Exposed As+ Donor x Figure 5Q17-1 In the presence of a temperature gradient, there is an internal field and a voltage difference. The Seebeck coefficient is defined as dV/dT, the potential difference per unit temperature. Solution a In a p-type semiconductor (SC), we can assume that we only have holes as the mobile charge carriers. The acceptors are negatively charged. The same temperature gradient as in Figure 5Q17-1 results in the diffusion of holes (instead of electrons) from the hot to cold end. This exposes negative acceptors in the hot region (instead of positive charge as in the n-type SC). Thus in a p-type semiconductor, the Seebeck effect has the reverse sign, or the polarity of the Seebeck voltage is reversed with respect to that for an n-type SC for the same temperature gradient. This provides a convenient way to identify whether a semiconductor is doped n-type or p-type. If the hot side is positive with respect to the cold side then the semiconductor is n-type. If the hot side is negative then it is p-type. The simplest test is to touch the test leads of a voltmeter (1-10 mV range) to a semiconductor with one lead made hot. In reality, the semiconductor and the copper lead form a thermocouple but the Seebeck coefficient of the semiconductor is much greater than that of the metal lead. Note: The potential of the cold side with respect to the hot side is taken as the sign of the Seebeck voltage and hence the Seebeck coefficient S. Thus, if electrons diffuse from hot to cold, then the Seebeck voltage (and hence S) is negative, and if holes diffuse from hot to cold then the Seebeck voltage is positive. Equation 2 only gives the magnitude of S. b The following is a proof of Equation 2 and is given as reference. We consider a small distance, dx, at x where the temperature is T as shown Figure 5Q17-1. The temperature difference across dx is dT. Suppose that we take a single electron from a hot to a cold region across dx. Electrical work done is simply –edV. Suppose that we are measuring the energy of the electron from EF. Then the energy of the electron at x is (Ec - EF) + 3/2kT (or, PE + KE) so that when we take it across dT, its energy changes by d(Ec - EF) + 3/2kdT We should remember that as the temperature increases so does (Ec - EF). The above energy change must come from the electrical work done (–edV) so that –edV = d(Ec - EF) + 3/2kdT We can divide through by dT and then use the definition of Sn in Eqn. 1 to obtain 1 3k d ( Ec − EF ) Sn = ± + e 2 dT From ( E − EF ) n = Nc exp − c kT we have (Ec - EF) = kTln(Nc/n) 5.30 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 5 The temperature dependence of the term in parenthesis is slower than the pre-exponential term kT so that we can differentiate this quite simply, to obtain, d ( E c − EF ) ( Ec − EF ) = dT T Substituting in the expression for Sn we get, 1 3k E − EF Sn = ± + c e 2 T A more rigorous analysis makes the numerical factor 2 instead of 3/2 so that k E − EF Sn = ± 2 + c (Seebeck Coefficient) [2] e kT Sn in Equation 2 depends on (Ec–EF) which depends on the doping concentration as shown in the table below. Ec - EF = ∆E can be found as follows (sample calculation is for Nd = 1014 cm-3): ∆E Nd = Nc exp − kT ∴ N ∆E = − kT ln d Nc ∴ 10 20 m −3 1 ∆E = −(1.381 × 10 −23 J/K )(300 K ) ln 25 −3 −19 2.8 × 10 m 1.602 × 10 J/eV ∴ ∆E = 0.3244 eV Substitute to find Sn: (0.3244 eV)(1.602 × 10 −19 J/eV) 1.381 × 10 −23 J/K Sn = ± 2 + 1.602 × 10 −19 C (1.381 × 10 −23 J/K)(300 K) S n = -1.25 mV K- 1 Table 5Q17-1 Summarized values for Sn. Nd (cm-3) (Ec - EF) (eV) S n (mV K-1) 1 × 1014 0.3244 –1.25 1 × 1016 0.2053 –0.857 1 × 1018 0.08618 –0.460 c In a typical semiconductor device such as an IC there will be many junctions between differently doped materials. A small local temperature fluctuation can give rise to a temperature induced signal of the order of few millivolts. In a dc op amp of large gain this will appear as a spurious signal in the output. One design cure is to use two different input devices at similar temperatures and use the output signal difference between them. From Table 5Q17-1 above, the p-side with Na = 1018 cm-3 has Sp = +0.46 mV/K; and n-side with Nd = 1014 cm-3 has Sn = -1.25 mV/K. For ∆T = 1 °C = 1 K, the magnitudes of the individual Seebeck voltages are |Vp| = Sp ∆T = (0.46 mV/K)(1 K) = 0.46 mV |Vn| = Sn ∆T = (1.25 mV/K)(1 K) = 1.25 mV 5.31 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Cold Cold ∆T p Vp p Vp Vpn = 0.46+1.25 = 1.71 mV Hot ∆T Chapter 5 ∆T Vpn = 1.25–0.46 = 0.79 mV n Vn n Vn Hot Cold Case I Case II Figure 5Q17-2 There are two possibilities for the temperature variation ∆T. In Case I, ∆T occurs between the junction and the electrodes. A practical example would be the thermal lag between the electrodes and the junction. If the electroded ends were to be cooled, the junction temperature will lag behind by ∆T due to the finite thermal conductivity of the semiconductor. Case II involves a temperature fluctuation across the whole pn junction. Thus Case I Vpn = |Vp| + |Vn| = 0.46 + 1.25 = 1.71 mV Case II Vpn = |Vp| + |Vn| = 0.46 – 1.25 = 0.79 mV The output drift in the worst case (Case I) from the op amp is 100 × 1.71 mV = 171 mV or 0.171 V (substantial drift). The output drift in the best case (Case II) from the op amp is 100 × 0.79 mV = 79 mV or 0.079 V. Before I came here I was confused about this subject. Having listened to your lecture I am still confused. But on a higher level. Enrico Fermi (1901-1954; Nobel Laureate, 1938) 5.32 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 Second Edition ( 2001 McGraw-Hill) Chapter 6: Solutions If current trends endure, future computers will consist of a single chip. No one will have the foggiest idea what is on it. Somewhere in the basement of Intel or its successor will be a huge computer file with chip's listing. The last electrical engineer will sit nearby, handcuffed to the disk drive in a scene out of Ben Hur. That engineer will be extremely well paid, and his or her every demand will be immediately satisfied. That engineer will be last keeper of the secret of the universe: E = IR. Robert Lucky (Spectrum, IEEE, May 1998 Issue, pg. 21) 6.1 The pn junction Consider an abrupt Si pn + junction that has 1015 acceptors cm-3 on the p-side and 1019 donors on the nside. The minority carrier recombination times are τe = 490 ns for electrons in the p-side and τh = 2.5 ns for holes in the n -side. The cross-sectional area is 1 mm2. Assuming a long diode, calculate the current, I, through the diode at room temperature when the voltage, V, across it is 0.6 V. What are V/I and the incremental resistance (rd) of the diode and why are they different? Solution Consider temperature, T = 300 K. Then kT/e = 0.02586 V. Drift Mobility(cm2 V-1s-1) 2000 1000 Holes Electrons 100 50 1015 1016 1017 1018 1019 1020 Dopant Concentration, cm-3 Figure 6Q1-1 The variation of the drift mobility with dopant concentration in Si for electrons and holes at 300 K. The general expression for the diffusion length is L = √[Dτ] where D is the diffusion coefficient and τ is the carrier lifetime. D is related to the mobility of the carrier, µ, via the Einstein relationship, D/µ = kT/e. We therefore need to know µ to calculate D and hence L. Electrons diffuse in the p-region and holes in the n-region so that we need µe in the presence of Na acceptors and µh in the presence of Nd donors. From the drift mobility, µ vs. dopant concentration graph for silicon (see Figure 6Q1-1) we have the following: with Na = 1015 cm-3, µe ≈ 1350 cm2 V-1 s-1, and with Nd = 1019 cm-3,and µh ≈ 65 cm2 V-1 s-1. Thus, De = kTµe/e ≈ (0.02586 V)(1350 cm2 V-1 s-1) = 34.91 cm2 s-1 6.1 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) and Chapter 6 Dh = kTµh/e ≈ (0.02586 V)(65 cm2 V-1 s-1) = 1.681 cm2 s-1 The diffusion lengths are Le = √[Deτe] = √[(34.91 cm2 s-1)(490 × 10-9 s)] = 4.136 × 10-3 cm or 41.36 µm and Lh = √[Dhτh] = √[(1.681 cm2 s-1)(2.5 × 10-9 s)] = 6.483 × 10-5 cm or 0.6483 µm To calculate the forward current when V = 0.6 V we need to evaluate both the diffusion and recombination components of the current. It is likely that the diffusion component will exceed the recombination component at this forward bias (this can be easily verified). Assuming that the forward current is due to minority carrier diffusion in neutral regions, I = Iso[exp(eV/kT) – 1] ≈ Isoexp(eV/kT) for V >> kT/e (≈ 0.02586 V) where Iso = AJso = Aeni2[(Dh/(LhNd)) + (De/(LeNa))] ≈ Aeni2De/(LeNd) as Nd >> Na. In other words, the current is mainly due to the diffusion of electrons in the p-region. Thus (ni = 1.45 × 1010 cm-3 from Table 5.1 in the textbook): Isoe ∴ (1 × 10 = −2 cm 2 )(1.602 × 10 −19 C)(1.45 × 1010 cm −3 ) (34.91 cm 2 s −1 ) 2 (4.14 × 10 −3 cm )(1015 cm −3 ) Isoe = 2.840 × 10-12 A or 2.840 pA (1 × 10 = −2 cm 2 )(1.602 × 10 −19 C)(1.45 × 1010 cm −3 ) (1.681 cm 2 s −1 ) 2 and Isoh ∴ Isoh = 8.738 × 10-16 A or 0.0008738 pA (6.48 × 10 −5 cm )(1019 cm −3 ) Clearly, as expected, Isoe >> Isoh and therefore Iso = Isoe. The forward current is then I = Isoexp[eV/(kT)] = (2.840 × 10-12 A)exp[(0.6 V)/(0.02586 V)] ∴ I ≈ 0.0339 A or 33.9 mA ∴ R = V/I = (0.6 V) / (0.0339 A) ≈ 17.7 Ω Author’s Note: This is not a true resistance but V/I for a nonlinear device. Current 1 = dI rd dV Tangent I+dI dI I dV Voltage 0 0.5 V V+dV Figure 6Q1-2 The dynamic resistance of the diode is defined as dV/dI, which is the inverse tangent at I. 6.2 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 The incremental diode resistance is (Equation. 6.27 in the textbook): rd = dV/dI = (kT/e)/I = (0.02586 V) / (0.0339 A) ≈ 0.763 Ω See Figure 6Q1-2, for the meaning for rd (rd = δV/δI) which is very different than V/I. *6.2 The Si pn junction Consider a long pn junction diode with an acceptor doping, N a , of 1018 cm-3 on the p-side and donor concentration of N d on the n-side. The diode is forward biased and has a voltage of 0.6 V across it. The diode cross-sectional area is 1 mm2. The minority carrier recombination time, τ, depends on the dopant concentration, Ndopant (cm-3), through the following approximate relation τ= 5 × 10 −7 1 + 2 × 10 −17 Ndopant ( ) a. Suppose that Nd = 1015 cm-3. Then the depletion layer extends essentially into the n-side and we have to consider minority carrier recombination time, τ h, in this region. Calculate the diffusion and recombination contributions to the total diode current. What is your conclusion? b. Suppose that N d = N a = 1018 cm-3. Then W extends equally to both sides and, further, τe = τ h . Calculate the diffusion and recombination contributions to the diode current. What is your conclusion? Solution Consider temperature, T = 300 K. kT/e = kT/e = 0.02586 V. Drift Mobility(cm2 V-1s-1) 2000 1000 Holes Electrons 100 50 1015 1016 1017 1018 Dopant Concentration, 1019 1020 cm-3 Figure 6Q2-1 The variation of the drift mobility with dopant concentration in Si for electrons and holes at 300 K. a This is a p+n diode: Nd = 1015 cm-3. Hole lifetime τh in the n-side is 5 × 10 −7 5 × 10 −7 τh = = = 490.2 ns 1 + 2 × 10 −17 Ndopant (1 + 2 × 10 −17 × 1015 cm −3 ) ( ) and, using the same equation with Ndopant = 1018 cm-3, electron lifetime in the p-side is τe = 23.81 ns. I. Diffusion component of diode current 6.3 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 From Figure 6Q2-1, with Na = 1018 cm-3, µe ≈ 250 cm2 V-1 s-1, and with Nd = 1015 cm-3, µh ≈ 450 cm2 V-1 s-1. Thus: De = kTµe/e ≈ (0.02586 V)(250 cm2 V-1 s-1) = 6.465 cm2 s-1 and Dh = kTµh/e ≈ (0.02586 V)(450 cm2 V-1 s-1) = 11.64 cm2 s-1 The diffusion lengths are then: Le = √[Deτe] = √[6.465 cm2 s-1)(23.81 × 10-9 s)] = 3.923 × 10-4 cm, or 3.923 µm and Lh = √[Dhτh] = √[(11.64 cm2 s-1)(490.2 × 10-9 s)] = 2.389 × 10-3 cm, or 23.89 µm The diffusion component of the current is I = Idiff = Iso[exp(eV/(kT)) – 1] ≈ Isoexp(eV/(kT)) for V >> kT/e (≈ 0.02586 V) where Iso = AJso = Aeni2[(Dh/(LhNd)) + (De/(LeNa))] ≈ Aeni2Dh/(LhNd) as Na >> Nd. In other words, the current is mainly due to the diffusion of holes in the n-region. Thus: Iso ∴ (1 × 10 = −2 cm 2 )(1.602 × 10 −19 C)(1.45 × 1010 cm −3 ) (11.64 cm 2 s −1 ) 2 (2.389 × 10 −3 cm )(1 × 1015 cm −3 ) Iso = 1.641 × 10-12 A or 1.641 pA The forward current due to diffusion is Idiff = Isoexp[(eV/(kT)] = (1.641 × 10-12 A)exp[(0.6 V)/(0.02586 V)] ∴ I diff = 0.0196 A or 19.6 mA II. Recombination component The built-in potential is (where ni is the intrinsic concentration, found in Table 5.1 in the textbook): Vo = (kT/e)ln(NdNa/ni2) = (0.02586 V)ln[(1018 cm-3 × 1015 cm-3)/(1.45 × 1010 cm-3)2] ∴ Vo = 0.7549 V The depletion region width W is mainly on the n-side (εr of Si is 11.9 from Table 5.1 in the textbook). 2ε ( Na + Nd )(Vo − V ) W= eNa Nd 1/ 2 2ε (Vo − V ) ≈ eNd 1/ 2 ∴ 2(11.9)(8.854 × 10 −12 F m −1 )(0.7549 V − 0.6 V) W= (1.602 × 10 −19 C)(1021 m −3 ) i.e. W ≈ 0.4514 × 10-6 m or 0.4514 µm 1/ 2 The recombination time τr here is not necessarily τh but let us assume that it is roughly. Then τr = τh which leads to: −2 2 −19 −4 10 −3 AeWni (1 × 10 cm )(1.602 × 10 C)(0.4514 × 10 cm )(1.45 × 10 cm ) Iro = = 2τ r 2( 490 × 10 −9 s) ∴ Iro = 1.070 × 10-9 A 6.4 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 The forward current due to recombination is Irecom = Iroexp(eV/2kT) = (1.070 × 10-9 A)exp[(0.6 V)/(2 × 0.02586 V)] I recom = 1.17 × 10-4 A or 0.117 mA ∴ Clearly, the diffusion component dominates the recombination component. This is a symmetrical pn diode: Nd = Na = 1018 cm-3. Hole lifetime τh in the n-side is b τh = τe = 5 × 10 −7 5 × 10 −7 = = 23.81 × 10-9 s 1 + 2 × 10 −17 Ndopant (1 + 2 × 10 −17 × 1018 cm −3 ) ( ) I. Diffusion component From Figure 6Q2-1, with Na = 1018 cm-3, µe ≈ 250 cm2 V-1 s-1, and with Nd = 1018 cm-3, µh ≈ 130 cm V s-1. Thus: 2 -1 Thus De = kTµe/e ≈ (0.02586 V)(250 cm2 V-1 s-1) = 6.465 cm2 s-1 and Dh = kTµh/e ≈ (0.02586 V)(130 cm2 V-1 s-1) = 3.362 cm2 s-1 Diffusion lengths are Le = √[Deτe] = √[6.465 cm2 s-1)(23.81 × 10-9 s)] = 3.923 × 10-4 cm or 3.923 µm and Lh = √[Dhτh] = √[(3.362 cm2 s-1)(23.81 × 10-9 s)] = 2.829 × 10-4 cm or 2.829 µm The diffusion component of the current is Idiff = I = Iso[exp(eV/kT) – 1] ≈ Isoexp(eV/kT) for V >> kT/e (≈ 0.02586 V) where Iso = AJso = Aeni2[(Dh/(LhNd)) + (De/(LeNa))] Iso = (1 × 10 −2 cm 2 )(1.602 × 10 −19 C)(1.45 × 1010 cm −3 ) 2 3.362 cm 2 s −1 ) 6.465 cm 2 s −1 ) ( ( × + −4 18 −3 −4 18 −3 (2.829 × 10 cm )(10 cm ) (3.923 × 10 cm )(10 cm ) ∴ Iso = 9.554 × 10-15 A The forward current due to diffusion is Idiff = Isoexp(eV/kT) = (9.554 × 10-15 A)exp(0.6 V/0.02586 V) ∴ I diff = 1.14 × 10-4 A or 0.114 mA II. Recombination component The built-in potential is Vo = (kT/e)ln(NdNa/ni2) = (0.02586 V)ln[(1018 cm-3 × 1018 cm-3)/(1.45 × 1010 cm-3)2] ∴ Vo = 0.9335 V The depletion region width is symmetrical about the junction as Na = Nd. 2ε ( Na + Nd )(Vo − V ) W= eNa Nd 1/ 2 6.5 4ε (Vo − V ) = eNd 1/ 2 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ 4(11.9)(8.854 × 10 −12 F m −1 )(0.9335 V − 0.6 V) W= (1.602 × 10 −19 C)(1024 cm −3 ) i.e. W ≈ 2.962 × 10-8 m or 0.02962 µm Chapter 6 1/ 2 The recombination time τr here is not necessarily τh but let us assume that it is roughly. Then τr = τh = τe which leads to: −2 2 −19 −4 10 −3 AeWni (1 × 10 cm )(1.602 × 10 C)(0.02962 × 10 cm )(1.45 × 10 cm ) Iro = = 2τ r 2(23.81 × 10 −9 s) ∴ Iro = 1.457 × 10-9 A The forward current due to recombination is Irecom = Iroexp(eV/2kT) = (1.457 × 10-9 A)exp[(0.6 V)/(2 × 0.02586 V)] I recom = 1.59 × 10-4 A or 0.159 mA ∴ Clearly, the recombination component dominates the diffusion component. 6.3 Junction capacitance of a pn junction The capacitance (C) of a reverse-biased abrupt Si p+n junction has been measured as a function of the reverse bias voltage, Vr, as listed in Table 6Q3-1. The pn junction cross-sectional area is 500 µm × 500 µm. By plotting 1/C2 versus Vr, obtain the built-in potential, Vo, and the donor concentration, Nd, in the n-region. What is Na? Table 6Q3-1 Capacitance at various values of reverse bias (Vr) V r (V) 1 2 3 5 10 15 20 C (pF) 38.3 30.7 26.4 21.3 15.6 12.9 11.3 Solution The depletion capacitance of an abrupt pn junction is given by Equation. 6.24 (in the textbook), eε ( Na Nd ) Cdep 2( Na + Nd ) Taking reverse bias, V = -Vr and Na >> Nd, we get 1/ 2 εA A = = 1/ 2 W (Vo − V ) A eεNd 1/ 2 (Vo + Vr ) 2 1/ 2 Cdep = Rearranging we get: 2 2 1 = V + Vo 2 2 r C dep eεNd A eεNd A2 which is like: y = mx + c 2 where y = 1/Cdep , x = Vr, m is the slope and c is the intercept on the y-axis. Thus a plot of y = 1/Cdep2 vs. Vr should be a straight line y = mx + c, with an intercept Vr = -Vo. The data is plotted below. The best straight line gives a slope of m = 3.776 × 1020 and a y-intercept of b = 6.6 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 3.119 × 1020, or 1/C 2 = (3.776 × 1020)V r + 3.119 × 1020. This gives an x-intercept -V o so that V o = 0.826 V. 2 Slope = m = = 3.776 × 10 20 F -2 V -1 2 e N A ε d Thus: Substituting ε = εoer, εr = 11.9 (Table 5.1, pg. 334 in the textbook), e = 1.602 × 10-19 C, A = (500 × 10-6 m)2, Nd = i.e. ∴ Nd = 2 meεA2 (3.776 × 10 20 -2 F V -1 )(1.602 × 10 2 −19 C)(11.9)(8.854 × 10 −12 F/m )(500 × 10 −6 m ) 4 N d = 5.02 × 1021 m-3 or 5.02 × 1015 cm- 3 we can calculate: The built-in potential is, Vo = (kT/e)ln(NdNa/ni2) thus, (1.45 × 1010 cm −3 ) exp 0.826 V ≈ 3.12 × 1018 cm- 3 n2 eV Na = i exp o = 0.02586 V kT (5.02 × 1015 cm −3 ) Nd 1/C2 (F-2) 2 9.00E+21 8.00E+21 7.00E+21 6.00E+21 5.00E+21 4.00E+21 3.00E+21 2.00E+21 1.00E+21 0 0 5 10 15 20 25 Vr (V) Figure 6Q3-1 Plot of 1/C2 versus reverse voltage. 6.4 Temperature dependence of diode properties a. Consider the reverse current in a pn junction. Show that δIrev Eg δT ≈ Irev ηkT T where η = 2 for Si and GaAs, in which thermal generation in the depletion layer dominates the reverse current, and η = 1 for Ge, in which the reverse current is due to minority carrier diffusion to the depletion layer. It is assumed that Eg >> kT at room temperature. Order the semiconductors Ge, Si, and GaAs according to the sensitivity of the reverse current to temperature. b. Consider a forward-biased pn junction carrying a constant current I. Show that the change in the voltage across the pn junction per unit change in the temperature is given by 6.7 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 V −V dV =− g dT T where Vg = Eg/e is the energy gap expressed in volts. Calculate typical values for dV/dT for Ge, Si and GaAs assuming that, typically, V = 0.2 V for Ge, 0.6 V for Si, and 0.9 V for GaAs. What is your conclusion? Can one assume that, typically, dV/dT ≈ –2 mV °C-1 for these diodes? Solution a The total reverse current is the sum of the diffusion and thermal generation components: eDh eDe 2 eAWni Irev = A + ni + τg Lh Nd Le Na (1) Neglecting the temperature dependences of parameters other than ni, the reverse current is, Irev = B1ni2 + B2 ni where B1 and B2 are constants. The intrinsic concentration is given by ni = ( Nc Nv ) 1/ 2 E exp − g 2 kT (2) Thus the reverse current can be written as: E E Irev = C1 exp − g + C2 exp − g kT 2 kT where C1 and C2 are constants. The two exponential terms in the right hand side can be combined into one convenient expression that can be written as E Irev = C exp − g ηkT (3) where η may be 1 or 2 depending on whether the diffusion or the thermal generation contribution respectively dominates the reverse current. Taking the derivative of the reverse current with respect to temperature E E E dIrev d C exp − g = g 2 C exp − g = ηkT ηkT ηkT dT dT i.e. dIrev Eg = Irev dT ηkT 2 so that δIrev Eg δT = Irev ηkT T (4) This is the fractional change in the current (δIrev/Irev) due to a fractional change in temperature (δT/T). Small changes have been represented by differentials i.e. dT ≈ δT etc. The ratio of Eg /η for Ge, Si and GaAs are 0.66, 0.55 and 0.71 respectively (Eg from Table 5.1, η = 1 for Ge and η = 2 for Si and GaAs). Therefore the fractional change in Irev for Ge and GaAs will be slightly higher than that for Si. If we consider the actual change of current with temperature, it will be a function of Irev as well as Eg and η. Since the reverse current for Ge is on the order of µA, for Si on the 6.8 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 order of nA and for GaAs on the order of pA, the sensitivity of Irev to temperature can be written in the same order: Ge, Si and GaAs. b Consider the forward bias current equation with V >> kT/e (Total diode current equation, page 428 of the textbook): eDh eDe 2 eV eAW n exp eV I = A + i ni exp + 2 kT kT 2τ r Lh Nd Le Na (5) Defining Vg = Eg/e, Equation (2) can be written as eV ni2 = ( Nc Nv ) exp − g kT ( e V − Vg I = I1 exp kT which for convenience can be written as then Eqn. (5) is: ( e V − Vg I = I1 exp ηkT ) + I ( e V − Vg exp 2 2 kT ) ) where η may be 1 or 2 depending on whether the diffusion or the recombination contribution to the forward current is more dominant respectively. Since the current is constant, the term in the exponential is constant as well. ( e V − Vg or ηkT ) = Constant (V − V ) = C = Constant g T Taking the derivative of the both side with respect to temperature: ( − V − Vg T 2 ) + 1 dV = 0 T dT V −V dV =− g (Constant current) (6) dT T Typical values of dV /dT can be calculated at 300 K using values for Eg = Vg/e from Table 5.1 (in the textbook): Rearranging: dV 0.66 V − 0.2 V =− = -0.00153 V/K or V/°C dT 300 K dV 1.10 V − 0.6 V For Si: =− = -0.00167 V/K or V/°C dT 300 K dV 1.42 V − 0.9 V For GaAs: =− = -0.00173 V/K or V/°C dT 300 K This shows that the rate of change of voltage with temperature is very similar for all three semiconductors. It should also be noted that the actual values are slightly higher than those calculated above. Therefore we can simply assume that it is typically of the order of -2 mV/°C. For Ge: 6.9 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 6.5 Avalanche breakdown Consider a Si p+n junction diode that is required to have an avalanche breakdown voltage of 25 V. Given the breakdown field Ebr in Figure 6Q5-1, what should be the donor doping concentration? Solution The reverse breakdown voltage requirement is Vbr = 25 V. From Table 5.1 (in the textbook), the relative permittivity of Si is εr = 11.9, and from Table 5.1 (in the textbook), the intrinsic concentration is ni = 1.45 × 1016 m-3. Figure 6Q5-1 below shows the relationship between the breakdown field Ebr and the doping Nd. Ebr and Vbr are related by (See Example 6.7 in the textbook): Vbr = ∴ Ebr = ε r ε oEbr 2 2eNd 2eNd Vbr ε rε o (1) 200 Ebr (V/ m) Avalanche Eqn. (1) 100 Nd (cm 3) 0 1014 1015 1017 1016 1018 Figure 6Q5-1 The breakdown field, Ebr, in the depletion layer for the onset of reverse breakdown vs. doping concentration, Nd, in the lightly doped region in a one-sided (p+n or pn+) abrupt pn junction. Three points from Eqn. (1) are also shown. The intersection is roughly at Nd = 6 × 1016 cm-3. We should plot Ebr vs. Nd from Eqn. (1) on the same graph and find the intersection point for operation. Given the small figure, this will not be terribly accurate. For four values of Nd calculated below, the plot is shown in Figure 6Q5-1 above. The intersection is roughly at Nd = 6 × 1016 cm-3. An acceptable alternative is to calculate the various Ebr by entering a series of Nd values into Eqn. (1) until by trial and error one finds a value of Ebr that matches Ebr in Figure 6Q5-1. Nd = 1020 m-3: Ebr = 2(1.602 × 10 −19 C)(10 20 m −3 )(25 V) (11.9)(8.854 × 10 −12 F/m ) = 2.757 × 106 V/m Other values for Ebr at different values for Nd can be calculated similarly. 6.10 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 Table 6Q5-1 Values of Ebr at different Nd. Nd (m-3) Ebr (V/m) 20 2.757 × 106 1022 2.757 × 107 5 × 1022 6.165 × 107 1023 8.719 × 107 10 Since the two graphs intersect at Nd = 6 × 1016 cm-3, this is the required donor doping for the allowed breakdown voltage. 6.6 Design of a pn + junction diode Design an abrupt Si pn+ junction that has a reverse breakdown voltage of 100 V and provides a current of 10 mA when the voltage across it is 0.6 V. Assume that, if N dopant is in cm-3, the minority carrier recombination time is given by τ= 5 × 10 −7 (1 + 2 × 10−17 Ndopant ) Mention any assumptions made. Solution For this design, we are required to have a reverse breakdown voltage of Vbr = 100 V, and at V = 0.6 V the current provided should be I = 10 mA. We will assume a room temperature of T = 300 K. The relative permittivity of Si is εr = 11.9 and the intrinsic concentration is ni = 1.45 × 1016 m-3 (Table 5.1 in the textbook). Drift Mobility(cm2 V-1s-1) 2000 1000 Holes Electrons 100 50 1015 1016 1017 1018 1019 1020 Dopant Concentration, cm-3 Figure 6Q6-1 The variation of the drift mobility with dopant concentration in Si for electrons and holes at 300 K. Figure 6Q6-2 shows the relation between Ebr and the doping Nd. Nd, the dopant concentration, is equal to the acceptor concentration because we have a pn+ junction and the depletion region is in the pside. 6.11 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 Ebr (V/ m) 200 100 Eqn. (1) Avalanche Tunneling Nd (cm 3) 0 1014 1015 1017 1016 1018 Figure 6Q6-2 The breakdown field, Ebr, in the depletion layer for the onset of reverse breakdown vs. doping concentration, Nd, in the lightly doped region in a one-sided (p+n or pn+) abrupt pn junction. Three points from Eqn. (1) are also shown. The intersection is roughly at Nd = 5 × 1015 cm-3. Ebr and Vbr are related by (see Example 6.7 in the textbook): Vbr = ∴ Ebr = ε r ε oEbr 2 2eNd 2eNd Vbr ε rε o (1) We should plot Ebr vs. Nd from Eqn. (1) on the same graph and find the intersection point for operation. Given the small figure, this will not be terribly accurate. For three values of Nd calculated below, the plot is shown in Figure 6Q6-2 above. The intersection is roughly at Nd = 5 × 1015 cm-3. An acceptable alternative is to calculate the various Ebr by entering a series of Nd values into Eqn. (1) until by trial and error one finds a value of Ebr that matches Ebr in Figure 6Q6-2. 20 -3 Nd = 10 m : Ebr = 2(1.602 × 10 −19 C)(10 20 m −3 )(100 V) (11.9)(8.854 × 10 −12 F/m) = 5.5147 × 106 V/m Other values for Ebr at different values for Nd can be calculated similarly. Table 6Q6-1 Values of Ebr at different Nd. Nd (m-3) 10 20 Ebr (V/m) 5.514 × 106 5 × 1021 3.899 × 107 1022 5.514 × 107 Since the two graphs intersect at Nd = 5 × 1015 cm-3 = 5 × 1021 m-3, this is the required dopant (acceptor) concentration for the allowed breakdown voltage. In a pn+ junction, electron diffusion in the p-side gives the diode current. We need the electron lifetime in the p-side. Using the given equation: 6.12 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) τ= 5 × 10 −7 5 × 10 −7 = (1 + 2 × 10 −17 Nd ) 1 + 2 × 10 −17 5 × 1015 cm −3 ( [ ]) Chapter 6 = 4.545 × 10-7 s At Nd (acceptor) = 5 × 1015 cm-3 doping, the electron drift mobility (Figure 6Q6-1) is almost the same as at room temperature. Therefore µe = 1350 × 10-4 m2 V-1 s-1. The diffusion coefficient of electrons (De) in the p-side is given by: −23 2 −1 −1 kTµe (1.381 × 10 J/K )(300 K )(0.135 m V s ) De = = = 0.003491 m2/s -19 e 1.602 × 10 C The diffusion length of the electrons is then: Le = Deτ = (0.003491 m /s)(4.545 × 10 s) = 3.983 × 10 −7 2 -5 m Note that Na = Nd (doping concentration, not “donor”). The forward current is (remember eV/(kT) >> 1) AeDe ni 2 eV I= exp kT Le Na Everything but A is given and therefore we can find the required area A: A= ∴ ∴ A= ILe Na eV eDe ni 2 exp kT (0.010 A)(3.983 × 10 −5 m)(5 × 10 21 m −3 ) (1.602 × 10 −19 C)(0.003491 m /s)(1.45 × 10 m 2 16 ) −3 2 (1.602 × 10 −19 C)(0.6 V) exp −23 (1.381 × 10 J/K )(300 K ) A = 1.42 × 10-6 m2 If we make a circular device with a radius r, the area is πr2. A = π r2 ∴ r= A π = 1.42 × 10 −6 m 2 π = 0.000672 m or 0.672 mm 6.7 Minority carrier profiles (the hyperbolic functions) Consider a pnp BJT under normal operating conditions in which the EB junction is forward biased and the BC junction is reverse biased. The field in the neutral base region outside the depletion layers can be assumed to be negligibly small. The continuity equation for holes, pn(x), in the n-type base region is d 2 pn pn − pno Dh − =0 dx 2 τh [1] where pn(x) is the hole concentration at x from just outside the depletion region and p no and τh are the equilibrium hole concentration and hole recombination lifetime in the base. a. What are the boundary conditions at x = 0 and x = W B , just outside the collector region depletion layer? (Consider the law of the junction). b. Show that the following expression for pn(x) is a solution of the continuity equation 6.13 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) eV pn ( x ) = pno exp kT x WB − x sinh sinh Lh Lh − 1 + pno 1 − WB WB sinh sinh Lh Lh Chapter 6 [2] where V = VEB and Lh = √[Dhτh]. c. Show that Equation 2 satisfies the boundary conditions. Solution The continuity equation for the holes in the n-type base region is Dh d 2 pn pn − pno − =0 dx 2 τh Eqn. 1 a The boundary conditions (hole concentrations) at x = 0 (in the base just outside the EB depletion region) and x = WB (in the base just outside the BC depletion region) are: At x = 0, eV pn (0) = pno exp kT Law of the junction pn(WB) = 0 At x = WB, b Substituting Eqn. 2 into Eqn. 1 will show that Eqn. 1 is satisfied. We, of course, have to differentiate sinh(x) and cosh(x) with respect to x: d[sinh(x)]/dx = cosh(x) and d[cosh(x)]/dx = sinh(x). W − x x sinh B sinh Lh Lh eV pn = pno exp − 1 + pno 1 − W WB kT sinh B sinh Lh Lh ∴ WB − x 1 x −1 cosh cosh Lh L Lh dpn eV L = pno exp − 1 h − pno h dx W W kT sinh B sinh B Lh Lh ∴ 1 WB − x 1 x 2 sinh 2 sinh L Lh Lh d pn eV L = pno exp − 1 h − pno h 2 dx W W kT sinh B sinh B Lh Lh 2 Substituting into the left hand side (LHS) of Eqn. 1 and using Lh2 = Dhτh we get, LHS = Dh d 2 pn pn − pno − dx 2 τh 6.14 Eqn. 2 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 1 x 1 WB − x sinh sinh 2 Lh L2h Lh Lh 2 eV Lh LHS = pno exp − 1 − pno τ h kT W W sinh B sinh B Lh Lh ∴ W − x x sinh B sinh Lh Lh eV − pno + pno 1 − pno exp − 1 WB WB kT sinh sinh Lh Lh − τh Simplifying, e.g. multiplying through by τh, and canceling terms we get, LHS = 0 which verifies Eqn. 1. Thus Eqn. 2 must be a solution of Eqn. 1. Mathematicians would prefer to solve the differential equation in Eqn. 1 directly and hence find Eqn. 2 which is a more rigorous approach. c Given sinh(0) = 0 then at x = 0, and at x = WB we have At x = 0, W sinh B Lh eV eV pn (0) = pno exp − 1 + pno [1 − 0] = pno exp kT W kT sinh B Lh WB 0 sinh sinh Lh Lh eV pn = pno exp − 1 At x = WB, = 0 + pno [1 − 1] = 0 + pno 1 − WB WB kT sinh sinh Lh Lh Both are the boundary conditions stated in a. *6.8 The pnp bipolar transistor Consider a pnp transistor in a common base configuration and under normal operating conditions. The emitter-base junction is forward biased and the base-collector junction is reverse biased. The emitter, base, and collector dopant concentrations are N a(E), N d(B), and N a(C) respectively where Na(E) >> Nd(B) ≥ N a(C). For simplicity, assume uniform doping in all the regions. The base and emitter widths are W B and W E ,respectively, both much shorter than the minority carrier diffusion lengths, L h and L e. The minority carrier lifetime in the base is the hole recombination time τ h. The minority carrier mobility in the base and emitter are denoted by µh and µe respectively. The minority carrier concentration profile in the base can be represented by Equation 6.68 (in the textbook). a. Assuming that the emitter injection efficiency is unity show that 1. IE ≈ eADh ni2 coth(WB Lh ) eV exp EB Lh Nd ( B ) kT 6.15 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) 2. IC ≈ Chapter 6 eADh ni2 cosech(WB Lh ) eV exp EB Lh Nd ( B ) kT W 3. α ≈ sech B Lh 4. β ≈ b. τh τt where τ t = WB2 is the base transit time. 2 Dh Consider the total emitter current, IE, through the EB junction, which has diffusion and recombination components as follows: eV eV IE = IE ( so ) exp EB + IE ( ro ) exp EB kT 2 kT Only the hole component of the diffusion current (first term) can contribute to the collector current. Show that when Na(E) >> Nd(B), the emitter injection efficiency, γ, is given by I eV γ ≈ 1 + E ( ro ) exp − EB 2 kT IE ( so ) −1 How does γ < 1 modify the expressions derived in part (a)? What is your conclusion (consider small and large emitter currents, or VEB = 0.4 and 0.7 V)? Solution a We are given the minority carrier concentration profile in the base: x W − x sinh B sinh Lh Lh eV pn = pno exp EB − 1 Eqn. 1 + pno 1 − WB WB kT sinh sinh Lh Lh Under normal operating conditions, EB is forward biased and eVEB/(kT) >> 1 and exp(eVEB/(kT)) >> 1. 1. The emitter current IE is the diffusion current at x = 0, i.e. in the base just outside the EB depletion region. −1 WB 1 cosh (1) Lh Lh dp eV L − pno IE = ±eADh n = ±eADh pno exp EB − 1 h kT WB WB dx x = 0 sinh sinh Lh Lh Using pno = ni2/Nd(B), eVEB/kT >> 1 (forward bias) and Lh >> WB (small recombination in the base) so that cosh(WB/Lh) ≈ 1, we get, W cosh B W Lh eV eADh ni2 eV eAD n IE ≈ exp EB = coth B exp EB kT Lh Nd ( B ) kT Lh Nd ( B ) W Lh sinh B Lh 2 h i 6.16 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) 2. Chapter 6 The collector current IC is determined by the concentration gradient at x = WB, dp IE = ±eADh n dx x = WB 1 WB −1 cosh cosh(0) L Lh eV L IE = ±eADh pno exp EB − 1 h − pno h kT WB WB sinh sinh Lh Lh ∴ Using pno = ni2/Nd(B), eV/kT >> 1 (forward bias) and Lh >> WB (small recombination in the base) so that cosh(WB/Lh) ≈ 1 and cosh(WB/Lh)/sinh(WB/Lh) ≈ 0 and we get, IC ≈ eADh ni2 Lh Nd ( B ) W eV eADh ni2 eV 1 exp EB = cosech B exp EB kT Lh Nd ( B ) kT W Lh sinh B Lh 3. The current gain, α = IC/IE. Substituting for IC and IE and simplifying by using sech(θ) = 1/cosh(θ) we obtain: α = cosech(WB/Lh) / coth(WB/Lh) = [1/sinh(WB/Lh)][sinh(WB/Lh)/cosh(WB/Lh)] α = 1/[cosh(WB/Lh)] = sech(WB/Lh) ∴ 4. Assume (WB/Lh) is small. For small θ, sech(θ) ≈ 1 – (1/2)θ2 (by Taylor expansion). Then: β ≈ 1/(1 – α) = 1/[1 – sech(WB/Lh)] ≈ 1/[(1/2)(WB/Lh)2] = 2Lh2/WB2 Given that Lh2 = Dhτh and WB2 = 2Dhτt, substituting we find, β ≈ τh/τt b If Na >> Nd then we can forget about the electron injection component of the emitter diffusion current (electron injection from the base into the emitter) and consider only the hole diffusion component. The emitter current has two components, diffusion (Shockley) and recombination as stated in total diode current equation on page 428 (in the texttbok): eV Diffusion component: IEs = IE ( so ) exp EB kT Recombination: eV IEr = IE ( ro ) exp EB 2 kT The emitter injection efficiency (γ) is given by: γ = IEs IEs + IEr γ is less than unity. Only IEs is useful in the transistor action. The equations in (a) assumed that γ = 1, i.e. full injection of holes via the diode action (law of the junction). When γ < 1, α and hence β are reduced. The new current gain is α′ = γα. Now consider γ: 6.17 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) γ = 1 = I 1 + Er IEs 1 eV IE ( ro ) exp EB 2 kT 1+ eV IE ( so ) exp EB kT = Chapter 6 1 eV IE ( ro ) exp − EB 2 kT 1+ IE ( so ) Typically, IE(ro) >> IE(so). Assume an order of magnitude for IE(so) = 10-13 A and IE(ro) = 10-11 A (100 times bigger than IE(so)) (see Example 6.4, pg. 432 in the textbook). Assume that temperature T = 300 K. We can then calculate γ at two VEB voltages to observe the effect of VEB. VEB = 0.4 V: γ = 1 eV IE ( ro ) exp − EB 2 kT 1+ IE ( so ) ∴ γ = 0.9580 VEB = 0.7 V: γ = 0.9999 = (10 1+ −11 1 1.602 × 10 −19 C)(0.4 V) ( A ) exp − −23 2(1.381 × 10 J/K )(300 K ) (10 −13 A) Clearly, γ is lower when VEB is lower. Injection is more efficient at higher VEB when the diffusion component of IE dominates over the recombination component. By the way, changing the value of IE(so) but keeping IE(ro) > IE(so) does not change the conclusion. 6.9 Characteristics of an npn Si BJT Consider an idealized silicon npn bipolar transistor with the properties listed in Table 6Q9-1. Assume uniform doping in each region. The emitter and base widths are between metallurgical junctions (not neutral regions). The cross-sectional area is 100 µm × 100 µm. The transistor is biased to operate in the normal active mode. The base-emitter forward bias voltage is 0.6 V and the reverse bias basecollector voltage is 18 V. Table 6Q9-1 Properties of an npn BJT Emitter Emitter Width Doping 10 µm 2 × 1018 cm-3 Hole Lifetime in Emitter 10 ns Base Width 4 µm Base Doping 1 × 1016 cm-3 Electron Lifetime in Base 400 ns Collector Doping 1 × 1016 cm-3 a. Calculate the depletion layer width extending from the collector into the base and also from the emitter into the base. What is the width of the neutral base region? b. Calculate α and hence β for this transistor, assuming unity emitter injection efficiency. How do α and β change with VCB? c. What is the emitter injection efficiency and what are α and β , taking into account that the emitter injection efficiency is not unity? d. What are the emitter, collector, and base currents? e. What is the collector current when VCB = 19 V but VEB = 0.6 V? What is the incremental collector output resistance defined as ∆VCB/∆IC? 6.18 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 Solution a Figure 6Q9-1 shows the principle of operation of the npn BJT and also defines various device characteristics such as the depletion widths and the base widths. With VBC >> Vo, the reverse bias across the base-collector junction Vr = VBC + Vo ≈ VBC. Thus, the depletion layer WBC at the base-collector junction is given by; BC ≈ W 2ε ( Na + Nd )VBC eNa Nd 1/ 2 1/2 i.e. WBC 2(8.854 × 10 -12 F m -1 )(11.9)(10 22 + 10 22 m -3 )(18 V) = (1.602 × 10 −19 C)(10 22 m -3 )(10 22 m -3 ) W B C = 2.18 × 10-6 m or 2.18 µ m WBCp WEB E WBCn B C WB WB IE IC Electron diffusion A n+ Holes Input Drift p IB VEB n A Output VCB An npn transistor operated in the normal mode, in the active region, in the common base (CB) configuration (Notation: W = width, = electric field). Figure 6Q9-1 Only a portion of WBC is in the base side. Suppose that WBCp and WBCn are the depletion widths in the base and collector sides of the SCL respectively. Since the total charge on the p and n-sides of the SCL must be the same, N aW BCp = N dW BCn and since W BC = W BCp + W BCn we can find WBCp, WBCp = Nd 1016 WBC = 16 (2.17 µm ) = 1.09 µm Na + Nd 10 + 1016 Since Nd(E) >> Na(B), the depletion layer width WEB is almost totally in the p-side (in the base). With forward bias, VEB = 0.6 V across the emitter-base junction, WEB is given by 1/ 2 2ε (Vo − VEB ) WEB = eNa We first need to calculate the built-in voltage Vo between the emitter and base, 6.19 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 (2 × 1018 )(1 × 1016 ) kT Na Nd Vo = ln = (0.02586 V)ln 10 2 e ni2 (1.5 × 10 ) i.e. Vo = 0.830 V Then, 2ε (Vo − VEB ) WEB = eNa i.e. 2(8.854 × 10 -12 F m -1 )(11.9)(0.830 ± 0.6 V) WEB = (1.602 × 10 −19 C)(10 22 m -3 ) or W E B = 1.74 × 10-7 m or 0.174 µ m 1/ 2 1/2 Notice that due to the forward bias across the EB junction, WEB is an order of magnitude smaller than W BCp. If WB is the base width between emitter and collector metallurgical junctions, then the width W′B of the neutral region in the base between the depletion regions is given by, W′B = WB – WBCp – WEB so that W ′B = 4 µm – 1.09 µm – 0.174 µm = 2.74 µm b The electron drift mobility µe in the base is determined by the dopant (acceptor) concentration here. For Na = 1 × 1016 cm-3, µe = 1250 cm2 V-1 s-1 and the diffusion coefficient De from the Einstein relationship is, De = kTµe/e = (0.02586 V)(1250 × 10-4 m2 V-1 s-1) = 3.23 × 10-3 m2 s-1 The electron diffusion length Le in the base is Le = (Deτe)1/2 = [(3.23 × 10-3 m2 s-1)(400 × 10-9 s)]1/2 i.e. Le = 36.0 × 10-6 m or 36.0 µm In order to calculate α, first we need to find the transit (diffusion) time τt through the base τt = WB′ 2 (2.74 × 10 −6 m )2 = = 1.161 × 10-9 s −3 2 −1 2 De 2(3.23 × 10 m s ) If we assume unity injection (γ = 1), then α = αB, base transport factor: α = αB = 1− i.e. Transit (diffusion) time across base τ =1− t Minority carrier recombination time in base τe α = 1 – (1.161 ns)/(400 ns) = 0.9971 The current gain β is β= α 0.9971 = = 343 1 − α 1 − 0.9971 c The hole drift mobility µh in the emitter is determined by the dopant (donor) concentration here. For Nd = 2 × 1018 cm-3, µh ≈ 100 cm2 V-1 s-1 and the diffusion coefficient Dh from the Einstein relationship is, Dh = kTµh/e = (0.02586 V)(100 × 10-4 m2 V-1 s-1) = 2.59 × 10-4 m2 s-1 6.20 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 The hole diffusion length Lh in the base is Lh = (Dhτh)1/2 = [(2.59 × 10-4 m2 s-1)(10 × 10-9 s)]1/2 Lh = 1.61 × 10-6 or 1.61 µm that Thus the hole diffusion length is much shorter than the emitter width. The emitter current is given by electron diffusion in the base and hole diffusion in the emitter so IE = IE(electron) + IE(hole) where for electrons diffusing in the base, eV IE (electron) = Isoe exp EB ; kT Isoe = eADe ni2 Na WB and holes diffusing in the emitter, eV IE (hole) = Isoh exp EB ; kT Isoh eADh ni2 Nd Lh where we used Lh instead of WE because WE >> Lh (emitter width is 10 µm and hole diffusion length is 1.62 µm). Substituting the values we find, Isoe = i.e. (1.602 × 10 −19 C)(1 × 10 −8 m 2 )(3.23 × 10 −3 m 2 s −1 )(1.5 × 1016 m −3 )2 (1 × 10 22 m −3 )(2.74 × 10 −6 m) Isoe = 4.267 × 10-14 A or 42.67 fA (1.602 × 10 −19 C)(1 × 10 −8 m 2 )(2.59 × 10 −4 m 2 s −1 )(1.5 × 1016 m −3 )2 = (2 × 10 24 m −3 )(1.61 × 10 −6 m) and Isoh i.e. Isoh = 2.93 × 10-17 A or 0.0293 fA The emitter injection efficiency is the fraction of the emitter current that is due to minority carriers injected into the base; those that diffuse across the base towards the collector. γ = i.e. IE (electron) Isoe = IE (electron) + IE (hole) Isoe + Isoh 4.267 × 10 −14 = 0.99931 4.267 × 10 −14 + 2.93 × 10 −17 The current gains, taking into account the emitter injection efficiency, are γ= α = γα B = (0.99931)(0.9971) = 0.99641 and d β = α/(1–α)= 0.99641/(1–0.99641) = 278 The emitter current with VEB = 0.6 V is IE = (Isoe + Isoh)exp(eVEB/kT) IE = (4.267 × 10-14 A + 2.93 × 10-17 A)exp(0.6 V/0.2586 V) I E = 5.13 × 10-4 A or 0.513 mA 6.21 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 The collector current is determined by those minority carriers in the base that reach the collector junction. Only γIE of IE is injected into the base as minority carriers and only a fraction αB make it to the collector, IC = αBγIE = αIE = (0.99641)(0.513 mA) = 0.511 mA The base current is given by, IB = IC/β = (0.511 mA)/278 = 1.83 × 10-3 mA or 1.83 µ A Table 6Q9-2 Characteristics of BJT with VCB = 18 and 19 V. VCB W ′B α β IE IC 18 V 2.74 µm 0.99641 278 0.513 mA 0.511 mA 19 V 2.71 µm 0.99649 283 0.517 mA 0.515 mA e Suppose that we increase VCB to 19 V and repeat all the calculations above. We then find results tabulated in Table 6Q9-2. We can calculate the small signal collector incremental resistance from, rc = 19 V − 18 V ∆VCB = 250 kΩ = 0.515 mA − 0.511 mA ∆IC We can also calculate the BJT characteristics using the hyperbolic expressions given in Question 6.8 in the textbook. The base transport factor αB is given by, W 2.74 µm α B ≈ sech B = sech = 0.99711 36.0 µm Le which is, within calculation errors, almost identical to the simplified theory. The emitter current is given by IE (electron) = Isoe exp( Isoe = i.e. W eVEB eADe ni2 ); Isoe = coth B kT Na Le Le (1.602 × 10 −19 C)(1 × 10 −8 m 2 )(3.23 × 10 −3 m 2 s −1 )(1.5 × 1016 m −3 )2 2.74 µm (36 × 10 −6 m)(1 × 10 22 m −3 ) tanh 36 µm Isoe = 4.260 × 10-14 A or 42.60 fA which, for all practical purposes, is identical to the simplified theory. NOTE: We have ignored any bandgap narrowing in the emitter due to heavy doping in this region. 6.10 The JFET Pinch-off Voltage Consider the symmetric n-channel JFET shown in Figure 6Q10-1. The width of each depletion region extending into the n-channel is W . The thickness, or depth, of the channel, defined between the two metallurgical junctions, is 2a. Assuming an abrupt pn junction and VDS = 0, show that when the gate to source voltage is –VP the channel is pinched off where 6.22 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 a 2 eNd − Vo 2ε where Vo is the built-in potential between p+n junction and Nd is the donor concentration of the channel. Calculate the pinch-off voltage of a JFET that has an acceptor concentration of 1019 cm-3 in the p+ gate, a channel donor doping of 1016 cm-3, and a channel thickness (depth), 2a, of 2µm. VP = Gate W Source Depletion region p+ Drain a n-channel Channel thickness p+ Figure 6Q10-1 A symmetric JFET. Solution Assume we have a Si JFET. The depletion region of a p+n junction extends essentially into n-side. W depends on the reverse bias Vr = –VGS between the p+ and n-channel, 2ε (Vo − VGS ) W= eNd As VGS is negative Vo–VGS is greater than Vo and W extends more into the channel than the VGS = 0 case. As VGS is made more negative, W extends more into the channel. Typically the magnitude of VGS is greater than the built-in potential Vo. The channel is totally pinched off when W extends halfway into the channel thickness, when W = a. Then 12 a2 = 2ε (Vo − VGS ) eNd Rearranging we obtain, a 2 eNd VGS = Vo − 2ε With VDS = 0, the channel is pinched off when, by definition, VGS = –VP. Thus a 2 eNd − Vo 2ε The built-in potential is VP = Vo = (kT/e)ln(NdNa/ni2) = (0.02586 V)ln[(1019 cm-3)(1016 cm-3)/(1.45 × 1010 cm-3)2] Vo = 0.8740 V The pinch-off voltage is, VP (1 × 10 = −6 m ) (1.602 × 10 −19 C)(10 22 m -3 ) 2 2(11.9)(8.854 × 10 −12 F m -1 ) − 0.8740 = 6.73 V It is also apparent that VP is highly sensitive to the channel doping. We would expect device to device variation between different batches of fabrication. 6.23 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 6.11 The JFET Consider an n-channel JFET that has a symmetric p+n gate-channel structure as shown in Figures 6Q111a and 6Q11-2. Let L be the gate length, Z the gate width, and 2a the channel thickness. The pinch-off voltage is given by Question 6.10. The drain saturation current, IDSS, is the drain current when VGS = 0. This occurs when VDS = VDS(sat) = VP (Figure 6Q11-3) so IDSS = VPGch, where Gch is the conductance of the channel between the source and the pinched-off point (Figure 6Q11-4). Taking into account the shape of the channel at pinch-off, if Gch is about 1/3 of the conductance of the free or unmodulated (rectangular) channel, show that 1 (eµe Nd )(2 a) Z I DSS = VP L 3 A particular n-channel JFET with a symmetric p+n gate-channel structure has a pinch-off voltage of 3.9 V and an IDSS of 5.5 mA. If the gate and channel dopant concentrations are Na = 1019 cm-3 and Nd = 1015 cm-3 respectively, find the channel thickness 2a and Z/L. If L = 10 µm, what is Z? What is the gate-source capacitance when the JFET has no voltage supplies connected to it? Gate G p+ Basic structure Source Drain S n n-channel D S G p+ (a) Depletion region G p+ Cross section Depletion regions Metal electrode Insulation (SiO2) n p n-channel n n S Channel thickness D p+ D n-channel (b) p+ G Circuit symbol for n-channel FET S D Figure 6Q11-1 (a) The basic structure of the junction field transistor (JFET) with an n-channel. The two p+ regions are electrically connected and form the gate. (b) A simplified sketch of the cross section of a more practical n-channel JFET. Gate W Source Depletion region p+ Drain a n-channel Channel thickness p+ Figure 6Q11-2 A symmetric JFET. 6.24 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ID (mA) VDS(sat) = VP IDSS 10 Chapter 6 VGS = 0 5 IDS VGS = –2 V VGS = –4 V VGS = –5 V VDS(sat) = VP+VGS 0 0 4 VDS 8 12 Figure 6Q11-3 Typical ID versus VDS characteristics of a JFET for various fixed gate voltages VGS. G Pinched off channel ID = 10 mA S P A Lch D po VDS > 5 V Figure 6Q11-4 The pinched-off channel and conduction for VDS > VP (= 5 V) Solution The conductivity of the channel is: σ = eNdµe The channel width is Z and the depth is 2a. Therefore the area A is A = 2aZ. The conductance of the channel is given as 1/3 of the conductance of the free channel, therefore: 1 σA 3 L 2 eNd µe aZ Gch = Substitute: 3 L The voltage across the channel is the pinch-off voltage VP (see Figure 6Q11-5). At pinch-off the drain current is IDSS, given as: IDSS = VPG ch Gch = ∴ 2 eNd µe aZ I DSS = VP 3 L (1) 6.25 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 G Vch ID = 10 mA VDS S 0 (a) A B VGS = 0 G p+ n S n (b) VDS = VP = 5 V ID = 6 mA G D B A D x ID = 10.1 mA Depletion region S n-channel D A P VDS = 1 V (c) Pinched off channel VDS = 10 V Figure 6Q11-5 (a) The gate and source are shorted (VGS = 0) and VDS is small. (b) VDS has increased to a value that allows the two depletion layers to just touch, when VDS = VP (= 5 V) and the p+n junction voltage at the drain end, VGD = -VDS = -VP = -5 V. (c) VDS is large (VDS > VP), so a short length of the channel is pinched off. Assume temperature T = 300 K and that the JFET is Si. The relative permittivity of Si is εr = 11.9 (Table 5.1 in the textbook) and the intrinsic concentration is ni = 1.45 × 1010 cm-3. The channel donor concentration Nd is given as 1015 cm-3 and the gate acceptor concentration Na = 1019 cm-3. The electron drift mobility with Nd = 1015 cm-3 is approximately µe = 1350 cm2 V-1 s-1 (see Table 5.1 in the textbook, the dopant concentration is too low to affect µe). Vo can be calculated as follows: Vo = ∴ −3 −3 21 25 −23 kT Nd Na (1.381 × 10 J/K )(300 K ) (10 m )(10 m ) ln = ln −3 2 16 e Ni 2 (1.602 × 10 −19 C) (1.45 × 10 m ) Vo = 0.8145 V We can calculate a from the given pinch-off voltage, VP = 3.9 V: VP = a 2 eNd − Vo 2ε ∴ a= 2ε (VP + Vo ) = eNd ∴ a = 2.490 × 10-6 m or 2.490 µm 2(11.9)(8.854 × 10 −12 F/m )(3.9 V + 0.8145 V) (1.602 × 10 −19 C)(10 21 m −3 ) Therefore the channel width (2a) is 4.98 µm. IDSS is given as 5.5 mA. The Z/L ratio can then be found from Eqn. (1) above: 2 eNd µe aZ I DSS = VP 3 L ∴ Z 3 I DSS = L 2VP eNd µe a 6.26 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) 3(5.5 × 10 −3 A ) Z = L 2(3.9 V)(1.602 × 10 −19 C)(10 21 m −3 )(0.135 m 2 V −1 s −1 )(2.49 × 10 −6 m ) ∴ ∴ Chapter 6 Z = 39.28 L The channel length L is given as 10 µm, therefore: Z = 39.28(10 × 10-6 m) = 3.93 × 10-4 m or 393 µm The gate area is Agate and is equal to Z × L = (3.93 × 10-4 m)(10 × 10-6 m) = 3.93 × 10-9 m2. The depletion capacitance per unit area is: 1 Cdep eεNd Na 2 = Agate 2[ Nd + Na ]Vo and the gate capacitance Cgate is twice Cdep as the JFET is symmetric and the two gates are connected in parallel as in Figure 6Q11-1. 1 Cgate = 2(3.93 × 10 −9 (1.602 × 10 −19 C)(11.9)(8.854 × 10 −12 F/m )(10 21 m −3 )(10 25 m −3 ) 2 2 m ) 2 10 21 m −3 + 10 25 m −3 (0.81486 V) [ ] C gate = 8.00 × 10-13 F ∴ This neglects stray capacitances (e.g. between gate and source leads, gate and drain leads etc.). 6.12 The JFET amplifier Consider an n-channel JFET that has a pinch-off voltage (VP) of 5 V and IDSS = 10 mA. It is used in a common source configuration as in Figure 6Q12-1a in which the gate to source bias voltage (VGS) is – 1.5 V. Suppose that VDD = 25 V. a. If a small signal voltage gain of 10 is needed, what should be the drain resistance (RD)? What is VDS? b. If an ac signal of 3 V peak-to-peak is applied to the gate in series with the dc bias voltage, what will be the ac output voltage peak-to peak? What is the voltage gain for positive and negative input signals? What is your conclusion? Solution Given, VP = 5 V, IDSS = 10 mA, VGS = VGG = -1.5 V. a If a small signal voltage gain of 10 is needed, what should be the drain resistance (RD)? See Example 6.11 (in the textbook): 2 I DS ∴ 2 VGS −1.5 V = I DSS 1 − = 4.90 mA = (10 mA )1 − −5 V −VP gm = dI DS 2 I DSS VGS 2(10 mA ) −1.5 V 1− = 2.80 × 10-3 A/V = 1 − = 5 V VGS VP −VP −5 V From Equation 6.56 (in the textbook), the small signal voltage gain AV = gmRD , so that RD = AV /gm = (10)/(2.80 × 10-3 A/V) = 3571 Ω 6.27 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 IDS (mA) 10 IDS RD C OUTPUT SIGNAL vds D INPUT SIGNAL vgs VGG G VGS 8 6 Q VDD id(t) Time 4 A VDS S B B A 2 +18 V VGS 0 –4 –2 0 –1.5 V A vgs(t) B (b) (a) Time Figure 6Q12-1 (a) Common source (CS) ac amplifier using a JFET. (b) Explanation of how ID is modulated by the signal, vgs in series with the dc bias voltage VGG. VDS is given by Eqn. 6.53 (in the textbook) (IDS = 4.9 mA from Table 6.1 in the textbook): VDS = VDD - IDSRD VDS = 25 V - (4.9 × 10-3 A)(3571 Ω) = 7.50 V b VDD is given as 25 V. Let RD = 3571 Ω. Negative input signal: When vsignal = -1.5 V at the input then, VGSmin = VGG + vsignal = -1.5 V - 1.5 V = -3 V 2 2 V −3 V = I DSS 1 − GS = (10 mA )1 − = 1.60 mA −5 V − V P ∴ I DS min ∴ VDSmax = VDD - IDSminRD = (25 V) - (1.60 mA)(3.571 kΩ) = 19.29 V For negative going signals, the gain is, AV − = ∴ Change in output voltage VDS max − VDS 19.29 V − 7.51 V = = Change in input voltage vsignal −1.5 V A V - = -7.85 Positive input signal: When vsignal = +1.5 V at the input then, VGSmax = VGG + vsignal = -1.5 V + 1.5 V = 0 V 2 ∴ I DS max 2 VGS 0 V = I DSS 1 − = 10.0 mA = (10 mA )1 − −5 V −VP 6.28 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Chapter 6 VDSmin = VDD - IDSmaxRD = 25 V - (10.0 mA)(3.571 kΩ) = -10.7 V This is nonsense as VDSmin can not be below zero. The peak to peak voltage is then 19.3 V - 0 V = 19.3 V. Therefore the JFET has saturated with VDS ≈ 0 and the voltage across RD being VDD. This occurs when IDS = IDSsat: The JFET amplifier saturates when IDS = IDSsat ≈ VDD/RD = 25 V / (3.571 kΩ) = 7.00 mA For positive going signals the output eventually becomes saturated and the gain is, AV + = ∴ Change in output voltage VDS min − VDS 0 − 7.5 = ≈ Change in input voltage vsignal +1.5 A V + = -5.00 The JFET amplifier is operating non-linearly. The negative going output signal will be clipped for the positive going input signal. The negative sign in both cases represents a phase shift of 180°. Can we find the signal vsignal for this saturation (clipping) condition in the positive going input signal? Normally this would be done using a load line with the JFET characteristics (as in electronics circuits courses). It may be thought that we can at least estimate VGSmin from 2 V I DS = I DSS 1 − GS −VP with IDS = IDSmax to find the required VGSmin. However this equation is not valid when VDS < VDS(sat), see Figure 6Q12-2, which will be the case when the drain current drops VDD across RD. As a very rough estimate, using the above equation with IDSmax = 10 mA, gives VGSmin = -0.82 V which can be interpreted as a rough condition for approaching saturation (clipping). Thus when VGG + vsignal ≈ -0.82 V, the output should be approaching saturation, or when vsignal ≈ +0.68 V. ID (mA) VDS(sat) = VP IDSS 10 VGS = 0 5 IDS VDS(sat) = VP+VGS 0 0 4 VDS 8 VGS = –2 V VGS = –4 V VGS = –5 V 12 Figure 6Q12-2 Typical ID versus VDS characteristics of a JFET for various fixed gate voltages VGS. 6.29 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 VDS 19.3 V 7.5V time 0V v signal 1.5 V time –1.5 V Figure 6Q12-3 6.13 The Enhancement NMOSFET amplifier Consider an n-channel Si enhancement NMOS that has a gate width (Z) of 150 µm, channel length (L) of 10 µm, and oxide thickness (tox) of 500 Å. The channel has µe = 700 cm2 V-1 s-1 and the threshold voltage (Vth) is 2 V (εr = 3.9 for SiO2). a. Calculate the drain current when VGS = 5 V and VDS = 5 V and assuming λ = 0.01. b. What is the small-signal voltage gain if the NMOSFET is connected as a common source amplifier as shown in Figure 6Q13-1, with a drain resistance RD of 2.2 kΩ, the gate biased at 5 V with respect to source (VGG = 5 V) and VDD is such that V DS = 5 V? What is VDD? What will happen if the drain supply is smaller? c. Estimate the most positive and negative input signal voltages that can be amplified if VDD is fixed at the above value in b. d. What factors will lead to a higher voltage amplification? Solution a Given the device geometry and fabrication characteristics we can calculate K for this device. Using Z = 150 µm, L = 10 µm, µe ≈ 700 cm2 V-1 s-1, ε = εoεr and εr = 3.9 for SiO2, and tox = 500 Å = 500 × 10-10 m, then, giving, K= Zµeε 2 Ltox K= (150 × 10 −6 m )(700 × 10 −4 m 2 V −1 s −1 )(3.9 × 8.854 × 10 −12 F m −1 ) 2(10 × 10 −6 m )(5 × 10 −8 m ) K = 0.363 × 10-3 A V-2 6.30 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) i.e. Chapter 6 Given Vth = 2 V, we note that VDS(sat) at VGS = 5 V is determined by VDS(sat) = VGS – Vth = 5 V – 2 V = 3 V Thus when VGS = 5 V and VDS = 5 V > VDS(sat), we have I DS = K (VGS − Vth ) (1 + λVDS ) 2 I DS = (3.63 × 10 −4 A V −2 )(5 V − 2 V) (1 + 0.01 V −1 × 5 V) 2 i.e. I DS = 3.42 × 10-3 A or 3.4 mA IDS RD C Output signal D G vgs Input signal VGS Blk vds VDS S VDD VGG An NMOSFET Amplifier b Figure 6Q13-1 The mutual conductance (or transconductance) is gm = 2 K (VGS − Vth )(1 + λVDS ) i.e. gm = 2(0.363 × 10-3 A V-2)(5 V – 2 V)(1 + 0.01 V-1 × 5 V) = 2.28 × 10-3 AV-1 The small signal voltage gain is, AV = i.e. vds − RDid R δI = = − D DS = ±gm RD δVGS vgs vgs AV = -gmRD = – (2.28 × 10-3 AV-1)(2200 Ω) = –5.05 Since VDS = VDD - IDSRD we can find VDD. V DD = VDS + IDSRD = 5 V + (3.4 mA)(2.2 kΩ) = 12.5 V A smaller drain supply will reduce the gain and also limit the voltage swing. c The output VDS can only change between 0 and VDD. Obviously, VDS = VDD when ID = 0, which corresponds to VGSmin = Vth. This means that the most negative input voltage vgs has to be vgsmin = –(VGS – Vth) = –(5 V - 2 V) = -3 V As the input voltage vgs and hence VGS increase, the drain current IDS also increases and VDS decreases. As soon as VDS reaches VDS(sat), any further increase in VGS will lead VDS < VDS(sat) which means ID ≠ IDS, or ID is no longer the saturated drain current and therefore no longer given by IDS = K(VGS - Vth)2. One can assume that once VDS < VD(sat), the MOSFET amplifier will become highly non-linear and will be operating in the non-desirable range, that is, further increases in VGS will cause the output VDS to change by small amounts. Suppose that VGSmax is the GS voltage that takes VDS to VDS(sat) so that the channel is just pinched off. Then just at pinch-off, 6.31 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 VGSmax – VDS(sat) = Vth I DS = and VDD − VDS( sat ) RD ≈ K (VGSmax − Vth )2 Combining the two equations we find a quadratic equation in VGSmax, (KR D)VGSmax2 + (1– 2KR DVth)VGSmax + (KR DVth2–VDD–Vth) = 0 Substituting for K = 0.363 × 10-3 A V-2, RD = 22000 Ω, VDD = 12.5 V, Vth = 2 V, (0.7977) VGSmax2 + (-2.19)VGSmax + (-11.31) = 0 and solving, VGSmax ≈ 5.38 V This corresponds to an the input signal is vgsmax = V GSmax – V GG = 5.38 V – 5 V = 0.38 V d If we set VDS = 1/2VDD (typical design for a symmetrical swing about the dc value), then IDS = VDD/(2RD) and the magnitude of voltage gain is, 1/ 2 AV = gm RD ∝ [ KI DS ] 1/ 2 or V RD = K DD 2 RD 1/ 2 KR V RD = D DD 2 AV ∝ [ KRD VDD ]1 / 2 Thus we can increase the gain by increasing RD, VDD or the device constant K. For the latter, we can use a wider gate width (greater Z), narrower channel (smaller L), or a thinner oxide thickness (smaller tox). *6.14 Ultimate limits to device performance a. Consider the speed of operation of an n-channel FET-type device. The time required for an electron to transit from the source to the drain is τt = L/v d, where L is the channel length and v d is the drift velocity. This transit time can be shortened by shortening L and increasing v d . As the field increases, the drift velocity eventually saturates at about vdsat = 105 m s-1 when the field in the channel is equal to Ec ≈ 106 V m-1. A short τt requires a field that is at least Ec. 1. What is the change in the PE of an electron when it traverses the channel length L from source to drain if the voltage difference is VDS? 2. This energy must be greater than the energy due to thermal fluctuations, which is of the order of kT. Otherwise, electrons would be brought in and out of the drain due to thermal fluctuations. Given the minimum field and V DS , what is the minimum channel length and hence the minimum transit time? b. Heisenberg's uncertainty principle relates the energy and the time duration in which that energy is possessed through a relationship of the form (Chapter 3) ∆E∆t > h. Given that during the transit of the electron from the source to the drain its energy changes by eVDS, what is the shortest transit time, τ, satisfying Heisenberg's uncertainty principle? How does it compare with your calculation in part (a)? c. How does electron tunneling limit the thickness of the gate oxide and the channel length in a MOSFET? What would be typical distances for tunneling to be effective? (Consider Example 3.10 in the textbook). 6.32 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 Solution Assume temperature T = 300 K. The saturation velocity is given as vdsat = 105 m/s and the saturation field is given as Ec = 106 V/m. a(1) The change in the PE is ∆PE. This is the charge times the voltage, i.e. ∆PE = eVDS. (2) The lower limit to ∆PE due to thermal fluctuations is kT. Therefore, substituting into the equation above: eVDS = kT ∴ VDS = kT/e = (1.381 × 10-23 J/K)(300 K)/(1.602 × 10-19 C) = 0.02586 V This is the lower limit to VDS. The minimum channel length L can now be found from the minimum electric field, given by Ec = VDS/L: ∴ L = VDS/E c = (0.02586 V)/(106 V/m) = 2.586 × 10-8 m The minimum transit time τt is then: τt = L/vdsat = (2.586 × 10-8 m)/(105 m/s) = 2.586 × 10-13 s The above limit is the thermal fluctuation limit (thermal noise limit). b Consider the Heisenberg uncertainty principle. Let τ = ∆t be the transit time. During this time the energy changes by ∆E = eVDS. We are given ∆E∆t > h, therefore, substituting for the shortest transit time: eVDSτ = h τ= ∴ h 1.055 × 10 −34 J s = 2.55 × 10-14 s = −19 eVDS (1.602 × 10 C)(0.02586 V) The uncertainty limit allows a shorter transit time down to 0.0255 ps. Thus thermal fluctuation limit will operate at room temperature. c If the oxide becomes too thin then the electron tunneling will allow gate charge to tunnel into the channel. This will prevent the gate from holding the necessary charge and will lead to a gate current. The field effect will fail. From Example 3.10, one can guess that the thickness should be less than 1 nm or 10 Å depending on various material properties. 6.15 LED Output Spectrum Given that the width of relative light intensity vs. photon energy spectrum of a LED is typically around ~3kT, what is the linewidth ∆λ in the output spectrum in terms of wavelength? Solution The emitted wavelength λ is related to the photon energy Eph by λ= c hc = υ E ph If we differentiate λ with respect to the photon energy Eph we get 6.33 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 dλ hc =− 2 dE ph E ph We can represent small changes or intervals (or ∆) by differentials, e.g. ∆λ dλ ≈ , ∆E ph dE ph then ∆λ ≈ hc ∆E ph 2 E ph We are given the energy width of the output spectrum, ∆E ph = h ∆ν ≈ 3kB T Then using the later and substituting for Eph in terms of λ we find ∆λ ≈ λ2 3kT hc 6.16 LED Output Wavelength Variations Show that the change in the emitted wavelength λ with temperature T from a LED is given by dλ hc dE = − 2 g dT Eg dT where Eg is the band gap. Consider a GaAs LED. The band gap of GaAs at 300 K is 1.42 eV which changes (decreases) with temperature as dEg/dT = -4.5 × 10-4 eV K-1. What is the change in the emitted wavelength if the temperature change is 10 °C? Solution The emitted photon energy is approximately equal to the band gap energy Eg. The emitted wavelength is then λ= c hc = υ Eg Differentiating λ with respect to temperature (note that Eg = f (T )) we receive −34 J s)(3 × 108 m s −1 ) dλ hc dE (6.626 × 10 = − 2 g = −4.5 × 10 −4 × 1.602 J K −1 ) ( −19 dT Eg dT (1.42 × 1.602 × 10 J) so that dλ = 2.77 × 10-10 m K-1 = 0.277 nm K-1 dT The change in the wavelength for ∆T = 10 °C is ∆λ = dλ ∆T = (0.277 nm K-1)(10 K) ≈ 2.8 nm dT Since Eg decreases with temperature, the wavelength increases with temperature. This calculated change is within 10% of typical values for GaAs LEDs quoted in the data books. 6.34 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 6.17 Solar Cell Driving a Load a. A Si solar cell of area 1 cm2 is connected to drive a load R as in Figure 6Q17-1a. It has the I-V characteristics in Figure 6Q17-1b under illumination of 500 W m-2. Suppose that the load is 20 Ω and it is used under a light intensity of 1 kW m-2. What are the current and the voltage in the circuit? What is the power delivered to the load? What is the efficiency of the solar cell in this circuit? b. What should be the load to obtain maximum power transfer from the solar cell to the load at 1 kW m2 illumination? What is this load at 500 W m-2? c. Consider using a number of such solar cells to drive a calculator that needs a minimum of 3 V and draws 3.0 mA at 3-4 V. It is to be used at a light intensity of about 500 W m-2. How many solar cells would you need and how would you connect them? I (mA) Voc V L 0 V I 0.6 0.2 0.4 V R I –10 Isc= –Iph I-V for a solar cell under an illumination of 500 Wm-2. Slope = – 1/R Operating Point I P The Load Line for R = 30 Ω (I-V for the load) –20 (a) (b) Figure 5Q17-1 (a) When a solar cell drives a load R, R has the same voltage as the solar cell but the current through it is in the opposite direction to the convention that current flows from high to low potential. (b) The current I′ and voltage V′ in the circuit of (a) can be found from a load line construction. Point P is the operating point (I′, V′). The load line is for R = 30 Ω. Solution a The total current through the solar cell is given by Equation 6.64 (in the textbook) which can be written as V I500 = − I500 ph + I0 exp − 1 , ηVt (1) kT . There are three unknowns in Equation (1) - I500 ph, I0 and ηVt . From Figure 5Q17-1b e we can determine that: where Vt = V, [V] I, [mA] 0 -16.2 .45 -13.1 .49 0 6.35 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 It is clear that Iph = 16.2 mA. In order to determine I0 and ηVt we have to solve the following system of equations: 0.49 −3 I0 exp − 1 = 16.2 × 10 ηVt 0.45 −3 I0 exp − 1 = (16.2 − 13.1) × 10 V η t The solution of this system is: I0 = 2.58 × 10-11 A and ηVt = 0.024 Now we are ready to scale I-V characteristics of the solar cell for illumination of 1000 W m-1. Using Equations 6.63 and 6.64 (in the textbook), it is easy to show that for this light intensity the current through the solar cell will be V I1000 = −2 I500 ph + I0 exp (2) − 1 V η t Figure 6Q17-2 shows the I-V curves for both illuminations together with the load lines for R = 30Ω and R = 20 Ω. Now we can find the interception of the solar cell I-V curve for 1000 W m-2 with the load line for R = 20 Ω solving the equation V V −2 I500 ph + I0 exp − 1 = − R ηVt The solution of this equation can be obtained by several iterations or graphically using Figure 6Q17-2. The result is V’ = 0.475 V. At this voltage the current trough the circuit will be I’ = 0.475/20 = 0.024 A and the power delivered to the load is P’ =0.475 × 0.024 = 0.011 W. This is not the maximum power available from the solar cell. The input sun-light power is Pin = ( Light intensity) (Surface area) = (1000 W m −2 )(1 × 10 −4 m 2 ) = 0.1 W The efficiency of the solar cell is then ηsolar = P© (0.011) × 100% = × 100% =11 % Pin (0.1) 6.36 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 0 R= R Current, mA -10 = 14 Ω R 30 Ω =2 R= 0.2 0.3 0Ω 28 Ω 500 W m-2 -20 -30 1000 W m-2 0 0.1 0.4 0.5 Voltage, V Figure 6Q17-2 b The power delivered to the load for the case of 1000 W illumination as a function of the voltage across the load is V P(V ) = I V = V −2 I500 ph exp (3) − 1 ηVt The maximum efficiency point corresponds to the extrema in expression (3). Differentiating Eqn. 3 and setting the derivative equal to zero we receive V V I0 V exp − 2 I500 ph + I0 exp − 1 = 0 V ηVt η ηVt t and the solution of that equation is Vm = 0.436 V. Substituting this value in Eqn. (2) we receive the current at that operating point: Im = 0.031 mA. To achieve this operation point we must connect to the solar cell load with resistance of Rm 1000 = Vm (0.436 V) = = 14.07 Ω Im (0.031 mA ) Now it is easy to find the fill factor of the solar cell for illumination of 1000 W m.-2. Setting Eqn. (2) equal to zero we can calculate the open circuit voltage Voc = 0.507 V. The fill factor (Equation 6.66 in the textbook) is then FF1000 = Im Vm (0.031 A)(0.436 V) = = 0.823 I sc Voc (0.0324 A )(0.507 V) Performing similar calculations we can find that when the illumination is 500 W m-2 the most efficient operating point is Vm = 0.42 V and Im = 0.015 A, which corresponds to load Rm 500 = 28 Ω 6.37 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 6 (very close to 30 Ω). The fill factor for this case is FF500 = 0.794. The most efficient load lines are also plotted in Figure 6Q17-2. c We need to connect the solar cells in series to obtain the necessary voltage. Assume that all the solar cells are identical and in series. Each has the same current. The short circuit current from Figure 6Q17-2 is much higher than the calculator current. From Figure 6Q17-2, when I = 3 mA, V is around 0.48 V. We need at least 3/0.48 = 6.25 or 7 solar cells. 6.18 Open Circuit Voltage A solar cell under an illumination of 100 W m-2 has a short circuit current Isc of 50 mA and an open circuit voltage Voc, of 0.55 V. What are the short circuit current and open circuit voltage when the light intensity is halved? Note: For more examples and problems on LEDs and solar cells, see Optoelectronics and Photonics: Principles and Practices. S. O. Kasap (Prentice Hall, 2001) chs 3 and 6. Solution The general I-V characteristics under illumination are given by Equation 6.64 (in the textbook) eV I = − I ph + I0 exp − 1 η kT Setting I = 0 for open circuit we have eV − I ph + I0 exp − 1 = 0 η kT Assuming that Voc >> ηkT and rearranging the above equation we can find Voc Voc = η kT I ph ln e Io (1) In Eqn. (1), the photocurrent Iph , depends on the light intensity I , via Iph = K I At a given temperature, then, the change in Voc is Voc 2 − Voc 1 = η kT I ph 2 η kT I2 ln = e ln I e I ph 1 1 The short circuit current is the photocurrent, so that at halved intensity this is Isc 2 = Isc 1 1 I2 = (50 mA ) = 25 mA 2 I1 Assuming η = 1 and T = 300 K the new open circuit voltage is Voc 2 = Voc 1 + η kT I2 1 ln = (0.55 V) + (0.02585 V)ln = 0.532 V 2 e I1 "Al Asuli writing in Bukhara some 900 years ago divided his pharmacopoeia into two parts, 'Diseases of the rich’ and 'Diseases of the poor'." Abdus Salam (1926-1997; Nobel Laureate, 1979) 6.38 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 Second Edition ( 2001 McGraw-Hill) Chapter 7 7.1 Relative permittivity and polarizability a. Show that the local field is given by Eloc = E εr + 2 3 Local field b. Amorphous selenium (a-Se) is a high resistivity semiconductor that has a density of approximately 4.3 g cm-3 and an atomic number and mass of 34 and 78.96 g/mol respectively. Its relative permittivity at 1 kHz has been measured to be 6.7. Calculate the relative magnitude of the local field in a-Se. Calculate the polarizability per Se atom in the structure. What type of polarization is this? How will εr depend on the frequency? c. If the electronic polarizability of an isolated atom is given by αe ≈ 4πεoro3 where ro is the radius of the atom, then calculate the electronic polarizability of an isolated Se atom, which has r0 = 0.12 nm, and compare your result with that for an atom in a-Se. Why is there a difference? Solution a The polarization, P, is given by: P = (ε o [ε r − 1])E where E is the electric field. The local field Eloc is given by: Eloc = E + Substitute for P: Eloc = E + ∴ Eloc = P 3ε o ( ε [ε o r − 1])E 3ε o (ε − 1) 3 + ε r − 1 E = 1 + r E = 3 3 εr + 2 E 3 b The relative magnitude of the local field refers to the local field compared to the applied field, i.e.: Eloc / E. Therefore, with εr = 6.7: Eloc ε r + 2 6.7 + 2 = = = 2.9 3 3 E If D is the density then the concentration of Se atoms N is 7.1 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 3 3 23 −1 DN A ( 4.3 × 10 kg/m )(6.022 × 10 mol ) N= = Mat (78.96 × 10 −3 kg/mol) = 3.279 × 1028 m-3 The Clausius-Mossotti equation relates the relative permittivity to the electronic polarizability, αe: εr − 1 N = αe ε r + 2 3ε o ∴ −12 F/m )(6.7 − 1) 3ε o (ε r − 1) 3(8.854 × 10 αe = = 28 N (ε r + 2 ) (3.279 × 10 m −3 )(6.7 + 2) ∴ α e = 5.31 × 10-40 F m2 This would be a type of electronic polarization, as Se is a covalent solid. εr is flat up to optical frequencies. The Se atom has a radius of about ro = 0.12 nm. Substituting into the given equation: c αe′ ≈ 4πεoro3 = 4π(8.854 × 10-9 F/m)(0.12 × 10-9 m)3 ∴ α e ′ ≈ 1.92 × 10-40 F m2 Comparing this value and our previous value: α e 5.30 × 10 −40 F m 2 = = 2.76 α e′ 1.92 × 10 −40 F m 2 The observed polarizability per Se atom in the solid is 2.8 times greater than the polarizability of the isolated Se atom. In the solid, valence electrons are involved in bonding and these electrons contribute to electronic bond polarization (the field can displace these electrons). 7.2 Relative permittivity, bond strength, bandgap and refractive index Diamond, silicon, and germanium are covalent solids with the same crystal structure. Their relative permittivities are shown in Table 7Q2-1. a. Explain why εr increases from diamond to germanium. b. Calculate the polarizability per atom in each crystal and then plot polarizability against the elastic modulus Y (Young's modulus). Should there be a correlation? c. Plot the polarizability from part b against the bandgap energy, Eg. Is there a relationship? d. Show that the refractive index n is √εr. When does this relationship hold and when does it fail? e. Would your conclusions apply to ionic crystals such as NaCl? Table 7Q2-1 Properties of diamond, Si, and Ge Mat Density εr (g cm-3 ) Diamond 5.8 12 3.52 Si 11.9 28.09 2.33 Ge 16 72.61 5.32 αe 7.2 Y (GPa) 827 190 75.8 Eg (eV) 5.5 1.12 0.67 n 2.42 3.45 4.09 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 Solution a In diamond, Si, and Ge, the polarization mechanism is electronic (bond). There are two factors that increase the polarization. First is the number of electrons available for displacement and the ease with which the field can displace the electrons. The number of electrons in the core shells increases from diamond to Ge. Secondly, and most importantly, the bond strength per atom decreases from diamond to Ge, making it easier for valence electrons in the bonds to be displaced. b For diamond, atomic concentration N is: 3 3 23 −1 DN A (3.52 × 10 kg/m )(6.022 × 10 mol ) N= = = 1.766 × 1029 m-3 −3 Mat × 12 10 kg/mol ( ) The polarizability can then be found from the Clausius-Mossotti equation: εr − 1 N = αe ε r + 2 3ε o ∴ −12 F/m )(5.8 − 1) 3ε o (ε r − 1) 3(8.854 × 10 αe = = 29 N (ε r + 2 ) (1.766 × 10 m −3 )(5.8 + 2) ∴ α e = 9.256 × 10-41 F m2 The polarizability for Si and Ge can be found similarly, and are summarized in Table 7Q2-2: Table 7Q2-2 Polarizability values for diamond, Si and Ge α e (F m2 ) Diamond 1.766 × 1029 m-3 9.256 × 10-41 F m2 Si 4.995 × 1028 m-3 4.170 × 10-40 F m2 Ge 4.412 × 1028 m-3 5.017 × 10-40 F m2 Polarizability per atom (F m2) N (m-3) 6.00 10-40 5.00 10-40 4.00 10-40 3.00 10-40 2.00 10-40 1.00 10-40 0 0 200 400 600 800 Young's modulus (GPa) 1000 = 5.311 10-40 - (5.327 10-43)Y Correlation coefficient = 0.9969 Figure 7Q2-1 Plot of polarizability per atom versus Young’s modulus. 7.3 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 As the polarization mechanism in these crystals is due to electronic bond polarization, the displacement of electrons in the covalent bonds depends on the flexibility or elasticity of these bonds and hence also depends on the elastic modulus. Polarizability per atom (F m2) c 6.00 10-40 5.00 10-40 4.00 10-40 3.00 10-40 2.00 10-40 1.00 10-40 0 0 1 2 3 4 5 Bandgap, Eg (eV) 6 = 5.325 10-40 - (8.0435 10-41)Eg Correlation coefficient = 0.9873 Figure 7Q2-2 Plot of polarizability versus bandgap energy. There indeed seems to be a linear relationship between polarizability and bandgap energy. d To facilitate this proof, we can plot a graph of refractive index, n, versus relative permittivity, εr. 5 Refractive index, n Log-log plot: Power law 2 4 10 Relative permittivity, 20 r Figure 7Q2-3 Logarithmic plot of refractive index versus relative permittivity. The log-log plot exhibits a straight line through the three points. The best fit line is n = Aεrx (Correlation coefficient is 0.9987) where x = 0.513 ≈ 1/2 and A = exp(–0.02070) ≈ 1. Thus n = √(εr). The refractive index n is an optical property that represents the speed of a light wave, or an electromagnetic wave, through the material (v = c/n). The light wave is a high frequency electromagnetic wave where the frequency is of the order of 1014 – 1015 Hz (ƒoptical). n and polarizability (or εr) will be related if the polarization can follow the field oscillations at this frequency (ƒoptical). This will be the case in electronic polarization because electrons are light and rapidly respond to the fast oscillations of the field. 7.4 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 The relationship between n and εr will not hold if we take εr at a low frequency (<< ƒoptical) where other slow polarization contributions (such as ionic polarization, dipolar polarization, interfacial polarization) also contribute to εr. n = ε r would apply to ionic crystals if εr is taken at the corresponding optical frequency rather than at frequencies below ƒoptical. Tabulated data for ionic crystals typically quote εr that includes ionic polarization and hence this data does NOT conform to n = ε r . e 7.3 Dipolar liquids Given the static dielectric constant of water as 80, its high frequency dielectric constant (due to electronic polarization) as 4, its density as 1 g cm-3 calculate the permanent dipole moment po per water molecule assuming that it is the orientational and electronic polarization of individual molecules that gives rise to the dielectric constant. Use both the simple relationship in Equation 7.14 (in the textbook) where the local field is the same as the macroscopic field and also the Clausius-Mossotti equation and compare your results with the permanent dipole moment of the water molecule which is 6.1 × 10-30 C m. What is your conclusion? What is εr calculated from the Clasius-Mossotti equation taking the true po (6.1 × 10-30 C m ) of a water molecule? (Note: Static dielectric constant is due to both orientational and electronic polarization. The Clausius-Mossotti equation does not apply to dipolar materials because the local field is not described by the Lorentz field.) Solution We first need the number H2O molecules per unit volume. The molecular mass of water Mmol is 18 × 10-3 kg mol-1, its density is d = 103 kg m-3. The number of H2O molecules per unit volume is N= 3 −3 23 −1 d N A (10 kg m )(6.022 × 10 mol ) = = 3.35 × 1028 m- 3 −3 −1 Mmol × 18 10 kg mol ( ) Using the high frequency dielectric constant εr_HV which is due only to electronic polarization and the Clausius-Mossotti equation (Equation 7.15 in the textbook), we can calculate the electronic polarizability αe of water molecules αe = ( ) = 3(8.85 × 10 3ε 0 ε r HF − 1 ( ) −12 F m −1 )( 4 − 1) (4 + 2)(3.35 × 10 28 m −3 ) ε r HF + 2 N = 3.964 × 10-40 F m2 Now using this result and the static dielectric constant εr Stat which is due both to electronic and dipolar polarization, we can solve the Clausius-Mossotti equation for the dipolar polarizability αd of water αd = = [3ε (ε 0 r Stat ) ( − 1 − N α e ε r Sat + 2 (ε r Sat ) +2 N )] = 3(8.85 × 10 −12 F m −1 )(80 − 1) − (3.35 × 10 28 m −3 )(3.96 × 10 −40 F m 2 )(80 + 2) (80 + 2)(3.35 × 10 28 m −3 ) = 3.674 × 10-40 F m2 The permanent dipolar moment per water p0 molecule can be calculated from Equation 7.19 (in the textbook) 7.5 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 p0 = 3kT α d = 3 (1.38 × 10 −23 J K −1 )(300 K )(3.964 × 10 −40 F m 2 ) = 2.137 × 10-30 C m This result is around 3 times smaller than the real permanent dipolar moment of the water molecule. The same calculations performed on the base of the simple relationship in Equation 7.14 (in the textbook) will result in αe = αd = ( ( ) = (8.85 × 10 ε 0 ε r HF − 1 N N −12 F m −1 )( 4 − 1) (3.35 × 10 e F m −1 )( 4 − 1) (3.35 × 1028 m −3 ) ) − α = (8.85 × 10 ε 0 ε r Stat − 1 −12 28 m −3 ) = 7.928 × 10-40 F m2 − (7.298 × 10 −40 F m 2 ) = 2.009 × 10-38 F m2 and p0 = 3kT α d = 3 (1.38 × 10 −23 J K −1 )(300 K )(2.009 × 10 −38 F m 2 ) = 1.58 × 10-29 C m which is about 3 times bigger than the actual value. Both are unsatisfactory calculations. The reasons for the differences are two fold. First is that the individual H2O molecules are not totally free to rotate. In the liquid, H2O molecules cluster together through hydrogen bonding so that the rotation of individual molecules is then limited by this bonding. Secondly, the local field can neither be totally neglected nor taken as the Lorentz field. A better theory for dipolar liquids is based on the Onsager theory which is beyond the scope of this book. Interestingly, if one uses the actual po = 6.1 × 10-30 C m in the Clasius-Mossotti equation, then εr turns out to be negative, which is nonsense. 7.4 Dipole moment in a nonuniform electric field Figure 7Q4-1 shows an electric dipole moment p in a nonuniform electric field. Suppose the gradient of the field is dE/dx at the dipole p, and the dipole is oriented to be along the direction of increasing E as in figure 7Q4-1. Show that the net force acting on this dipole is given by F=p dE dx Net force on a dipole Which direction is the force? What happens to this net force when the dipole moment is facing the direction of increasing field? Given that a dipole normally also experiences a torque (as described in Section 7.3.2 of the textbook), explain qualitatively what happens to a randomly placed dipole in a nonuniform electric field. Explain the experimental observation of bending a flow of water by a nonuniform field from a charged comb as shown in the photograph in Figure 7Q4-1? (Remember that a dielectric medium placed in a field develops polarization P directed along the field.) 7.6 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 E p F Figure 7Q4-1 Left: A dipole moment in a nonuniform field experiences a net force F that depends on the dipole moment p and the field gradient dE/dx. Right: When a charged comb (by combing hair) is brought close to a water jet, the field from the comb polarizes the liquid by orientational polarization. The induced polarization vector P and hence the liquid is attracted to the comb where the field is higher. Solution E P F E E′ = E + δx(dE/dx) E F- −Q +Q P Figure 7Q4-2 When a dielectric is placed in a nonuniform field it develop a polarization P along the direction of the field. We can represent this polarization by two oppositle charges +Q and −Q separaed by δx. F+ δx x x′ = x + δx x We can represent the dipole p as two opposite charges +Q and -Q separated by a distance δx as in Figure 7Q4-2. Thus p = Qδx. Let -Q be at x, then +Q is at x′ = x + δx. The field is nonuniform. If the field is E at x and then it is E′ at x′, that is E′ = E + δx(dE/dx) The force F- acting on -Q is along the -x direction and its magnitude is given by F- = QE and that on +Q is along +x and is given by F+= QE′ = Q[E + δx(dE/dx)]. 7.7 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 The net force along the +x direction is F = F+ - F- = Q[E + δx(dE/dx)] - QE or F = Qδx(dE/dx) = p(dE/dx). The force is along the +x direction. The force changes direction if p is pointing in the opposite direction. When a dipole moment is placed in a random direction in an electric field, it will always rotate to align with the electric field. Thus it will rotate until p is along E. Since the field is nonuniform, it will then experience a net force towards higher electric fields. Thus, p rotates and moves towards higher fields. When a dielectric is placed in a nonuniform electric field, it develops a polarization P along the field as shown in Figure 7Q4-2. We can of course represent the polarized dielectric by the surface charges +Q and -Q as in Figure 7Q4-2. Following the above lines of argument, since induced P is along the field direction, the dielectric experiences a net force F towards higher fields along +x direction, F = P(dE/dx) The water jet near a charged comb is attracted to the comb because water is a dielectric, develops a polarization P along the field and becomes attracted towards the higher field region which is near the comb. 7.5 Ionic and electronic polarization Consider a CsBr crystal that has the CsCl unit cell crystal structure (one Cs+–Br- pair per unit cell) with a lattice parameter (a) of 0.430 nm. The electronic polarizability of Cs+ and Cl- ions are 3.35 × 10-40 F m2 and 4.5 × 10-40 F m2 respectively, and the mean ionic polarizability per ion pair is 5.8 × 10-40 F m 2. What is the low frequency dielectric constant and that at optical frequencies? Solution The CsBr structure has a lattice parameter given by a = 0.430 nm, and there is one CsBr ion pair per unit cell. If n is the number of ion pairs in the unit cell, the number of ion pairs, or individual ions, per unit volume (N) is N= 1 n 28 = m-3 3 = 1.258 × 10 3 −9 a (0.430 × 10 m) At low frequencies both ionic and electronic polarizability contribute to the relative permittivity. Thus, from Equation 7.20 (in the textbook), (where αi is the mean ionic polarizability per ion pair, αeCs is the electronic polarizability of Cs+ and αeBr is the electronic polarizability of Br-): ε r ( low ) − 1 1 = ( Nα i + Nα eCs + Nα eBr ) ε r ( low ) + 2 3ε o Remember that (Nαi + NαeCs + NαeBr) should be written as (Niαi + NCsαeCs + NBrαeCl), but since there is a one-to-one ratio between the number of molecules and ions in CsCl, we can take all the N’s to be the same. ∴ ε r ( low ) = ( ) 1 ( Nα i + Nα eCs + Nα eBr ) ε r (low ) + 2 + 1 3ε o 7.8 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 ∴ ε r ( low ) − 1 2 Nα i + Nα eCs + Nα eBr )ε r ( low ) = ( ( Nα i + Nα eCs + Nα eBr ) + 1 3ε o 3ε o Isolate and simplify: ε r ( low ) = 2 N (α i + α eCs + α eBr ) + 3ε o 3ε o − N (α i + α eCs + α eBr ) ε r ( low ) = 2(1.258 × 10 28 m −3 )(5.8 × 10 −40 F m 2 + 3.35 × 10 −40 F m 2 + 4.50 × 10 −40 F m 2 ) + 3(8.854 × 10 −12 F/m ) 3(8.854 × 10 −12 F/m ) − (1.258 × 10 28 m −3 )(5.8 × 10 −40 F m 2 + 3.35 × 10 −40 F m 2 + 4.50 × 10 −40 F m 2 ) ε r(low) = 6.48 ∴ At optical frequencies there is no contribution from ionic polarization. We only consider electronic polarization of individual ions and therefore the relative permittivity at optical frequencies, εr(op), is: ε r ( op ) = 2(1.258 × 10 28 m −3 )(3.35 × 10 −40 F m 2 + 4.50 × 10 −40 F m 2 ) + 3(8.854 × 10 −12 F/m ) 3(8.854 × 10 −12 F/m ) − (1.258 × 10 28 m −3 )(3.35 × 10 −40 F m 2 + 4.50 × 10 −40 F m 2 ) ε r(op) = 2.77 ∴ 7.6 Electronic polarizability and KCl KCl has the NaCl crystal structure with a lattice parameter of 0.629 nm. Calculate the relative permittivity of a KCl crystal at optical frequencies given that the electronic polarizability of K+ is 1.264 × 10-40 F m2 and that of Cl– is 3.408 × 10-40 F m 2. How does this compare with the measured value of 2.19? Solution The CsCl structure has a lattice parameter given by a = 0.629 nm, and there are 4 KCl ion pairs per unit cell (see Table 1.3, pg. 48 in the textbook). The number of ion pairs, or individual ions, per unit volume (N) is therefore: N= 4 4 28 m-3 = 3 = 1.607 × 10 3 − 9 a (0.629 × 10 m) The electronic polarizability of the K+ ion is given as αeK = 1.264 × 10-40 F m2, and polarizability of the Cl- ion is given as αeCl = 3.408 × 10-40 F m2. From Equation 7.20 (in the textbook), the relative permittivity at optical frequencies, εr(op), can be found (see solution for question 7.5 for derivation): ε r ( op ) = ∴ ∴ ε r ( op ) = 2 N (α eK + α eCl ) + 3ε o 3ε o − N (α eK + α eCl ) 2(1.607 × 10 28 m −3 )(1.264 × 10 −40 F m 2 + 3.408 × 10 −40 F m 2 ) + 3(8.854 × 10 −12 F/m ) 3(8.854 × 10 −12 F/m ) − (1.607 × 10 28 m −3 )(1.264 × 10 −40 F m 2 + 3.408 × 10 −40 F m 2 ) ε r(op) = 2.18 This value is very close to the experimental value of 2.19. 7.9 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 7.7 Equivalent circuit of a polyester capacitor Consider a 1 nF polyester capacitor that has a polymer (PET) film thickness of 1 µm. Calculate the equivalent circuit of this capacitor at 50 °C and at 120 °C for operation at 1 kHz. What is your conclusion? Solution The capacitance is given as 1 nF at room temperature and also at 50 ˚C where εr′ is constant as shown in Figure 7Q7-1. The relative permittivities at 50 ˚C and 120 ˚C can be found approximately from Figure 7Q7-1. Upon inspection, at 50 ˚C, εr′ = 2.6 and at 120 ˚C εr′ = 2.8. 0.1 PET at f = 1kHz 2.8 0.01 tan r' 2.7 0.001 2.6 r' DEA 2.5 0 50 Loss tangent, tan 2.9 100 150 200 0.0001 250 Temperature (°C) Figure 7Q7-1 Real part of the dielectric constant εr′ and loss tangent, tan δ, at 1 kHz versus temperature for PET. SOURCE: Data obtained by Kasap and Maeda (1955) using a dielectric analyzer (DEA). From the equation for capacitance, the area of the dielectric can be found (where d is the thickness of the film): ∴ C= ε r′ε o A d A= (1 × 10 −6 m)(1 × 10 −9 F) = 4.344 × 10-5 m2 dC = ε r′ε o (2.6)(8.854 × 10 −12 F/m ) Conductance = Gp = 1/Rp C Figure 7Q7-2 Equivalent circuit of a capacitor at one given frequency. 7.10 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 Let tanδ equal the loss tangent or loss factor. From Figure 7Q7-1, at 50 ˚C, tanδ = 0.001 and at 120 ˚C tanδ = 0.01. We need the equivalent parallel conductance, Gp, (or resistance Rp) at 50 ˚C and 120 ˚C. For temperature T = 50 ˚C: Gp = ωC tan δ = 2π (1000 Hz)(1 × 10 −9 F )(0.001) = 6.28 × 10-9 Ω - 1 R p = 1 / G p = 1.59 × 108 Ω or 159 MΩ At 120 ˚C, the capacitance is different (C′) due to the change in εr′. The loss is higher which means a lower equivalent parallel resistance Rp. Assuming no change in area A: C′ = (2.8)(8.854 × 10 −12 F/m)(4.344 × 10 −5 m 2 ) (1 × 10 −6 m) = 1.077 × 10-9 F A value 7.7% higher than at the lower temperature. The new conductance and resistance are: Gp = ωC tan δ = 2π (1000 Hz)(1.077 × 10 −9 F )(0.01) = 6.77 × 10-8 Ω - 1 R p = 1 / G p = 1.48 × 107 Ω or 14.8 MΩ 7.8 Dielectric loss per unit capacitance Consider the three dielectric materials listed in Table 7Q8-1 with the real and imaginary dielectric constants, εr' and ε r ''. At a given voltage, which dielectric will have the lowest power dissipation per unit capacitance at 1 kHz and at an operating temperature of 50 °C? Is this also true at 120 °C? Table 7Q8-1 Dielectric properties of three insulators at 1 kHz T =50 °C ′ Material εr ε r″ Polycarbonate 2.47 0.003 PET 2.58 0.003 PEEK 2.24 0.003 T = 120 °C εr 2.535 2.75 2.25 ′ ε r″ 0.003 0.027 0.003 Solution Since we are merely comparing values, assume voltage V = 1 V for calculation purposes. From example 7.5 (in the textbook), the power dissipated per unit capacitance (Wcap) is given by: Wcap = V 2ω ε r′′ ε r′ where ω is the angular frequency (2πf) and εr′ and εr″ represent the real and imaginary components of the relative permittivity εr, respectively. As a sample calculation, the power dissipated in polycarbonate is: Wcap = (1 V) [2π (1000 Hz)] 2 (0.003) = 7.63 W/F (2.47) Therefore, 7.63 W per F are dissipated at 50 ˚C at 1 V. The values for the other materials at both 50 ˚C and 120 ˚C are listed below in Table 7Q8-2: 7.11 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 Table 7Q8-2 Power dissipated at different temperatures for the given materials. 50 ˚C 120 ˚C Power Dissipated Power Dissipated (W per F) (W per F) Polycarbonate 7.63 7.44 PET 7.31 61.7 PEEK 8.41 8.38 Material At 50 ˚C, all three are comparable in magnitude but PET has the lowest power dissipation. At 120 ˚C, polycarbonate has the lowest dissipation, while PET is almost ten times worse. *7.9 TCC of a polyester capacitor Consider the parallel plate capacitor equation C= ε oε r xy z where εr is the relative permittivity (or εr'), x and y are the side lengths of the dielectric so that xy is the area A, and z is the thickness of the dielectric. The quantities ε r, x, y and z change with temperature. By differentiating this equation with respect to temperature, show that the temperature coefficient of capacitance (TCC) is TCC = 1 dC 1 dε r = +λ C dT ε r dT Temperature coefficient of capacitance where λ is the linear expansion coefficient defined by λ= 1 dL L dT where L stands for any length of the material (x, y or z). Assume that the dielectric is isotropic, λ is the same in all directions. Using εr' versus T behavior in Figure 7Q9-1 and taking λ = 50 × 10-6 K-1 as a typical value for polymers, predict the TCC at room temperature and at 10 kHz. Solution The real part of the relative permittivity, εr′, is usually simply written as εr. We are given xy = area (A) and z = thickness. From the definition of the linear expansion coefficient λ: dx = λx dT dy = λy dT dz = λz dT Now differentiate C with respect to T (remember that εr depends on temperature): 7.12 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) C= Chapter 7 ε oε r xy z ε xy dε r + ε ε y dx + ε ε x dy z − ε ε xy dz o r o r o r dC o dT dT dT dT = 2 z dT ∴ Substitute for the derivatives of x, y and z as according to the definition of λ and simplify: dC = dT dε r + λε oε r xy dT z ε o xy The temperature coefficient of capacitance is defined as: TCC = substitute for dC/dT: TCC = substitute for C: TCC = simplify: TCC = 1 dC C dT ε o xy dε r + λε oε r xy dT Cz dε r + λε oε r xy dT ε oε r xy z z ε o xy 1 dε r +λ ε r dT 2.60 2.5925 2.59 r' 2.58 2.57 2.56 PET, f = 10 kHz 20 30 40 60 50 Temperature (°C) 70 80 90 Figure 7Q9-1 Temperature dependence of εr′ at 10 kHz. From the slope of the straight line in Figure 7Q9-1, we can estimate the value of dεr/dt: dε r 2.5925 − 2.57 = = 0.000375 ! C −1 or K-1 dT 80 ! C − 20 ! C Using the given value of λ = 50 × 10-6 K-1 and εr = 2.57 (εr at room temperature from inspecting Figure 7Q9-1): TCC = ∴ 1 dε r 1 0.000375 K −1 ) + 50 × 10 −6 K −1 +λ = ( ε r dT (2.57) TCC = 0.000196 K-1 or ˚C- 1 This is 196 ppm per ˚C. PET capacitors are quoted to have typically 200 ppm/˚C. 7.13 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 7.10 Dielectric breakdown of gases and Paschen curves Dielectric breakdown in gases typically involves the avalanche ionization of the gas molecules by energetic electrons accelerated by the applied field. The mean free path between collisions must be sufficiently long to allow the electrons to gain sufficient energy from the field to impact-ionize the gas molecules. The breakdown voltage, V br , between two electrodes depends on the distance, d, between the electrodes as well as the gas pressure, P, as shown in Figure 7Q10-1. V br versus Pd plots are called Paschen curves. We consider gaseous insulation, air and SF6, in an HV switch. a. What is the breakdown voltage between two electrodes of a switch separated by a 5 mm gap with air at 1 atm when the gaseous insulation is air and when it is SF6? b. What are the breakdown voltages in the two cases when the pressure is 10 times greater? What is your conclusion? c. At what pressure is the breakdown voltage minimum? d. What air gap spacing, d, at 1 atm gives the minimum breakdown voltage? e. What would be the reasons for preferring gaseous insulation over liquid or solid insulation? Solution a At pressure P = 1 atm = 1.013 × 105 Pa and air gap d = 5 mm, P × d = (1.013 × 105 Pa)(0.005 m) = 506.5 Pa m. From Figure 7Q10-1, the corresponding values of breakdown voltage for air (Vair) and for SF6 (VSF6) are: V air = 21000 V or 21.0 kV V SF6 = 50000 V or 50.0 kV 105 12 5 105 5 104 Breakdown voltage (V) 105 2.1 104 104 SF6 Air 103 5 102 10-1 1 101 102 5 5 103 5 Pressure Spacing (Pa m) Figure 7Q10-1 Breakdown voltage versus (pressure × electrode spacing) (Paschen curves) b At P = 10 atm = 1.013 × 106 Pa and d = 5 mm, P × d = (1.013 × 106 Pa)(0.005 m) = 5065 Pa m. Using linear extrapolation on Figure 7Q10-1: V air = 500000 V or 500 kV V SF6 = 1200000 V or 1200 kV 7.14 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 Pressure increases by 10 times but the breakdown voltages increase by a factor of about 25 times this is a good improvement. c With a gap length of 5 mm, we need to know the pressure at which the breakdown voltage is a minimum. From the graph, the minimum breakdown voltage of air is about Vair = 250 V, and the minimum for SF6 is about VSF6 = 420 V. The corresponding values for P × d are (P × d)air = 0.62 Pa m and (P × d)SF6 = 0.2 Pa m. From these we can determine the values of pressure needed for minimum breakdown voltage: Pair d = 0.62 Pa m PSF6 d = 0.2 Pa m 0.62 Pa m 0.005 m ∴ Pair = PSF6 = ∴ Pair = 124 Pa or 0.00122 atm 0.2 Pa m 0.005 m PSF6 = 40.0 Pa or 0.000395 atm A low pressure is needed for minimum breakdown which explains why discharge tubes operate at a low pressure. d At a set pressure P = 1 atm = 1.013 × 105 Pa, the air gap spacing d for minimum breakdown voltage can be found in a similar manner to the one above, using the same values for P × d: Pdair = 0.62 Pa m 0.62 Pa m 1.013 × 10 5 Pa ∴ dair = ∴ d air = 6.12 × 10-6 m This value corresponds to a breakdown voltage of 250 V. Therefore a gap of about 6 µm will only need 250 V for breakdown. e HV and high current switches or relays that have moving parts cannot be practically insulated using solid dielectrics. Liquid dielectrics are not as efficient as gaseous dielectrics because some undergo chemical changes under partial discharges. Further, they have a higher viscosity than gases that may affect the efficiency of the moving parts. Gas naturally permeates all the necessary space or locations where insulation is critical. *7.11 Capacitor design Consider a nonpolarized 100 nF capacitor design at 60 Hz operation. Note that there are three candidate dielectrics, as listed in Table 7Q11-1. a. Calculate the volume of the 100 nF capacitor for each dielectric, given that they are to be used under low voltages and each dielectric has its minimum fabrication thickness. Which one has the smallest volume? b. How is the volume affected if the capacitor is to be used at a 500 V application and the maximum field in the dielectric must be a factor of 2 less than the dielectric strength? Which one has the smallest volume? c. At a 500 V application, what is the power dissipated in each capacitor at 60 Hz operation? Which one has the lowest dissipation? 7.15 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Table 7Q11-1 Comparison of dielectric properties at 60 Hz (typical values) Polymer Film Ceramic PET TiO2 Name Polyester Polycrystalline titania 3.2 90 εr ′ tan δ 5 × 10-3 4 × 10-4 -1 150 50 Ebr (kV cm ) Typical minimum 1-2 µm 10 µm thickness Chapter 7 High-K Ceramic (BaTiO 3 based) X7R 1800 5 × 10-2 100 10 µm Solution Note: All sample calculations are for Polymer film (PET). All methods of calculation for the other materials are identical, and the obtained values are summarized in Table 7Q11-2. a To find the volume needed for C = 100 nF given that the dielectric has the minimum practical thickness, d (Table 7Q11-1), find the capacitance per unit volume (Cvol): Cvol −12 F/m )(3.2) ε oε r (8.854 × 10 = 2 = = 28.33 F/m 3 2 − 6 d (1 × 10 m) The volume V can now be found as follows: V = C / C vol = (100 × 10-9 F) / (28.33 F/m3) = 3.53 × 10-9 m3 This is the volume at low voltage operation based on the minimum practical thickness. b Suppose that d is the minimum thickness (in m) which gives a maximum field of half of Ebr at 500 V. Then: 1 V Ebr = max 2 d ∴ d=2 Vmax 500 V −5 = 2 = 6.667 × 10 m 7 1.50 × 10 V/m Ebr Now the capacitance per unit volume can be found: Cvol = ∴ −12 F/m )(3.2) ε oε r (8.854 × 10 = = 0.006374 F/m 3 2 2 −5 d (6.667 × 10 m) V = C / C vol = (100 × 10-9 F) / (0.006374 F/m3) = 1.57 × 10-5 m3 This is the dielectric volume at 500 V. c The power dissipation in the capacitor at 500 V (60 Hz operation) can be found by first obtaining the power lost per unit volume Wvol. It is given by: Wvol = Ebr 2 ωε oε r′ tan δ η2 where η is the safety factor (assumed to be equal to 2) and ω = 2πf is the angular frequency. Evaluating: 7.16 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Wvol ∴ (1.5 × 10 = 7 (2 ) 2 V/m ) 2 (2π (60 Chapter 7 Hz))(8.854 × 10 −12 F/m )(3.2)(5 × 10 −3 ) Wvol = 3004 W/m3 The power dissipated (W) is therefore: W = WvolV = (3004 W/m3)(1.57 × 10-5 m3) ∴ W = 0.0472 W Table 7Q11-2 Summarized values for volume and power of given capacitors. Polymer Film Ceramic High-K Ceramic PET TiO2 (BaTiO3 based) a Low voltage volume (m3) 3.53 × 10- 9 1.25 × 10- 8 6.27 × 10- 1 0 b High voltage volume (m3) 1.57 × 10- 5 5.02 × 10- 6 6.27 × 10- 8 c Power dissipated (W) 0.0472 0.00377 0.471 Upon inspection we see that for part a and part b, high-K ceramic has the smallest volumes, and for part c, ceramic has the lowest power dissipation. *7.12 Dielectric breakdown in a coaxial cable Consider a coaxial underwater high-voltage cable as in Figure 7Q12-1a. The current flowing through the inner conductor generates heat, which has to flow through the dielectric insulation to the outer conductor where it will be carried away by conduction and convection. We will assume that steady state has been reached and the inner conductor is carrying a dc current I. Heat generated per unit second, Q′ = dQ /dt, by joule heating of the inner conductor is Q′ = RI 2 = ρLI 2 πa 2 Rate of heat generation [1] where ρ is the resistivity, a the radius of the conductor, and L the cable length. This heat flows radially out from the inner conductor through the dielectric insulator to the outer conductor, then to the ambient. This heat flow is by thermal conduction through the dielectric. The rate of heat flow Q′ depends on the temperature difference Ti – T o , between the inner and outer conductors; on the sample geometry (a, b and L); and on the thermal conductivity κ of the dielectric. From elementary thermal conduction theory, this is given by Q′ = (Ti − To ) 2πκL b ln a Rate of heat conduction [2] The inner core temperature, Ti, rises until, in the steady state, the rate of joule heat generation by the electric current in Equation 1 is just removed by the rate of thermal conduction through the dielectric insulation, given by Equation 2. a. Show that the inner conductor temperature is 7.17 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ρI 2 b Ti = To + 2 2 ln a 2π a κ Chapter 7 [3] Steady state inner conductor temperature b. The breakdown occurs at the maximum field point, which is at r = a, just outside the inner conductor, and is given by (see Example 7.10 in the textbook) Emax = V b a ln a [4] Maximum field in a coaxial cable The dielectric breakdown occurs when Emax reaches the dielectric strength Ebr . However the dielectric strength Ebr for many polymeric insulation materials depends on the temperature, and generally it decreases with temperature, as shown for a typical example in Figure 7Q12-1b. If the load current, I, increases, then more heat, Q′, is generated per second and this leads to a higher inner core temperature, Ti, by virtue of Equation 3. The increase in Ti with I eventually lowers Ebr so much that it becomes equal to Emax and the insulation breaks down (thermal breakdown). Suppose that a certain coaxial cable has an aluminum inner conductor of diameter 10 mm and resistivity 27 nΩ m. The insulation is 3 mm thick and is a polyethylene-based polymer whose long-term dc dielectric strength is shown in Figure 7Q12-1b. Suppose that the cable is carrying a voltage of 40 kV and the outer shield temperature is the ambient temperature, 25 °C. Given that the thermal conductivity of the polymer is about 0.3 W K-1 m-1, at what dc current will the cable fail? [ ] Dielectric V a b Ti Heat Q' To Dielectric Strength (MV/m) c. Rederive Ti in Equation 3 by considering that r depends on the temperature as ρ = ρ0 1 + α 0 (T − T0 ) (Chapter 2 of the textbook). Recalculate the maximum current in b given that αo = 3.9 × 10-3 °C-1 at 25 °C. 60 50 40 30 20 10 0 -50 0 50 100 Temperature (°C) 150 (b) (a) Figure 7Q12-1 (a) The joule heat generated in the core conductor flows outward radially through the dielectric material. (b) Typical temperature dependence of the dielectric strength of a polyethylene-based polymeric insulation. Solution a The resistance R of the inner conductor is: R= ρL ρL = A πa 2 where ρ is the resistivity of the conductor, L is the length and a is the radius. 7.18 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 Joule heating power generated by the current I through the conductor (P) is given by: P = I2R = I 2 ρL πa 2 Eqn. [1] The rate of heat conduction (Q′) from inner conductor to outer conductor is: Q′ = (Ti − To ) 2πκL b ln a Eqn. [2] In the steady state, the rate of Joule heating of the inner conductor, P, is equal to the rate of heat flow, Q′, through the insulator. Therefore: I2 ρL 2πκL = (Ti − To ) 2 b πa ln a ρL ∴ (Ti − To ) = I 2 2κπ 2 a 2 L ln a ∴ Ti = To + b ρI 2 b ln 2 2 2κπ a a Note: A major assumption is the inner conductor resistivity ρ and thermal conductivity κ are constant (do not change with temperature). b We are given the cable’s characteristics: resistivity ρ = 27 × 10-9 Ω, thermal conductivity κ = 0.3 -1 W K m-1 (polyethylene material at 25 ˚C), inner conductor radius a = 0.005 m, and total radius b = 0.005 m + 0.003 m = 0.008 m. The outer temperature is given as To = 25 ˚C + 273 = 298 K, and the applied voltage is 40 kV. The maximum field Emax depends on the voltage V by Equation 4: Emax V 40 × 10 3 V = = = 1.702 × 107 V/m or 17.02 MV/m . m b 0 008 a ln (0.005 m ) ln a 0.005 m 60 50 40 Dielectric Strength (MV/m) 30 20 90°C 10 0 -50 0 50 100 Temperature (°C) 150 Figure 7Q12-2 Temperature dependence of the dielectric strength of a polyethylene-based polymeric insulation. The breakdown field Ebr is equal to this maximum field Emax when the inner temperature Ti = 90 ˚C = 363 K (see Figure 7Q12-2 above). Now, from Equation 3: 7.19 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Ti = To + Chapter 7 ρI 2 b ln 2 2 2κπ a a 2(0.3 W K −1 m −1 )π 2 (0.005 m ) (363 K − 298 K ) m (27 × 10 −9 Ω m) ln 00..008 005 m 2κπ 2 a 2 (Ti − To ) = b ρ ln a ∴ I= ∴ I = 871 A 2 c As in a for the steady state, the rate of Joule heating of the inner conductor, P, is equal to the rate of heat flow, Q′, through the insulator but now ρ is temperature dependent. Therefore: I2 ρ0 [1 + α 0 (Ti − To )]L πa 2 = (Ti − To ) 2πκL b ln a (5) Solving for Ti and simplifying we receive Ti = To + I2 2π 2 a 2κ − I 2α 0 b ρ0 ln a Now we have to calculate at what current Ti will reach 90 °C and hence thermal breakdown will occur. Solving for I Equation (5) we have I= 2π 2 a 2κ (Ti − To ) b ρ0 [1 + α 0 (Ti − To )] ln a So that 2π 2 (5 × 10 −3 m ) (0.3 W K −1 m −1 )((90 − 25) K ) = 777.8 A I= 8 −9 −3 27 10 Ω m 1 3 . 9 10 90 25 K ln × + × − ( ) ) 5 ( ) ( )( 2 [ ] 7.13 Piezoelectricity Consider a quartz crystal and a PZT ceramic filter both designed for operation at fs = 1 MHz. What is the bandwidth of each? Given Young's modulus (Y), density (ρ ) of each, and that the filter is a disk with electrodes and is oscillating radially, what is the diameter of the disk for each material? For quartz, Y = 80 GPa and ρ = 2.65 g cm-3. For PZT, Y = 70 GPa and ρ = 7.7 g cm-3. Assume that the velocity of mechanical oscillations in the crystal is v = Y ρ and the wavelength λ = v/f s. Consider only the fundamental mode (n = 1). 7.20 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 Solution We are given the first resonance frequency, fs = 1 MHz. There is a second resonance frequency, fa, related to the first by (see example 7.13, pg. 565 in the textbook)): fa = fs 1 − K2 where K is the electromechanical conversion factor. From Table 7.7 (pg. 561 in the textbook), Kquartz = 0.1 and KPZT = 0.72. The second resonance frequencies can then be found: Quartz: fa = fs 1− K 2 = (1 × 10 6 Hz) 1 − (0.1) 2 = 1.005 × 106 Hz The bandwidth Bquartz is defined as the difference between these two frequencies: ∴ B quartz = fa - fs = 1.005 × 106 Hz - 1 × 106 Hz = 5000 Hz PZT: fa = ∴ fs 1− K 2 (1 × 10 = 6 Hz) 1 − (0.72) 2 = 1.441 × 106 Hz B PZT = fa - fs = 1.441 × 106 Hz - 1 × 106 Hz = 4.41 × 105 Hz A much wider bandwidth than the quartz crystal. Next, we need to find the diameter of the disks of each material. We are given Young’s modulus (Yquartz = 80 × 109 Pa, YPZT = 70 × 109 Pa) and the density (ρquartz = 2650 kg/m3, ρPZT = 7700 kg/m3). From these we can determine the velocity and wavelength of the mechanical oscillations in the crystal, and then the diameter (L) from Equation 7.50 (in the textbook). Quartz: Yquartz vquartz = ρquartz (80 × 10 Pa) (2650 kg/m ) 9 = 3 = 5494 m/s vquartz 5494 m/s = = 0.005494 m fs 1 × 10 6 Hz ∴ λquartz = ∴ 1 1 Lquartz = n λquartz = 1 (0.005494 m ) = 0.00275 m or 2.75 mm 2 2 PZT: vPZT = YPZT = ρPZT (70 × 10 Pa) = 3015 m/s (7700 kg/m ) 9 3 vPZT 3015 m/s = = 0.003015 m fs 1 × 10 6 Hz ∴ λ PZT = ∴ 1 1 LPZT = n λ PZT = 1 (0.003015 m ) = 0.00151 m or 1.51 mm 2 2 7.21 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 7.14 Piezoelectric voltage coefficient The application of a stress T to a piezoelectric crystal leads to a polarization P and hence to an electric field E in the crystal such that E = gT Piezoelectric voltage coefficient where g is the piezoelectric voltage coefficient. If εoεr is the permittivity of the crystal, show that d ε oε r g= A BaTiO3 sample, along a certain direction (called 3), has d = 190 pC N-1, and its εr ≈ 1900 along this direction. What do you expect for its g coefficient for this direction and how does this compare with the measured value of approximately 0.013 C-1 m2? Solution F F Piezoelectric A L Piezoelectric V Piezoelectric F F (a) (b) Figure 7Q14-1 The piezoelectric spark generator. The electric field E in the piezoelectric sample due to induced voltage V over the sample (see Figure 7Q14-1a) is E= V L The induced voltage is proportional to the induced charge Q by Q= V , C where C is the capacitance of the sample. The induced charge in his turn is related to the polarization P by Q = AP And finally, the capacitance of parallel plate capacitor is given by C= ε 0ε r A L 7.22 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 Combining these expressions we receive E= P V Q AP = = = A L CL ε ε L ε0 εr 0 r L (1) The applied stress T and the induced polarization P are related through the piezoelectric voltage coefficient d P=dT Thus substituting in (1) we can obtain the relation between the electric field E and T E= d T ε 0ε r A direct comparison between this expression and the piezoelectric voltage coefficient definition shows that g= d ε 0ε r The piezoelectric voltage coefficient for the BaTiO3 sample described in the problem is 190 × 10 −12 C N −1 ) ( d g= = = 0.0113 C -1 m2 ε oε r (8.8534 × 10 −12 F m −1 )(1900) The received value is in pretty good agreement with the experimental one. 7.15 Piezoelectricity and the piezoelectric bender a. Consider using a piezoelectric material in an application as a mechanical positioner where the displacements are expected to be small (as in a scanning tunneling microscope). For the piezoelectric plate shown in Figure 7Q15-1a, we will take L = 20 mm, W = 10 mm, and D (thickness) = 0.25 mm. Under an applied voltage of V, the plate changes length, width, and thickness according to the piezoelectric coefficients dij, relating the applied field along i to the resulting strain along j. Suppose we define direction 3 along the thickness D and direction 1 along the length L, as shown in the figure. Show that the changes in the thickness and length are δD = d33 V Piezoelectric effects L δL = d31V D Given d33 ≈ 500 × 10-12 m V-1 and d31 ≈ –250 × 10-12 m V-1, calculate the changes in the length and thickness for an applied voltage of 100 V. What is your conclusion? b. Consider two oppositely poled and joined ceramic plates, A and B, forming a bimorph, as shown in Figure 7Q15-1b. This piezoelectric bimorph is mounted as a cantilever; one end is fixed and the other end is free to move. Oppositely poled means that the electric field elongates A and contracts B, and the two relative motions bend the plate. The displacement h of the tip of the cantilever is given by h= 3 L2 d31 V 2 D Piezoelectric bending What is the deflection of the cantilever for an applied voltage of 100 V? What is your conclusion? 7.23 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) W Chapter 7 V D Piezoelectric V L 3 D 2 L 2 A B 1 h 0 (a) (b) Figure 7Q15-1 (a) A mechanical positioner using a piezoelectric plate under an applied voltage of V. (b) A cantilever-type piezoelectric bender. An applied voltage bends the cantilever. Solution a The strain along direction 3 is given by the change in thickness divided by the thickness, S3 = δD/D, and the electric field along 3 is given by E3 = V/D where V is the voltage. The piezoelectric effect relates the strain S and the field E via Equation 7.47 (in the textbook), such that: S 3 = d 33E 3 ∴ δD V = d33 D D ∴ δD = d33V Along direction 1, the strain is the change in length divided by the length, S1 = δL/L. By the piezoelectric effect (Equation 7.47 in the textbook): S 1 = d 31E 3 ∴ δL V = d31 L D ∴ L δL = d31V D Now, using the given values, we can find the change in thickness and length for 100 V: δD = d33V = (500 × 10 −12 m/V)(100 V) = 5.00 × 10-8 m L 0.02 m δL = d31V = −250 × 10 −12 m/V)(100 V) = -2.00 × 10-6 m D 0.00025 m ( There is a much greater change in the length than in the thickness of the plate. b The cantilever tip is displaced by h, whose magnitude is: 2 h= ∴ 2 3 3 0.02 m L d31 V = ( −250 × 10 −12 m/V) (100 V) 0.00025 m D 2 2 h = 0.000240 m or 0.240 mm The cantilever configuration provides a greater displacement than the above cases. 7.24 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 7.16 Piezoelectricity The wavelength, λ, of mechanical oscillations in a piezoelectric slab satisfies 1 n λ = L 2 where n is an integer, L is the length of the slab along which mechanical oscillations are set up, and the wavelength λ is determined by the frequency f and velocity υ of the waves. The ultrasonic wave velocity υ depends on Young's modulus Υ as 1/ 2 Y υ= ρ where ρ is the density. For quartz, Y = 80 GPa and ρ = 2.65 g cm-3. Considering the fundamental mode (n = 1), what are practical dimensions for crystal oscillators operating at 1 kHz and 1 MHz? Solution We are given the characteristics of quartz, Y = 80 × 109 Pa and ρ = 2650 kg/m3. We can then calculate the ultrasonic wave velocity, υ: υ= Y 80 × 10 9 Pa = = 5494 m/s 2650 kg/m 3 ρ First, consider f = 1 kHz: λ= υ 5494 m/s = = 5.494 m f 1000 Hz The length L of the quartz crystal at 1 kHz at the fundamental mode (n = 1) is therefore: 1 1 L = n λ = (1) × 5.494 m = 2.75 m 2 2 A huge crystal, very impractical. Now, consider f′ = 1 MHz: λ′ = ∴ υ 5494 m/s = 0.005494 m = f ′ 1 × 10 6 Hz 1 1 L′ = n λ = (1) × 0.005494 m = 0.00275 m or 2.75 mm 2 2 This is a much more practical value. Note: Using n > 1 increases the size of the quartz crystal. 7.25 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 7.17 Pyroelectric detectors Consider two different radiation detectors using PZT and PVDF as pyroelectric materials whose properties are summarized in Table 7Q17-1. The receiving area is 4 mm2. The thicknesses of the PZT ceramic and the PVDF polymer film are 0.1 mm and 0.005 mm respectively. In both cases the incident radiation is chopped periodically to allow the radiation to pass for a duration of 0.05 seconds. a. Calculate the magnitude of the output voltage for each detector if both receive a radiation of intensity 10 µW cm-2. What is the corresponding current in the circuit? In practice, what would limit the magnitude of the output voltage? b. What is the minimum detectable radiation intensity if the minimum detectable signal voltage is 10 nV? Table 7Q17-1 PZT PVDF Properties of PZT and PVDF Pyroelectric ε r’ Coefficient (× 10-6 C m-2 K-1) 290 380 12 27 Density (g cm-3 ) Heat Capacity (J K-1 g-1 ) 7.7 1.76 0.3 1.3 Solution a The change in voltage is given by Equation 7.54 (in the textbook): ∆V = pI∆t ρcε r ε o where p is the pyroelectric coefficient, ρ is the density, c is the specific heat capacity, I is the light intensity, and ∆t is the duration of radiation. The output voltages for the two materials can then be calculated: (380 × 10 C m (7700 kg/m )(300 J K −6 ∆VPZT = ∴ 3 −1 K −1 )(0.1 W/m 2 )(0.05 s) kg −1 )(290)(8.854 × 10 −12 F/m ) ∆V P Z T = 0.000320 V (27 × 10 C m K )(0.1 W/m )(0.05 s) = (1760 kg/m )(1300 J K kg )(12)(8.854 × 10 −6 ∆VPVDF ∴ −2 −2 −1 −1 3 2 −1 −12 F/m ) ∆V PVDF = 0.000555 V The corresponding currents can be found in two ways: (1) Neglecting thermal conduction, the current is equal to the change in polarization P over time, or: i= dP dT pdH =p = dt dt mcs dt where p is the pyroelectric coefficient, m is the mass, cs is the specific heat capacity and dH is the change in heat. The change in heat can be expressed as dH = IAdt, and the mass can be found from the density, ρ = m/V, i.e. m = ρAd. Substituting into the previous equation, we obtain: 7.26 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) i= pI ρdcs (380 × 10 C m K )(0.1 W m ) = 1.65 (7700 kg m )(0.1 × 10 m)(300 J K kg ) −6 iPZT = similarly, (2) Chapter 7 −2 −3 −1 −2 −3 −1 −1 × 10-7 A i PVDF = 2.36 × 10-7 A We can also estimate through the equation for current in a capacitor (where d is the thickness): i=C iPZT similarly, dV ∆V ε oε r A∆V ≈C ≈ dt ∆t d∆t (8.854 × 10 ≈ −12 F/m )(290)( 4 × 10 −6 m 2 )(0.000320 V) (0.1 × 10 −3 m )(0.05 s) = 6.57 × 10-13 A i PVDF ≈ 9.43 × 10-13 A Remember that these are only approximations and that the first method is more accurate. Heat losses, e.g. thermal conduction, would prevent the absorbed heat from raising the temperature indefinitely. The calculation assumes that the rise in the temperature is small so that the heat losses, proportional to the temperature difference, can be neglected. b Rearrange the equation for change in voltage and isolate I: I= ∆Vρcε r ε o ∆tp The minimum signal is given as ∆V′ = 10 × 10-9 V. Substituting to find the detectable light intensity: I PZT = ∴ −9 V)(7700 kg/m 3 )(300 J K −1 kg −1 )(290)(8.854 × 10 −12 F/m ) (0.05 s)(380 × 10 −6 C m −2 K −1 ) I PZT = 3.12 × 10-6 W/m2 I PVDF ∴ (10 × 10 (10 × 10 = −9 (Small - advantage of pyroelectric detectors) V)(1760 kg/m 3 )(1300 J K −1 kg −1 )(12)(8.854 × 10 −12 F/m ) (0.05 s)(27 × 10 −6 C m −2 K −1 ) I PVDF = 1.80 × 10-6 W/m2 (about the same order of magnitude as PZT) Note: Notice that we did not need the pyroelectric material thickness (Example 7.15 in the textbook). This is because we neglected the thermal conduction loss. If we were to include heat losses via thermal conduction then we would indeed need the thickness - this matter is treated in advanced texts. *7.18 Pyroelectric detectors Consider a typical pyroelectric radiation detector circuit as shown in Figure 7Q18-1. The FET circuit acts as a voltage follower (source follower). The resistance R1 represents the input resistance of the FET in parallel with a bias resistance that is usually inserted between the gate and source. C1 is the overall input capacitance of the FET including any stray capacitance but excluding the capacitance of the pyroelectric detector. Suppose that the incident radiation intensity is constant and equal to I. Emissivity 7.27 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 η of a surface characterizes what fraction of the incident radiation that is absorbed. ηI is the energy absorbed per unit area per unit time. Some of the absorbed energy will increase the temperature of the detector and some of it will be lost to surroundings by thermal conduction and convection. Let the detector receiving area be A, thickness be L, density be ρ , and specific heat capacity (heat capacity per unit mass) be c. The heat losses will be proportional to the temperature difference between the detector temperature, T, and the ambient temperature, T o , as well as the surface area A (much greater than L). Energy balance requires that Rate of increase in the internal energy (heat content) of the detector = Rate of energy absorption – Rate of heat losses that is, ( ALρ )c dT = AηI − KA(T − To ) dt where K is a constant of proportionality that represents the heat losses and hence depends on the thermal conductivity κ . If the heat loss involves pure thermal conduction from the detector surface to the detector base (detector mount), then K = κ/L. In practice, this is generally not the case and K = κ/L is an oversimplification. a. Show that the temperature of the detector rises exponentially as T = To + t ηI 1 − exp − K τ th Detector temperature where τth is a thermal time constant defined by τth = Lρc/K. Further show that for very small K , the above equation simplifies to T = To + ηI t Lρc b. Show that temperature change dT in time dt leads to a pyroelectric current, ip, given by i p = Ap t dT ApηI exp − = dt Lρc τ th Pyroelectric current where p is the pyroelectric coefficient. What is the initial current? c. The voltage across the FET and hence the output voltage v (t) is given by t t v(t ) = Vo exp − − exp − τ el τ th Pyroelectric detector output voltage where V 0 is a constant and τel is the electrical time constant given by R 1C t , where Ct , total capacitance, is (C1 + C det), where Cdet is the capacitance of the detector. Consider a particular PZT pyroelectric detector with an area 1 mm2, and thickness 0.05 mm. Suppose that this PZT has εr = 250, ρ = 7.7 g cm-3, c = 0.3 J K-1 g-1, and κ = 1.5 W K-1 m -1. The detector is connected to an FET circuit that has R1 = 10 MΩ and C1 = 3 pF. Taking the thermal conduction loss constant K as κ /L, and η = 1, calculate τth and τel. Sketch schematically the output voltage. What is your conclusion? 7.28 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Radiation Receiving surface, A Chapter 7 FET R1 L C1 v(t) Rs Cdet Figure 7Q18-1 A pyroelectric detector with an FET voltage follower circuit. Solution a Energy balance requires the following: ALρc dT = AηI − KA(T − To ) dt (1) Consider the given expression for the temperature with thermal time constant τth = Lρc/K: T = To + t ηI 1 − exp − K τ th (2) We can show that Eqn. (2) is a solution by substituting Eqn. (2) into Eqn. (1) and verifying that is indeed a solution. Mathematicians prefer solving Eqn. (1) directly and this is how Eqn. (2) was actually obtained in the first place. However, given the expression in Eqn. (2), the simplest method is to substitute Eqn. (2) into Eqn. (1) and show that it is satisfied. Substitute for T in Eqn. (1): ALρc t t d ηI ηI 1 exp I 1 exp = − + − − − T + − − A η KA T T o o o K dt K τ th τ th ∴ Lρc t t ηI 1 − − exp − = ηI − ηI 1 − exp − K τ th τ th τ th ∴ Lρc t ηI 1 t exp − = ηI exp − K τ th τ th τ th substitute for τth: Lρc t ηI 1 t exp − = ηI exp − K Lρc K τ th τ th simplify: ηI = ηI As the right hand side equals the left hand side, Eqn. (1) is satisfied and Eqn. (2) is a solution. For small K, we can expand the exponential of Eqn. (2) (exp(-t / τth) = exp(-Kt / LρC)), i.e.: Assuming x is small and neglecting second order and higher order terms (x2 and higher), exp(-x) = 1 - x. T = To + Kt ηI ηI Kt 1 − exp − 1 − 1 − = To + K K Lρc Lρc 7.29 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ T = To + ηI Kt K Lρc ∴ T = To + ηI t Lρc Chapter 7 Therefore, for small loss (K small) or for sufficiently short periods of time (t small), the temperature rise is linear with time. b The pyroelectric current ip is given by: ip = Ap dT dt This is so because in time dt, the temperature changes by dT. The change in the polarization is p = dP/dT. This changes the charge on the pyroelectric crystal by an amount dQ = AdP = A(pdT). The current ip is dQ/dt, hence the above equation. Substitute Eqn. (2) into the above: ip = Ap t ηI d To + 1 − exp − dt K τ th ∴ ip = Ap t t ηI 1 ηI 1 − − exp − − − exp − = Ap K τ th K Lρc K τ th τ th ∴ ip = t ApηI exp − Lρc τ th To find the initial current, substitute t = 0. ip (0) = ApηI ApηI exp(0) = Lρc Lρc c To find the electrical time constant τel, we need the capacitance of the detector Cdet, which can be found as follows (relative permittivity εr = 250, area A = 1 mm2, thickness L = 0.05 mm): Cdet = −12 F/m )(250)(1 × 10 −6 m 2 ) ε oε r A (8.854 × 10 = 4.427 × 10-11 F = L 0.05 × 10 -3 m Now the total capacitance Ct can be found (FET circuit capacitance C1 is given as 3 pF): Ct = C1 + Cdet = 3 × 10-12 F + 4.427 × 10-11 F = 4.727 × 10-11 F and the electrical time constant is then (where FET circuit resistance R1 = 10 MΩ): τel = R 1C t = (10 × 106 Ω)(4.727 × 10-11 F) = 0.000473 s To find the thermal time constant τth, we need the constant of proportionality K (where κ is the thermal conductivity and L is the thickness): K = κ / L = (1.5 W K-1 m-1) / (0.05 × 10-3 m) = 30000 W K-1 τth is therefore: 7.30 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 7 −3 3 −1 −1 Lρc (0.05 × 10 m )(7700 kg/m )(300 J K kg ) τth = = = 0.00385 s K 30000 W K −1 For the sketch, we can set Vo to 1 for the purpose of simplicity. Note: Intensity is not required as the output is a schematic sketch. The magnitude of the output voltage is: t t v(t ) = Vo exp − − exp τ el τ th The output peaks and drops. This is because R1 leaks the induced charge on the pyroelectric sample to ground at a rate determined by the electrical time constant τel. If we were to set τel = ∞ (R1 = ∞) then there would be no decay in the output voltage v, at least in theory. However, in practice there is always some leakage or finite R1. 0.8 v (V) 0.6 0.4 0.2 0.02 0.04 0.06 t (sec) Figure 7Q18-2 Schematic plot of output voltage. Note that the light intensity is switched on suddenly at t = 0, i.e. it is a step excitation. 7.19 Spark generator design Design a PLZT piezoelectric spark generator using two back-to-back PLZT crystals that provide a 60 µJ spark in an air gap of 0.5 mm from a force of 50 N. At 1 atmosphere in an air gap of 0.5 mm, the breakdown voltage is about 3000 V. The design will need to specify the dimensions of the crystal and the dielectric constant. Assume that the piezoelectric voltage coefficient is 0.023 V m N-1. Solution Given, F = applied force = 50 N to be applied through a mechanical lever arrangement. Two ceramics are used to obtain a greater energy in the spark. Energy stored in each ceramic just before breakdown has to be 30 µJ. The stress T induces an electric field E and hence a voltage V given by, E = gT (1) where g is the piezoelectric voltage coefficient. Thus, at breakdown when V = Vbr, Vbr F =g L A ∴ L Vbr = A gF 7.31 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) ∴ Chapter 7 L (3000 V) = = 2608 m-1. A (0.023 V m N -1 )(50 N) When V reaches Vbr and the air breaks down, the energy stored on the piezoelectric sample capacitance is very roughly the energy in the spark, Equation (2), E= εε A 1 CV 2 ≈ o r Vbr2 2 2L (2) 2E L 2( 30 × 10−6 J) = εr ≈ 2.6 × 10 3 m-1 ) = 1958 −12 2 -1 2 ( εoVbr A (8.85 × 10 F m )( 3000 V) This is the minimum dielectric constant needed. Recall the units relationship: [J] = [F][V]2. Taking a crystal length L of 5 mm gives, A = 0.0055/2.6 × 103 m2 or 1.92 mm2. This means a sample diameter of 1.56 mm. Note: g determines the force needed to spark the gap and hence determines the crystal dimensions. εr determines the amount of energy in the gap. This the reason for giving g and requesting L/A and εr. Alternatively one can give εr and request g. The problem can also be approached in terms of d and εr but it is more usual to use g when a voltage needs to be generated; of course g = d/(εoεr) "In Zipf's law the quantity under study is inversely proportional to the rank, that is, proportional to 1, 1/2, 1/3, 1/4, etc." Murray Gell-Mann (California Institute of Technology; Nobel Laureate 1969) 7.32 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 2001 McGraw-Hill) Second Edition ( Chapter 8 8.1 Inductance of a long solenoid Consider the very long (ideally infinitely long) solenoid shown in Figure 8Q1-1. If r is the radius of the core and l is the length of the solenoid, then l >> r. The total number of turns is N and the number of turns per unit length is n = N/l. The current through the coil wires is I. Apply Ampere's law around C, which is the rectangular circuit PQRS, and show that B ≈ µoµrnI Further, show that the inductance is L ≈ µoµrn2Vcore Inductance of long solenoid where Vcore is the volume of the core. How would you increase the inductance of a long solenoid? What is the approximate inductance of an air-cored solenoid with a diameter of 1 cm, length of 20 cm, and 500 turns? What is the magnetic field inside the solenoid and the energy stored in the whole solenoid when the current is 1 A? What happens to these values if the core medium has a relative permeability µr of 600? C S R P I r Q B r n = Turns per unit length Figure 8Q1-1 Solution We use Ampere's law in Equation 8.15 (in the textbook). Consider Figure 8Q1-2. If H is the field along a small length dl along a closed path C, then around C, ∫Hdl = total threaded current = Itotal = NI. I O C r dl P Ht Figure 8Q1-2 Ampere’s circuital law Assume that the solenoid is infinitely long. The rectangular loop PQRS has n(PQ) number of turns where n is the number of turns per unit length or n = N/l (See Figure 8Q1-1). The field is only inside the solenoid and only along the PQ direction (long solenoid assumption) and therefore the field along QR, RS and SP is zero. Assume that the field H is uniform across the solenoid core cross section. Then the path 8.1 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 integral of the magnetic field intensity H around PQRS is simply is Hl = H(PQ). Ampere's law ∫Hdl = Itotal is then H(PQ) = I(nPQ) i.e. H = nI The dimensions of the solenoid are such that length >> diameter. We can assume that H field is relatively uniform at all points inside the solenoid. Note: The approximate equality sign in the text (equation for B) is due to the fact that we assumed H is uniform across the core and, further, along the whole length of the solenoid from one end to the other. The ends of the solenoid will have different fields (lower). Let A be the cross-sectional area of the solenoid. The magnetic field B, the flux Φ and hence the inductance L are B = µoµrH ≈ µoµrnI ∴ Φ = BA ≈ µoµrnAI = µoµr (N/l)AI and L = (NΦ)/I = N[µoµr(N/l)AI]/I = µoµr(N2/l)A = µoµrn2(lA) ∴ L = µoµrn2Vcore where Vcore is the volume of the core. Inductance depends on n2, where n is the number of turns per unit length, on the relative permeability µr and on the volume of the core containing the magnetic flux. For a given volume inductor, L can be increased by using a higher µr material or increasing n, e.g. thinner wire to get more turns per unit length (not so thin that the skin effect diminishes the Q-factor, quality factor; see §2.8 in the textbook). The theoretical inductance of the coil is L = (4π × 10-7 H/m)(1)[(500)/(0.2 m)]2(0.2 m)(π)[(0.01 m)/(2)]2 ∴ L = 1.23 × 10-4 H or 0.123 mH and B ≈ (4π × 10-7 Wb A-1 m-1)(1)[(500)/(0.2 m)](1 A) = 3.14 × 10-3 T The energy per unit volume is, Evol = B2/(2µo) = (3.14 × 10-3 T)2/[2(4π × 10-7 Wb A-1 m-1)] ∴ Evol = 3.92 J / m3 The total energy stored is then, Etot = Evol ( Length × Area ) = (3.92 J/m 3 )π 2 0.01 m (0.2 m) = 61.6 µJ 2 Suppose that µr = 600 and suppose that the core does not saturate (an ideal ferromagnetic material) then, L ≈ (1.23 × 10 −4 H)(600) = 0.0738 H B ≈ (3.14 × 10 −3 T )(600) = 1.88 T and Evol ≈ (1.88 T)2 2( 4π × 10 −7 Wb/A ⋅ m )(600) so that = 2344 J/m 3 Etot = (Evol)(Volume) = 36.8 mJ This is a dramatic increase and shows the virtue of using a magnetic core material for increasing the inductance and the stored magnetic energy. 8.2 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 8.2 Magnetization Consider a long solenoid with a core that is an iron alloy (see Problem 8.1 for the relevant formulas). Suppose that the diameter of the solenoid is 2 cm and the length of the solenoid is 20 cm. The number of turns on the solenoid is 200. The current is increased until the core is magnetized to saturation at about I = 2 A and the saturated magnetic field is 1.5 T. a. What is the magnetic field intensity at the center of the solenoid and the applied magnetic field, µoH, for saturation? b. What is the saturation magnetization Msat of this iron alloy? c. What is the total magnetization current on the surface of the magnetized iron alloy specimen? d. If we were to remove the iron-alloy core and attempt to obtain the same magnetic field of 1.5 T inside the solenoid, how much current would we need? Is there a practical way of doing this? Solution a Applying Ampere’s law or Hl = NI we have, NI (200)(2 A ) = l 0.2 m Since I = 2 A gives saturation, corresponding magnetizing field is Hsat ≈ 2000 A/m H= Suppose the applied magnetic field is the magnetic field in the toroid core in the absence of material. Then Bapp = µo Hsat = ( 4π × 10 −7 Wb A -1 m -1 )(2000 A/m ) Bapp = 2.51 × 10-3 T ∴ b Apply Bsat = µo ( Msat + Hsat ) 1.5 T Bsat − Hsat = − 2000 A/m −7 4π × 10 Wb A -1 m -1 µo ∴ Msat = ∴ Msat ≈ 1.19 × 106 A/m c Since M is the magnetization current per unit length, Im = Msat ≈ 1.19 × 106 A/m Then Isurface = Total circulating surface current: ∴ Isurface = Im l = (1.19 × 10 6 A/m )(0.2 m ) = 2.38 × 10 5 A Note that the actual current in the wires, 2 A is negligible compared with Isurface. d Apply, B ≈ µo nI (for air) I≈ 1.5 T (4π × 10 −7 Wb A -1 200 m ) 0.2 m = 1194 A -1 Not very practical in every day life! Perhaps this current (thus field B = 1.5 T) could be achieved by using a superconducting solenoid. 8.3 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 8.3 Pauli spin paramagnetism Paramagnetism in metals depends on the number of conduction electrons that can flip their spins and align with the applied magnetic field. These electrons are near the Fermi level EF, and their number is determined by the density of states g(EF) at EF. Since each electron has a spin magnetic moment of β, paramagnetic susceptibility can be shown to be given by χpara ≈ µo β2 g(EF) Pauli spin paramagnetism where the density of states is given by Equation 4.10 (in the textbook). The Fermi energy of calcium, EF, is 4.68 eV. Evaluate the paramagnetic susceptibility of calcium and compare with the experimental value of 1.9 × 10-5. Solution ( ) 3 Apply, m g ( E ) = 8π 2 2e h so that 9.109 × 10 −31 kg g ( EF ) = 8π 2 2 −34 (6.626 × 10 J ⋅ s) ∴ g ( EF ) = 9.197 × 10 46 J -1 m −3 Then, χpara ≈ µo β2 g(EF) ( 2 E (Equation 4.10) ) 3 2 (4.68 eV)(1.602 × 10 −19 J/eV) χ para ≈ ( 4π × 10 −7 Wb A -1 m -1 )(9.273 × 10 −24 A m 2 ) (9.199 × 10 46 J −1 m −3 ) 2 χ para ≈ 0.994 × 10-5 ∴ This is in reasonable agreement within an order of magnitude with the experimental value of 1.9 × 10-5. 8.4 Ferromagnetism and the exchange interaction Consider dysprosium (Dy), which is a rare earth metal with a density of 8.54 g cm-3 and atomic mass of 162.50 g mol-1. The isolated atom has the electron structure [Xe] 4f106s2. What is the spin magnetic moment in the isolated atom in terms of number of Bohr magnetons? If the saturation magnetization of Dy near absolute zero of temperature is 2.4 MA m–1, what is the effective number of spins per atom in the ferromagnetic state? How does this compare with the number of spins in the isolated atom? What is the order of magnitude for the exchange interaction in eV per atom in Dy if the Curie temperature is 85 K? Solution In an isolated Dy atom, the valence shells will fill in accordance with the exchange interaction: 4f 10 ↑ ↓ ↑ ↓ ↑ ↓ ↑ 6s2 ↑ ↑ ↑ ↓ ↑ Obviously, there are 4 unpaired electrons. Therefore for an isolated Dy atom, the spin magnetic moment = 4β. 8.4 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 Atomic concentration in dysprosium (Dy) solid is (where ρ is the density, NA is Avogadro’s number and Mat is the atomic mass): nat = 3 3 23 -1 ρN A (8.54 × 10 kg/m )(6.022 × 10 mol ) = = 3.165 × 10 28 m −3 Mat 162.50 × 10 −3 kg/mol Suppose that each atom contributes x Bohr magnetons, then Msat = nat xβ x= Msat 2.4 × 10 6 A/m = 8.18 = nat β (3.165 × 10 28 m −3 )(9.273 × 10 −24 A m 2 ) This is almost twice the net magnetic moment in the isolated atom. Suppose that the Dy atom in the solid loses all the 4 electrons that are paired into the "electron gas" in the solid. This would make Dy+4 have 8 unpaired electrons and a net spin magnetic moment of 8β (this is an oversimplified view). Exchange interaction ∼ kTC = (8.617 × 10 −5 eV/K )(85 K ) = 0.00732 eV The order of magnitude of exchange interaction ~ 10-2 eV/atom for Dy (small). *8.5 Toroidal inductor and radio engineers toroidal inductance equation a. Consider a toroidal coil (Figure 8Q5-1) whose mean circumference is l and has N tightly wound turns around it. Suppose that the diameter of the core is 2a and l >> a. By applying Ampere's law, show that if the current through the coil is I, then the magnetic field in the core is µo µr NI [8.30] l where µr is the relative permeability of the medium. Why do you need l >> a for this to be valid? Does this equation remain valid if the core cross section is not circular but rectangular, a × b, and l >> a and b? b. Show that the inductance of the toroidal coil is B= µo µr N 2 A Toroidal coil inductance [8.31] l where A is the cross-sectional area of the core. c. Consider a toroidal inductor used in electronics that has a ferrite core size FT-37, that is, round but with a rectangular cross section. The outer diameter is 0.375 in (9.52 mm), the inner diameter is 0.187 in (4.75 mm), and the height of the core is 0.125 in (3.175 mm). The initial relative permeability of the ferrite core is 2000, which corresponds to a ferrite called the 77 Mix. If the inductor has 50 turns, then using Equation 8.31, calculate the approximate inductance of the coil. d. Radio engineers use the following equation to calculate the inductances of toroidal coils, L= AL N 2 Radio engineers inductance equation [8.32] 10 6 where L is the inductance in millihenries (mH) and AL is an inductance parameter, called an inductance index, that characterizes the core of the inductor. AL is supplied by the manufacturers of ferrite cores and is typically quoted as millihenries (mH) per 1000 turns. In using Equation 8.32, one simply substitutes the numerical value of AL to find L in millihenries. For the FT-37 ferrite toroid with the 77 Mix as the ferrite core, AL is specified as 884 mH/1000 turns. What is the inductance of the toroidal inductor in part (c) from the radio engineers equation in Equation 8.32? What is the L( mH) = 8.5 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 percentage difference in values calculated by Equations 8.32 and 8.31? What is your conclusion? (Comment: The agreement is not always this close). H C A I N turns Figure 8Q5-1 A toroidal coil with N turns. Solution a As in Figure 8Q5-1, if we choose a closed path C that runs along the geometric center of the core and if I is the current, N is the number of turns and l = mean circumference, then: ∫ H dl = Hl = NI H = (NI)/l or t C µo µr NI l This particular derivation only applies in the special case of l >> radius (a); that is, for a very long, narrow core. If the core is very long and narrow, it may be safely assumed that the magnetic flux density B is uniform across the entire width (2a) of the core. If B was not uniform, then applying Ampere’s Law to different (concentric) closed paths would yield different results. This above derivation for B is valid for a rectangular cross-sectioned core of area a × b, provided that l >> a and l >> b. The magnetic field is then, ∴ B = µo µr H = µo µr NI l The inductance by definition is given by, B= b L= NΦ N ( BA) = = I I N µo µr NAI µo µr N 2 A l = I l c W rinner router H Figure 8Q5-2 Toroidal core with rectangular cross section. Given outer radius = router = 0.00476 m, inner radius = rinner = 0.002375 m, height = H = 0.003175 m. We take the mean circumference l through the geometric center of the core so that the mean radius is: 8.6 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) rl = Chapter 8 router − rinner + rinner = 3.5675 mm 2 l = 2πrl = (2π )(3.5675 × 10 −3 m ) = 22.42 × 10 −3 m ∴ Width = W = router - rinner = 0.002385 m A = Cross sectional area = W × H = (0.002385 m)(0.003175 m) = 7.572 × 10-6 m2. Since l >> a and l >> b (at least approximately) we can calculate L as follows: L= L= ∴ (4π × 10 −7 H/m )(2000)(50) (7.572 × 10 −6 m 2 ) 2 22.42 × 10 −3 m L = 0.00212 H or 2.12 mH ∴ d µo µr N 2 A l Using the radio engineer’s equation, AL N 2 (884 mH)(50) = = 2.21 mH 10 6 10 6 2.21 mH − 2.12 mH so that % difference = × 100% = 4.07% 2.21 mH Equations 2 and 3 differ only by 4.1% in this case. This is a good agreement. 2 L(mH) = *8.6 A toroidal inductor a. Equations 8.31 and 8.32 (in the textbook) allow the inductance of a toroidal coil in electronics to be calculated. Equation 8.32 (in the textbook) is the equation that is used in practice. Consider a toroidal inductor used in electronics that has a ferrite core of size FT-23 that is round but with a rectangular cross section. The outer diameter is 0.230 in (5.842 mm), the inner diameter is 0.120 in (3.05 mm), and the height of the core is 0.06 in (1.5 mm). The ferrite core is a 43-Mix that has an initial relative permeability of 850 and a maximum relative permeability of 3000. The inductance index for this 43Mix ferrite core of size FT-23 is AL =188 (mH/1000 turns). If the inductor has 25 turns, then using Equations 8.31 and 8.32 (in the textbook), calculate the inductance of the coil under small-signal conditions and comment on the two values. b. The saturation field, Bsat, of the 43-mix ferrite is 0.2750 T. What will be typical dc currents that will saturate the ferrite core (an estimate calculation is required)? It is not unusual to find such an inductor in an electronic circuit also carrying a dc current? Will your calculation of the inductance remain valid in these circumstances? c. Suppose that the above toroidal inductor is in the vicinity of a very strong magnet that saturates the magnetic field inside the ferrite core. What will be the inductance of the coil? Solution a Provided that the mean circumference is much greater than any long cross sectional dimension (e.g. l >> diameter or l >> a and l >> b), then we can use, µo µr N 2 A l We need the mean circumference l which can be calculated from the mean radius rl, L≈ 8.7 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 router − rinner + rinner = 2.223 mm so that l = 2πrl = 13.97 mm 2 Width = W = router - rinner = 0.001396 m and Height = H = 0.0015 m. A = Cross sectional area = W × H rl = Since l >> a and l >> b (at least approximately) we can calculate L as follows: µo µr N 2 (WH ) L≈ l (4π × 10 L≈ −7 H/m )(850)(25) (1.396 × 10 −3 m )(1.5 × 10 −3 m ) 2 13.97 × 10 −3 m L ≈ 0.10 mH And, the radio engineer’s equation, Equation 8.32 (in the textbook), (radio engineers toroidal inductance equation, pg. 6.25 and data pg. 24.7 in The ARRL Handbook 1995) AL N 2 (188 mH)(25) = = 0.118 mH 10 6 10 6 There is a 15% difference between the two inductances; limitations of the approximation are apparent. b To estimate Hsat, we’ll take the maximum relative permeability µr max = 3000 as an estimate in order to find Hsat (see Figure 8Q6-1). We know that 2 L= NI and Bsat = µr max µo Hsat l µ µ NI Bsat = o r max sat l H= ∴ (4π × 10 T= ∴ 0.2750 ∴ Isat = 40.8 mA −7 Wb A -1 m -1 )(3000)(25) Isat 13.97 × 10 −3 m will saturate the core. B µr = 3000 B sat µ r = 850 H Hsat Figure 8Q6-1 B versus H curve for 43 - Mix ferrite. 8.8 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 If the core is saturated, the calculation of inductance is of course no longer valid as it used the initial permeability. The inductance now will be much reduced. Since under saturation ∆B = µ0∆H, effectively µr = 1. The use of an initial permeability such as µr = 850 implies that we have small changes (and also reversible changes) near around H = 0 or I = 0. When an inductor carries a dc current in addition to an ac current, the core will operate “centered” at a different part of the material’s B-H curve, depending on the magnitude of the dc current inasmuch as H ∝ I. Thus, a dc current I1 imposes a constant H1 and shifts the operation of the inductor to around H1. µr will depend on the magnitude of the dc current. c The same effect as passing a large dc current and saturating the core. When the core is saturated, the increase in the magnetic field B in the core is that due to free space, i.e. ∆B = µo∆H. This means we can use µr = 1 in Equation 8.31 (in the textbook) to find the inductance under saturation. It will be about 0.10 mH / 850 or 0.12 µH, very small. Author’s Note: Static inductance can be defined as L = Flux linked per unit dc current or L = NΦ/I. It applies under dc conditions and I = dc current. For ac signals, the current i will be changing harmonically (or following some other time dependence) and we use the definition dΦ N dt NδΦ = L= δi di dt Further, by Faraday’s law, since v is the induced voltage across the inductor by the changing total flux NdΦ/dt, we can also define L by, di v = L dt Moreover, since L = NδΦ/δi we see that the ac L is proportional to the slope of the B vs. H behavior. *8.7 The transformer a. Consider the transformer shown in Figure 8Q7-1a whose primary is excited by an ac (sinusoidal) voltage of frequency ƒ. The current flowing into the primary coil sets up a magnetic flux in the transformer core. By virtue of Faraday's law of induction and Lenz's law, the flux generated in the core is the flux necessary to induce a voltage nearly equal and opposite to the applied voltage. Thus, d ( Total flux linked) NAdB = dt dt where A is the cross-sectional area, assumed constant, and N is the number of turns in the primary. Show that if Vrms is the rms voltage at the primary (Vmax = Vrms√2) and Bm is the maximum magnetic field in the core, then υ= Vrms = 4.44 NAƒBm Transformer equation [8.33] Transformers are typically operated with Bm at the "knee" of the B-H curve, which corresponds roughly to maximum permeability. For transformer irons, Bm ≈ 1.2 T. Taking Vrms = 120 V and a transformer core with A = 10 cm × 10 cm, what should N be for the primary winding? If the secondary winding is to generate 240 V, what should be the number of turns for the secondary coil? b. The transformer core will exhibit hysteresis and eddy current losses. The hysteresis loss per unit second, as power loss in watts, is given by 8.9 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 Ph = KƒBmnVcore Hysteresis loss [8.34] where K = 150.7, ƒ is the ac frequency (Hz), Bm is the maximum magnetic field (T) in the core (assumed to be in the range 0.2 – 1.5 T), n = 1.6 and, Vcore is the volume of the core. The eddy current losses are reduced by laminating the transformer core as shown in Figure 8Q7-1b. The eddy current loss is given by d2 Pe = 1.65 f 2 Bm2 Vcore ρ [8.35] Eddy current loss where d is the thickness of the laminated iron sheet in meters (8Q7-1b) and ρ is its resistivity (Ω m). Suppose that the transformer core has a volume of 0.0108 m3 (corresponds to a mean circumference of 1.08 m). If the core is laminated into sheets of thickness 1 mm and the resistivity of the transformer iron is 6 × 10-7 Ω m, calculate both the hysteresis and eddy current losses at f = 60 Hz, and comment on their relative magnitudes. How would you achieve this? Windings Bm Primary Eddy currents Secondary N Vrms Lamination Irms d Insulation (a) B (b) Figure 8Q7-1 (a) A transformer with N turns in the primary. (b) Laminated core reduces eddy current losses. Solution a The induced voltage either at the primary or the secondary is given by Faraday’s law of induction (the negative sign indicates an induced voltage opposite to the applied voltage), that is, dB dt Suppose we apply this to the primary winding. Then υ = Vmsin(2πft) where f = 60 Hz and Vm is the maximum voltage, that is Vm = Vrms(√2). Then υ = −NA dB dt It is clear that B is also a sinusoidal waveform. Integrating this equation we find, Vm sin(2πft ) = − NA B= Vm cos(2 πft ) NA(2 πf ) which can be written as B = Bmcos(2πft) so that the maximum B is Bm given by Bm = Vm V 2 = rms 2πfNA 2πfNA 8.10 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 where we used Vm = Vrms(√2). Thus, Vrms = 4.44 NAfBm Let NP = Primary winding turns and assume f = 60 Hz, then NP = Vrms 120 V = = 38 turns on primary 2 4.44 AfBm ( 4.44)(0.1 m ) (60 Hz)(1.2 T ) Secondary winding turns are NS = b 240 V = 75 turns on secondary (4.44)(0.1 m)2 (60 Hz)(1.2 T) Part a gives Bm = 1.2 T. Then the hysteresis loss is Ph = KfBmnVcore Bm = 1.2 T Ph = (150.7)(60 Hz)(1.2 T ) 1.6 (0.0108 m 3 ) = 131 W Eddy current loss is d2 Pe = 1.65 f 2 Bm2 Vcore ρ Bm = 1.2 T (0.001 m )2 3 Pe = 1.65(60 Hz) (1.2 T ) (0.0108 m ) = 154 W −7 6 × 10 Ω m 2 2 With 1 mm lamination and at this low frequency (60 Hz) hysteresis loss seems to dominate the eddy current loss. Eddy current loss can be reduced with thinner laminations or higher resistivity core materials (e.g. ferrites at the expense of Bmax). Hysteresis loss can be reduced by using different core material but that changes B. Eqn. 8.34 is valid for only silicon steel cores only which have the required typical Bm for power applications. 8.8 Losses in a magnetic recording head Consider eddy current losses in a permalloy magnetic head for audio recording up to 10 kHz. We will use the following equation for the eddy current losses, d2 Pe = 1.65 f 2 Bm2 Vcore ρ where Vcore is the volume of the core. Consider a magnetic head weighing 30 grams and made from a permalloy with density 8.8 g cm-3 and resistivity 6 × 10-7 Ω m. The head is to operate at Bm of 0.5 T. If the eddy current losses are not to exceed 1 mW, estimate the thickness of laminations needed. How would you achieve this? Solution We will apply the eddy current loss equation in this problem though this equation has a number of assumptions so that answer is only an estimate. The eddy current loss is d2 Pe = 1.65 f B Vcore ρ 2 2 m where the core volume is 8.11 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Vcore = Then, d2 = Chapter 8 Mass 30 g = = 3.409 cm 3 or 3.409 × 10 −6 m 3 3 Density 8.8 g/cm Pe ρ 1.65 f 2 Bm2 Vcore (1 × 10 ∴ d= ∴ d = 2.07 µ m −3 W )(6 × 10 −7 W m ) (1.65)(10, 000 Hz)2 (0.5 T)2 (3.409 × 10 −6 m 3 ) Very thin (a page of a textbook is typically about 50 - 100 µm). *8.9 Design of a ferrite antenna for an AM receiver We consider an AM radio receiver that is to operate over the frequency range 530 - 1600 kHz. Suppose that the receiving antenna is to be a coil with a ferrite rod as core, as depicted in Figure 8Q9-1. The coil has N turns, its length is l, and the cross-sectional area is A. The inductance, L, of this coil is tuned with a variable capacitor C. The maximum value of C is 265 pF, which with L should correspond to tuning in the lowest frequency at 530 kHz. The coil with the ferrite core receives the EM waves, and the magnetic field of the EM wave permeates the ferrite core and induces a voltage across the coil. This voltage is detected by a sensitive amplifier, and in subsequent electronics it is suitably demodulated. The coil with the ferrite core therefore acts as the antenna of the receiver (ferrite antenna). We will try to find a suitable design for the ferrite coil by carrying out approximate calculations - in practice some trial and error experimentation would also be necessary. We will assume that the inductance of a finite solenoid is γµri µo AN 2 Inductance of a solenoid [8.36] L= l where A is the cross-sectional area of the core, l is the coil length, N is the number of turns, and γ is a geometric factor that accounts for the solenoid coil being of finite length. Assume γ ≈ 0.75. The resonant frequency f of an LC circuit is given by f = 1 2π ( LC )1 / 2 [8.37] a. If d is the diameter of the enameled wire to be used as the coil winding, then the length l ≈ Nd. If we use an enameled wire of diameter 1 mm, what is the number of coil turns, N, we need for a ferrite rod given its diameter is 1 cm and its initial relative permeability is 100? b. Suppose that the magnetic field intensity H of the signal in free space is varying sinusoidally, that is [8.38] H = Hmsin(2πƒt) where Hm is the maximum magnetic field intensity. H is related to the electric field E at a point by H = E/Zspace where Zspace is the impedance of free space given by 377 Ω. Show that the induced voltage at the antenna coil is Vm = Em d 2π 377Cfγ Induced voltage across a ferrite antenna [8.39] where ƒ is the frequency of the AM wave and Em is the electric field intensity of the AM station at the receiver point. Suppose that the electric field of a local AM station at the receiver is 10 mV m-1. What is the voltage induced across the ferrite antenna and can this voltage be detected by an amplifier? Would you use a ferrite rod antenna at short wave frequencies, given the same C but less N? 8.12 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 Ferrite rod Coil L C Figure 8Q9-1 A ferrite antenna of an AM receiver. Solution a Given Cmax = 265 pF and resonant frequency fo = 530 kHz (the lower frequency will need the largest inductance). From 1 fo = 2π LC L= 1 1 -4 2 = 2 = 3.403 × 10 H −12 3 C(2πfo ) π F × Hz 265 10 2 530 10 × ( )( ) Given L= γµri µo AN 2 γµri µo AN = since l coil ≈ Nd d l coil ∴ N= Ld = γµri µo A b (3.40 × 10 H)(0.001 m) 0.01 (0.75)(100)(4π × 10 H/m)(π ) 2 −4 −7 m 2 = 46 turns H-field is continuous everywhere. Let ω = 2πf where f is the frequency. H = Hm sin ωt ∴ B ≈ µr µo Hm sin ωt . This is the flux density inside the ferrite rod. The induced voltage is v(t ) = NA ∴ ∴ ∴ dB = µr µo NAωHm cos ωt dt 1 µr µoω 2 NAHm ; ω2 = ω LC Substitute in the final equation for N in part a: Vm = µr µoωNAHm = Ld µr µo AHm γµr µo A µr µo NAHm E H d Vm = = = m ; Hm = m LCω LCω 2πfγC 377 Vm = Em d 2π 377Cfγ Assume fo = 530 kHz, corresponding to C = 265 pF: (10 × 10 V/m)(1 × 10 m) (2π )(377 Ω)(265 × 10 F )(530 × 10 Hz)(0.75) −3 Vm = ∴ −3 −12 3 Vm = 40.1 µ V 40 µV can be detected without corruption by noise (see Example 1.8, pg. 39 where the noise voltage is at most typically a few microvolts). 8.13 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 Author’s Note: Suppose that the radio engineer uses the same tuning capacitor C for different bands. Then, since Vm depends inversely on f, the ferrite antennas will not be efficient at high frequencies. Changing the tuning capacitor C will obviously affect Vm. Note also that thicker d is better though there is another reason for using a thick d (Skin effect). *8.10 A permanent magnet with an air gap The magnetic field energy in the gap of a permanent magnet is available to do work. Suppose that Bm and Bg are the magnetic field in the magnet and the gap, Hm and Hg are the field intensities in the magnet and the gap and Vm and Vg are the volumes of the magnet and gap; show that, in terms of magnitudes, B gH gV g ≈ B m H m V m What is the significance of this result? Solution lm lg Figure 8Q10-1 A permanent magnet with a small air gap. Assume uniform cross-sectional area A in the magnet. Apply Ampere’s Law to the closed path l m + l g (refer to Figure 8Q10-1): ∫ Hdl = H l + Hg l g = Total Current = 0 m m lm +lg because there are no windings on the magnet. ∴ Hg = − l lm Hm which means Bg = − µo m Hm lg lg The magnetic flux, Φ = BA, must be continuous through the magnet and the air gap (a fundamental law in magnetism). Since the flux inside the magnet Φm also flows through the air gap, Φg = Φm. Also, if the gap length l g is sufficiently small, then fringing will be negligible. Then, since both the gap and the magnet have the same cross sectional area A, the flux density in the gap Bg ≈ Bm. Magnetic energy stored in the air gap is ( ) 1 1 Eg = Vg Bg Hg = Al g Bg Hg 2 2 Eg ≈ l 1 1 1 Al g Bm Hm m = ( Al m ) Bm Hm = Vm Bm Hm 2 2 lg 2 ( ) 8.14 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 Vg Bg Hg ≈ Vm Bm Hm Thus, This is the required result. If we only consider magnitudes without reference to a direction then BgHgVg = BmHmVm. The greater the magnet volume (Vm large) or smaller the air gap volume (Vg small), the greater is the magnetic field Bg in the gap. For an efficient permanent magnet, i.e. high Bg, we need a large magnet volume, small air gap volume, and a high magnetic field in the magnet which implies a high remanent magnetic field (Br) and a high coercive field intensity (Hc). 8.11 A permanent magnet with an air gap a. Show that the maximum energy stored in the air gap of a permanent magnet can be written very roughly as Egap ≈ 1/8BrHcVm where Vm is the volume of the magnet, which is much greater than that of the gap, Br is the remanent magnetic field, and Hc is the coercivity of the magnet. b. Using Table 8.5, compare the (BH)max with the product [(1/2Hc)][(1/2Br)] and comment on the closeness of agreement. c. Calculate the energy in the gap of a rare earth cobalt magnet that has a volume of 0.1 m3. Give an example of typical work (e.g., raising so many apples, each 100 grams, by so many meters) that could be done if all this energy could be converted to mechanical work. Solution a For most magnetic materials, to a fairly close approximation, (BH)max from the B-H characteristics would be (see Figure 8Q11-1 below): ( BH )max = Bm′ Hm′ ≈ Br Hc = Br Hc 2 2 4 1 1 1 Since Egap ≈ Em (from Question 8.10), 1 Egap ≈ ( BH )max Vm 2 1 ∴ Egap ≈ Br HcVm 8 where Vm is the volume of the magnet. This is the required result and it is an approximation. It is useful, obviously, when the B vs. H behavior of the magnet is not available but we know Br and Hc as below. 8.15 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) B g vs. Hm Magnet Bm vs. Hm Chapter 8 B Air gap Br P Bm ' Bm = Bg –Hm –Hc Hm O Hm ' Figure 8Q11-1 Magnet with air gap. Point P is the operating point of the magnet and determines the field inside the magnet and in the air gap. b (Hc/2)(Br/2) calculations are summarized in the table below. Table 8Q11-1 Summarized values and comments for different magnetic materials. Material Tabulated (BH)max kJ/m3 (Hc/2)(Br/2) kJ/m3 Comment Alnico (Fe-Al-Ni-Co-Cu) 50 34.0 30% difference Alnico (Columnar) 60 20.1 Substantial difference Strontium Ferrite (anisotropic) 24 - 34 21.5 - 34.2 Very close Rare Earth Cobalt 150 - 240 135.7 - 240.7 Very close NdFeB Magnets 200 - 275 179.0 - 238.7 Close c For rare-earth cobalt magnet, from Table 8.5 (in the textbook), (BH)max = 150 - 240 kJ/m3. Taking an average, Egap 1 1 150 + 240 kJ/m 3 3 ≈ ( BH )max Vm = (0.1 m ) = 9750 J 2 2 2 This corresponds to a mechanical work of raising 1000 apples (each 100 g) by 10 m. 8.12 Weight, cost, and energy of a permanent magnet with an air gap For a certain application, an energy of 1 kJ is required in the gap of a permanent magnet. There are three candidates, as shown in Table 8Q12-1. Which material will give the lightest magnet? Which will give the cheapest magnet? 8.16 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 Table 8Q12-1 Three permanent magnet candidates. Magnet (BH)max kJ m-3 50 200 30 Alnico Rare earth Ferrite Density g cm- 3 7.3 8.2 4.8 Yesterday's Relative Price (per unit mass) 1 2 0.5 Solution The energy in the magnet gap is, 1 Egap ≈ ( BH )max Vm 2 2 Egap 2(1000 J ) Vm ≈ = ( BH )max ( BH )max ∴ If ρ is the density, then the corresponding magnet mass is Mm = Volume × Density = Vm ρ = 2(1000 J )ρ ( BH )max (1) The magnet material cost is Cost = ( Price per unit mass)Mm (2) The table below lists the results of the calculations of magnet mass and cost using Eqns. (1) and (2) given (BH)max. Table 8Q12-2 Summarized results for magnet candidates. Magnet (BH)max kJ/m3 Density g/cm3 Yesterday’s Relative Price (per unit mass) Mass, Mm (kg) Relative cost Alnico 50 7.3 1 292 292 Rare Earth 200 8.2 2 82 164 Ferrite 30 4.8 0.5 320 160 Conclusions: (1) Rare earth magnet is lightest. (2) Ferrite magnet is cheapest. 8.13 Superconductivity and critical current density Consider two superconducting wires, tin (Sn; Type I) and Nb3Sn (Type II), each 1 mm in thickness. The magnetic field on the surface of a current-carrying conductor is given by µo I 2πr a. Assuming that Sn wire loses its superconductivity when the field at the surface reaches the critical field (0.2 T), calculate the maximum current and hence the critical current density that can be passed through the Sn wire near absolute zero of temperature. b. Calculate the maximum current and critical current density for the Nb3Sn wire using the same assumption as in part (a) but taking the critical field to be the upper critical field, Bc2, which is 24.5 at 0 K. How does your calculation of Jc compare with the critical density of about 1011 A m-2 for Nb3Sn at 0 K? B= 8.17 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 Solution a We assume that when the current through the wire generates a magnetic field at the wire surface that is equal to the critical field, then superconductivity is extinguished. Ic = (0.2 T)(2π ) Bc 2πr = µo 4π × 10 −7 ∴ Ic = 500 A ∴ Jc = 0.001 m 2 -1 -1 Wb A m 500 A Ic I 8 2 = c2 = 2 = 6.37 × 10 A/m 0 . 001 m A πr π 2 It turns out that this is indeed about the order of magnitude for the critical current density for Type I superconductors. The current that generates a field on the conductor surface that is equal to Bc extinguishes the superconductivity. b Type II superconductor, Nb3Sn. The critical field is the upper critical field, Bc2 Ic = and (24.5 T)(2π ) 0.001 m 2 = 61250 A -1 -1 Wb A m 4π × 10 −7 61250 A 10 Jc = A/m 2 2 = 7.80 × 10 . m 0 001 π 2 This value is close to experimental value of ~1011 A/m2. *8.14 Enterprising engineers in the high arctic building a superconducting inductor A current-carrying inductor has energy stored in its magnetic field that can be converted to electrical work. A group of enterprising engineers and scientists living in Resolute in Nunavut (Canada) have decided to build a toroidal inductor to store energy so that this energy can be used to supply a small community of 10 houses each consuming on average 3 kW of energy during the night (6 months). They have discovered a superconductor (Type II) that has a Bc2 = 100 T and a critical current density of Jc = 5 × 1010 A m-2 at night temperatures (it is obviously a novel high-Tc superconductor of some sort). Their superconducting wire has a diameter of 5 mm and is available in any desirable length. All the wiring in the community is done by superconductors except where energy needs to be converted to other forms (mechanical, heat, etc.). They have decided on the following design specification for their toroid: The mean diameter, Dtoroid, of toroid, (1/2)(Outside diameter + Inside diameter), is 10 times longer than the core diameter, Dcore. The field inside the toroid is therefore reasonably uniform to within 10%. The maximum operating magnetic field in the core is 35 T. Fields larger than this can result in mechanical fracture and failure. Assume that Jc decreases linearly with the magnetic field and that the mechanical engineers in the group can take care of the forces trying to blow open the toroid by building a proper support structure. Find the size of the toroid (mean diameter and circumference), the number of turns and the length of the superconducting wire they need, the current in the coil, and whether this current is sufficiently below the critical current at that field. Is it feasible? 8.18 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 Solution As a design question, there are several solutions. Design I (Author’s design) Critical field Bc = 100 T and the critical current density Jc = 5 × 1010 A/m2. Engineers choose B = 35 T based on the mechanical support technology for the solenoid. Assume that Jc decreases linearly with Bc so that when the actual field is B, Jc is J′c. J′c = Jc(Bc - B)/Bc J′c = (5 × 1010 A/m2)(100 T - 35 T)/(100 T) = 3.250 × 1010 A/m2 Power per house = 3000 W 1 night is 6 months and in seconds this is Time = (1 Nordic night)(6 months/night)(30 days/month)(24 hours/day) (60 minutes/hour)(60 seconds/minute) Time = 1.555 × 107 s Total energy requirement is Etotal = (Houses)(Power per house)(Time) Etotal = (10 houses)(3000 J/s per house)(1.555 × 107 s) = 4.665 × 1011 J The available energy density, Evolume, in the toroid is B2 (35 T) = = 4.874 × 108 J/m3 2 µo 2( 4π × 10 −7 Wb A −1 m -1 ) 2 Evolume = This provides the energy per unit volume in the toroid. Let V = volume of the toroidal core. By the definition of energy density, Evolume, V = Volume = 4.665 × 1011 J Etotal = 957.1 m3 = 8 −3 Evolume 4.874 × 10 J m which is the necessary toroid volume. We now need the maximum current Imax for destroying superconductivity. Let d = diameter of superconducting wire; d = 5 mm. Imax corresponding to J′c is 0.005 m 2 d 2 5 Imax = Jc′ π = (3.250 × 1010 A/m 2 )π = 6.381 × 10 A 2 2 The operating current must be less than this maximum current. Let L = mean circumference of the toroid and D = diameter of cross section of toroid core. Let the mean diameter of the toroid be Dtoroid, whereas D is the diameter of the core. We need Dtoroid > > D or by design choice: Dtoroid = 10 D Mean circumference is 8.19 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 L = π(Dtoroid) = 10π D or D = L/(10π) Now consider the volume of the core, ( L / 10π ) D L3 V = π L = π L= 2 2 400π 2 ∴ 2 L = ( 400πV ) 1/ 3 ( [ = 400π 957.1 m −3 ]) 1/ 3 = 106.3 m which is the mean circumference of the inductor. And: Dtoroid = L/π = 106.4 m/π = 33.9 m Let N = number of turns. If d is the diameter of the superconducting wire, then Nd ≈ Length (mean circumference) of the inductor = L L 106.3 = = 21260 turns d 0.005 Suppose that Iop is the operating current. Then the magnetic field inside the core of the toroid is N= µo NIop L BL (35 T )(106.3 m ) Iop = = = 1.39 × 105 A µo N ( 4π × 10 −7 Wb A -1 m -1 )(21260) B= so that The operating current is some 4.5 times less than the maximum allowed current, Imax, that extinguishes superconductivity. Length of superconducting wire needed = NL = (21260)(106.3 m)= 2.26 × 106 m = 2260 km Author’s Note: There are various assumptions in the above design. In addition, there are various technological problems such as setting up the current and tapping out the current as needed etc. Furthermore, there has to be some energy margin in the design (perhaps 50%). Feasible? The currents are not trivial. Any vanishing of superconductivity at any location would have disastrous consequences. Though not feasible today, why not in 50-70 years given the present rate of development in superconducting technologies? The students find the following analogy helpful in appreciating why we considered an inductive storage device rather than a capacitor. Suppose that we build a huge capacitor that is charged to the required voltage and hence to the required energy during the summer and then the energy is tapped as needed during the winter. If the voltage is 10 kV, then we need a capacitor of 104 F! (And, it must not leak.) Design II (Submitted by an instructor using the textbook) Consider the energy need of the village. Etot = Total energy needed Etot = (10 houses)(3000 W/house)(3600 s/hour)(24 hours/day)(180 days) Etot = 4.665 × 1011 J If Evol is the energy density, then Etot = 4.665 × 10 11 D J = Evol (πDtoroid )π core 2 2 where Dcore and Dtoroid are the diameters of the core and the toroid. Assume Dtoroid = 10Dcore. Then 8.20 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Etot = Evol 10π 2 Dcore ∴ Chapter 8 Dcore 2 4 5 3 Etot = π 2 Dcore Evol 2 4.665 × 1011 J = 1.891 × 10 7 J/m 3 5 2 3 5 2 3 π Dcore π (10 m ) 2 2 Now these engineers are guided by the mechanical size of the toroid and choose, Dcore = 10 m. In Design I, the electrical engineers instead chose B and left the mechanical size to mechanical and civil engineers. Select Dcore = 10 m Etot ∴ Evol = = ∴ Evol = 1.891 × 10 7 J/m 3 = ∴ B = 2( 4π × 10 −7 H/m )(1.891 × 10 7 J/m 3 ) = 6.894 T B2 2 µo Since Dtoroid is 10 times Dcore, we know Dtoroid is 100 m. The mean circumference L is therefore L = (100 m)π = 314 m. If the turns on the toroid touched each other along the inside edge of the toroid, there would be (Dtoroid - Dcore)π = (90π) m of wire side-by-side which leads to N= (90π ) m = 56549 turns (0.005) m Length of wire needed: l wire = NπDcore = (56549)(π )(10 m ) = 1776.5 km (just for the toroid itself; more would be needed to wire the houses). Bl toroid (6.89 T)(100 m)(π ) = 3.05 × 104 A = −7 -1 -1 µo N 4 10 Wb A m 56549 π × ( ) ( ) ∴ I= ∴ Jwire = I 3.05 × 10 4 A 9 2 = 2 = 1.55 × 10 A/m πr 2 . m 0 005 (π ) 2 which is OK. [Author’s Note: This design uses a larger toroid than the first - I’m open to other design suggestions.] 8.15 Magnetic storage media a. Consider the storage of video information (FM signal) on a video tape. Suppose that the maximum signal frequency to be recorded as a spatial magnetic pattern is 10 MHz. The heads helically scan the tape, and the relative velocity of the tape to head is about 10 m s-1. What is the minimum spatial wavelength of the stored magnetic pattern (information) on the tape? b. Suppose that the speed of an audio cassette tape in a cassette player is 5 cm s-1. If the maximum frequency that needs to be recorded is 20 kHz, what is the minimum spatial wavelength on the tape? Solution a Consider a video tape. The maximum frequency in the signal is fvideo = 10 MHz. The speed of the video tape is 10 m/s. The spatial wavelength is 8.21 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 8 λ = v/fvideo = (10 m/s) / (10 × 106 s-1) = 10-6 m or 1 µ m By comparison, typically, a page of a book is 50 - 100 µm thick and so is a typical human hair. b Consider an audio tape. The maximum frequency in the signal is faudio = 20 kHz. The speed of the video tape is 0.05 m/s. The spatial wavelength is 5 × 10 −2 m/s = 2.5 × 10-6 m or 2.5 µm λ= 3 20 × 10 Hz This is a spatial wavelength of 2.5 µm on the tape. "We have a habit in writing articles published in scientific journals to make the work as finished as possible, to cover up all the tracks, to not worry about the blind alleys or describe how you had the wrong idea first, and so on. So there isn't any place to publish, in a dignified manner, what you actually did in order to get to do the work." Richard P. Feynmann (Nobel Lecture, 1966) 8.22 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 2001 McGraw-Hill) Second Edition ( Chapter 9 9.1 Refractive index and relative permittivity Using n = ε r , calculate the refractive index n of the materials in the table given their low frequency relative permittivities εr(LF). What is your conclusion? Material → a-Se Ge NaCl MgO εr (LF) n (∼ 1 - 5 µm) 6.4 2.45 16.2 4.0 5.90 1.54 9.83 1.71 Solution The results from the calculations are summarized in Table 9Q1-1. For a-Se and Ge there is an excellent agreement between ε r ( LF ) and n. Both are covalent solids in which electronic polarization (electronic bond polarization) is the only polarization mechanism at low and high frequencies. Electronic polarization involves the displacement of light electrons with respect to positive ions. This process can readily respond to the field oscillations up to optical frequencies. For NaCl and MgO ε r ( LF ) is larger than n because at low frequencies both of these solids possess a degree of ionic polarization. The bonding has a substantial degree of ionic character which contributes to polarization at frequencies below far-infrared wavelengths. Table 9Q1-1 n - optical for (1 - 5) µm range Comment 2.53 2.45 Electronic bond polarization 16.2 4.02 4.0 Electronic bond polarization NaCl 5.9 2.43 1.54 Ionic polarization contributes to εr (LF) MgO 9.83 3.14 1.71 Ionic polarization contributes to εr (LF) Material εr (LF) a-Se 6.4 Ge ε r ( LF ) 9.2 Refractive index and bandgap Diamond, silicon, and germanium all have the same diamond unit cell. All three are covalently bonded solids. Their refractive indices (n) and energy bandgaps (Eg) are shown in the table below. (a) Plot n versus Eg and (b) Plot also n4 versus 1/Eg. What is your conclusion? According to Moss’s rule, very roughly, n 4 Eg ≈ K = const Moss’s rule What is the value of K? Material → Diamond Silicon Germanium Bandgap, Eg (eV) 5 1.1 0.66 n 2.4 3.46 4.0 8.1 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 Solution The plots of n versus Eg and of n4 versus 1/Eg are presented in Figures 9Q2-1 and 9Q2-2 respectively. The lines presented on both plots are the best fits of the Moss’s rule to the experimental data with K being the only parameter. On the base of the high values of r2 and Adj r2 we can conclude that there is a reasonable agreement between the experimental data and Moss’s rule. Fitting the Moss’s rule to the experimental data gives a value for K of about 164.3 eV. n = K 1/4 Eg-1/4 r 2=0.99841514 DF Adj2=0.99683028 r FitStdErr=0.032404072 Fstat=1259.9395 K=164.27936 4.5 4 3.5 n 3 2.5 2 0.5 1.5 2.5 3.5 Eg Figure 9Q2-1 8.2 4.5 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 n4=K*(1/Eg) r 2=0.99686953 DF Adj2=r 0.99373906 FitStdErr=6.2336564 Fstat= 636.88218 K=164.27936 300 250 200 n 4 150 100 50 0 0 0.5 1 1.5 1/Eg Figure 9Q2-2 *9.3 Temperature coefficient of refractive index Suppose that we could write the relationship between the refractive index n (at frequencies much less than ultraviolet light) and the band gap Eg of a semiconductor as suggested by Hervé and Vandamme, 2 A 2 n =1+ Eg + B where Eg is in eV, A = 13.6 eV and B = 3.4 eV. (B depends on the incident photon energy.) Temperature dependence in n results from dEg/dT and dB/dT. Show that the temperature coefficient of refractive index (TCRI) is given by1 , (n 2 − 1)3 / 2 dEg 1 dn TCRI = ⋅ =− + B′ Hervé-Vandamme relationship 2 13.6n dT n dT where B′ is dB/dT. Given that B′= 2.5 × 10-5 eV K-1, calculate TCRI for two semiconductors: Si with n ≈ 3.5, dEg/dT ≈ -3 × 10-4 eV K-1 and AlAs with n ≈ 3.2, dEg/dT ≈ -4 × 10-4 eV K-1. Solution We can start the proof of the Hervé-Vandamme relationship for TCRI from the empirical relation for suggested by them, which connects the refractive index and energy band gap 1 P.J.L. Hervé and L.K.J. Vandamme, J. Appl. Physics, 77, 5476, 1995, and references therein. 8.3 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) A n =1+ Eg + B Chapter 9 2 2 (1) Taking natural logarithm from both sides of (1) and differentiating with respect to temperature we subsequently receive A 2 2 ln(n) = ln 1 + Eg + B d dT 2 2 dn 1 d A 1 A d A 1 + = 2 = = n dT A 2 dT Eg + B n 2 Eg + B dT Eg + B 1 + Eg + B 2 2 A2 d A2 dEg dB + = − + E B =− 2 3 3 g 2 n E + B dT n E + B dT dT ( ) g ( ) ( g ) or 1 dn 1 A2 dEg dB =− 2 + 3 n dT n Eg + B dT dT ( (2) ) The left hand side of this equation is exactly the TCRI. Using Equation (1) we can easily show that 3 (E g A2 ) +B 3 (n 2 − 1) 2 = A and substituting in (2) we receive exactly Hervé-Vandamme relationship for TCRI (n TCRI = − − 1) 2 dEg dB (n2 − 1) 2 + = − 13.6 n 2 An 2 dT dT 3 3 2 dEg + B© dT Applying Hervé-Vandamme relationship for Si and for AlAs we receive: (3.5)2 − 1]2 [ −4 TCRI Si = − eV K −1 ) + (2.5 × 10 −5 eV K −1 )] = 6.23 × 10-5 K-1 2 [( −3 × 10 (13.6 eV) (3.5) 3 [(3.2) − 1] [(−4 × 10 (13.6 eV)(3.2) 2 TCRI AlAs = − 3 2 2 −4 ] eV K −1 ) + (2.5 × 10 −5 eV K −1 ) = 7.56 × 10-5 K-1 9.4 Dispersion (n vs. λ ) in diamond The dispersion relationship for diamond can be written as Aλ2 Bλ2 n2 = 1 + 2 + Sellmeier equation λ − λ21 λ2 − λ22 where A = 0.3306 and B = 4.3356, λ1 = 175 nm and λ2 = 106 nm. This type of n(λ) dispersion relation is called the Sellmeier equation and is quite common for describing the refractive index of many 8.4 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 materials; in general, there may be more terms on the right hand side. Calculate the refractive index at 500 nm. Solution The refractive index for diamond at 500 nm is Aλ2 Bλ2 (0.3306)(500) (0.4.3356)(500) = 2.432 + = 1+ + 2 2 2 2 λ − λ1 λ − λ2 (500)2 − (175)2 (500)2 − (106)2 2 n = 1+ 2 9.5 Dispersion (n vs. λ ) in GaAs By using the dispersion relation for GaAs, calculate the refractive index n and the group index Ng of GaAs at a wavelength of 1300 nm. Solution The dispersion relation for GaAs is given by Equation 9.13 (in the textbook) 3.78 λ2 (1) λ2 − 0.2767 where λ must be in µm. At λ =1300 nm = 1.3 µm for the refractive index of GaAs we receive n 2 = 7.10 + n = 7.10 + 3.78 (1.3)2 = 3.409 (1.3)2 − 0.2767 The group index is defined as (see Equation 9.18 in the textbook ) Ng = n − λ To calculate dn dλ (2) dn we have to differentiate both sides of (1) with respect to λ: dλ 2 2 dn 3.78 (2 λ )(λ − 0.2767) − 3.78 λ (2λ ) 2n = 2 dλ (λ2 − 0.2767) and dn 1 1.045926 λ 1 1.045926 (1.3) -1 =− 2 = − 2 = -0.1997 µ m 2 dλ n (λ − 0.2767) 3.409 ((1.3)2 − 0.2767) Substituting this value in (2) we can calculate the group index Ng = 3.409 − (1.3 µm ) ( −0.1997 µm −1 ) = 3.669 *9.6 Dispersion (n vs. λ) Consider an atom in the presence of an oscillating electric field as in Figure 9Q6-1. The applied field oscillates harmonically in the +x and -x directions and is given by E = Eoexp(jωt). The energy losses can be represented by a frictional force whose magnitude is proportional to the velocity, dx/dt. If γ is the proportionality constant per electron and per unit electron mass then Newton’s second law for Z electrons in the polarized atom is 8.5 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 d2x dx = − ZeEo exp( jωt ) − Zmeω o2 x − Zmeγ 2 dt dt where ωo =(β/Zme)1/2 is the natural frequency of the system composed of Z electrons and a +Ze nucleus and β is a force constant for the restoring Coulombic force between the electrons and the nucleus. Show that the electronic polarizability αe is p Ze 2 α e = induced = Electronic polarizability me (ω o2 − ω 2 + jγω ) E What does a complex polarizability represent? Since αe is a complex quantity, so is εr and hence the refractive index. By writing the complex refractive index Ν = (ε r ) and εr is related to αe by the Clasius-Mossotti equation, show that, Zme Ν2 − 1 NZe 2 = Complex refractive Ν 2 + 2 3ε o me (ω o2 − ω 2 + jγω ) where N is the number of atoms per unit volume. What is your conclusions? E = Eoe j t C x index O Z electrons in shell C Atomic nucleus Fr O Center of negative charge x pinduced (a) A neutral atom in E = 0. (b) Induced dipole moment in a field Figure 9Q6-1 Electronic polarization of an atom. Solution The induced electronic dipole moment is simply given by pinduced = −( Ze) x . Using this simple relation we can easily prove that if the expression for the electronic polarizability is correct, the magnitude of the displacement of the negative charge with respect to the positive charge in the atom is given by x=− eE me (ω − ω 2 + jγω ) 2 o Further taking into account that the electric field is given by E = E0 exp( jωt ) for the magnitude of the displacement we receive x=− eEo exp( jωt ) me (ω o2 − ω 2 + jγω ) This expression has to satisfy the differential equation representing Newton’s second law. We can prove that the expression is solution of the differential equation by substituting it in the equation and showing that it turns it into an identity. To do so, we need the first and the second derivative of the displacement with respect to time: 8.6 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 dx jωeEo exp( jωt ) =− dt me (ω o2 − ω 2 + jγω ) d2x ω 2 eEo exp( jωt ) = dt 2 me (ω o2 − ω 2 + jγω ) Substituting the derivatives into the displacement differential equation we receive Zω 2 eEo exp( jωt ) Zω o2 eEo exp( jωt ) ZγjωeEo exp( jωt ) ZeE exp( j ω t ) = − + + o (ω o2 − ω 2 + jγω ) (ω o2 − ω 2 + jγω ) (ω o2 − ω 2 + jγω ) Zω 2 e Zω o2 e Zγjωe Ze = − + + 2 2 2 2 2 (ω o − ω + jγω ) (ω o − ω + jγω ) (ω o − ω 2 + jγω ) Zω 2 e = − Ze(ω o2 − ω 2 + jγω ) + Zω o2 e + Zγ jωe Zω 2 e = − Zeω o2 + Zeω 2 − Zejγω + Zω o2 e + Zγ jωe Zω 2 e = Zω 2 e Thus the substitution turns the equation into an identity and this actually proves that the expression for the electronic polarizability is correct, as the expression for the displacement was derived directly from it. The Clausius-Mossotti relation (Equation 7.15 in the textbook) relates εr and αe as follows: ε r − 1 Nα e = ε r + 2 3ε 0 Ze 2 Substituting εr with N and αe with we receive exactly the relation we wanted to prove me (ω o2 − ω 2 + jγω ) 2 N2 − 1 N Ze 2 = N 2 + 2 3ε 0 me (ω o2 − ω 2 + jγω ) The complex polarizability accounts for some energy losses in the media. If γ is zero, there are no losses in the media and we have the usual expression for lossless media. 9.7 Dispersion and diamond Consider applying the simple electronic polarizability and Clasius-Mossotti equations to diamond. Neglecting losses, Ze 2 αe = me (ω o2 − ω 2 ) εr − 1 NZe 2 = Dispersion in diamond and ε r + 2 3ε o me (ω o2 − ω 2 ) For diamond we can take Z = 4 (valence electrons only as these are the most responsive), N = 1.8 × 1029 atoms m-3, εr(DC) = 5.7. Find ωo and then find the refractive index at λ = 0.5 µm and 5 µm. Solution We can obtain the value of ω0 from the dispersion relation in diamond using that for DC ω = 0. The dispersion relation then reduces to 8.7 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 ε r DC − 1 NZe 2 = ε r DC + 2 3ε o meω o2 Solving for ω0 we have (1.8 × 10 m )(4)(1.6022 × 10 3(8.85 × 10 F m )(9.1 × 10 NZe 2 ε r DC + 2 = ωo = 3ε o me ε r DC − 1 −3 29 −12 −1 This corresponds to linear frequency of fo = −19 −31 C) (5.7) + 2 × 1016 s-1 = 3.54× kg) (5.7) − 1 2 ωo = 5.63 × 1015 Hz and wavelength of 2π c = 53.2 nm fo λo = If we neglect the losses the refractive index is simply given by n = ε r and the value of εr for given ω can be found from the dispersion relation solving for εr εr = 2 NZe 2 + 3ε o me (ω o2 − ω 2 ) 3ε o me (ω o2 − ω 2 ) − NZe 2 The refractive index is then 2 NZe 2 + 3ε o me (ω o2 − ω 2 ) n= 3ε o me (ω o2 − ω 2 ) − NZe 2 The circular frequency corresponding to λ = 0.5 µm is 8 −1 2πc 2π (3 × 10 m s ) ω= = = 3.767 × 1015 s −1 −6 λ (0.5 × 10 m) and the value of the refractive index is 2 n= [ 2(1.8 × 10 29 m −3 )( 4)(1.6022 × 10 −19 C) + 3(8.85 × 10 −12 F m −1 )(9.1 × 10 −31 kg) (3.54 × 1016 s −1 ) − (3.77 × 1015 s −1 ) 3(8.85 × 10 −12 F m −1 )(9.1 × 10 −31 [ kg) (3.54 × 10 s 16 ) − (3.77 × 10 −1 2 15 s ) ] − (1.8 × 10 −1 2 2 29 m −3 )(4)(1.6022 × 10 −19 2 C) ] = 2.417 Analogously for λ = 5 µm we have ω= 8 −1 2πc 2π (3 × 10 m s ) = = 3.767 × 1014 s −1 −6 λ (5 × 10 m) and n = 2.388. 9.8 Electric and magnetic fields in light The intensity (irradiance) of the red laser beam from a He-Ne laser in air has been measured to be about 1 mW cm-2. What are the magnitudes of the electric and magnetic fields? What are the magnitudes if this 1 mW cm-2 beam were in a glass medium with a refractive index n = 1.45 and still had the same intensity? Solution 8.8 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 The average irradiance according to Equation 9.24 (in the textbook) is given by 1 cε o nEo2 2 For vacuum or air n = 1 and we can calculate the magnitude of the electric field from the above relation: I= Eo = 2I = cε o n 2(10 W m −2 ) (3 × 108 m s−1 )(8.85 × 10 −12 F m −1 )(1) = 86.772 V m-1 The corresponding magnetic field is −1 nEo (1)(86.772 V m ) Bo = = = 2.892 × 10-7 T =0.2892 µ T 8 −1 c (3 × 10 m s ) If this beam was traveling in a glass medium of n = 1.45 and still had the same intensity (1 mW cm2 ), then Eo = 2I = cε o n (3 × 10 2(10 W m −2 ) 8 m s −1 )(8.85 × 10 −12 F m −1 )(1.45) = 72.06 V m-1 and −1 n Eo (1.45)(86.772 V m ) Bo = = = 3.483 × 10-7 T =0.3483 µ T. c (3 × 108 m s−1 ) 9.9 Reflection of light from a less dense medium (internal reflection) A ray of light which is traveling in a glass medium of refractive index n1 = 1.450 becomes incident on a less dense glass medium of refractive index n2 = 1.430. Suppose that the free space wavelength (λ) of the light ray is 1 µm. a. What should be the minimum incidence angle for TIR? b. What is the phase change in the reflected wave when θi = 85° and when θi = 90°? c. What is the penetration depth of the evanescent wave into medium 2 when θi = 85° and when θi = 90°? Solution a The critical angle qc for TIR is given by Equation 9.26 (in the textbook) sinθc = n2/n1 = 1.430/1.450 so that θc = 80.47°. b Since the incidence angle θi > θc, there is a phase shift in the reflected wave. The phase change in Er, ⊥ is given by φ⊥ (Equation 9.39 in the textbook). With n1 = 1.450, n2 = 1.430 and θi = 85°, the phase change is For the Er,// 2 1/ 2 1.43 2 (sin 85°) − (sin 2 θ i − n 2 )1 / 2 1.45 = 2 arctan ° φ ⊥ = 2 arctan = 116.45° cosθ i cos 85° component (Equation 9.40 in the textbook), the phase change is 8.9 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 (sin 2 θ i − n 2 )1 / 2 1 − π = φ // = 2 arctan 2 n cos θ 2 i 2 1/ 2 1.43 2 (sin 85°) − 1.45 1 ° = 2 arctan 2 − 2 π = -62.1° 1.43 cos 85° 1.45 (Note: If we were to invert the reflected field, this phase change would be 117.9°° ) . We can repeat the calculation with θi = 90° to find φ⊥ = 180° and φ// = 0°. Note that as long as θi > θc, the magnitude of the reflection coefficients are unity. Only the phase changes. c The amplitude of the evanescent wave as it penetrates into medium 2 is Et,⊥(y,t) ~ Eto,⊥exp(–α2y) We ignore the z-dependence, expj(ωt - kzz), as this only gives a propagating property along z. The field strength drops to e-1 when y = 1/α2 = d, which is called the penetration depth. The attenuation constant α2 (Equation 9.42 in the textbook) is 1 2 2 2πn2 n1 2 α2 = sin θ i − 1 λ n2 1 2 2 2π (1.43) (1.45) 2 6 -1 sin 85 1 i.e. α2 = − ° ( ) = 1.28 × 10 m . −6 1 . 43 1 10 m × ( ) 6 -1 so that the penetration depth is, d = 1/α2 = 1/(1.28×10 m ) = 7.8 × 10-7 m, or 0.78 mm. For 90°, repeating the calculation we find, α2 = 1.5 × 106 m-1, so that d = 1/α2 = 0.66 mm. We see that the penetration is greater for smaller incidence angles. 9.10 Internal and external reflection at normal incidence Consider the reflection of light at normal incidence on a boundary between a GaAs crystal medium of refractive index 3.6 and air of refractive index 1. a. If light is traveling from air to GaAs, what is the reflection coefficient and the intensity of the reflected light in terms of the incident light? b. If light is traveling from GaAs to air, what is the reflection coefficient and the intensity of the reflected light in terms of the incident light? Solution a The light travels in air and becomes partially reflected at the surface of the GaAs crystal which corresponds to external reflection. Thus n1 = 1 and n2 = 3.6. Then according to Equation 9.37 (in the textbook), n − n2 1 − 3.6 = = -0.565 r// = r⊥ = 1 n1 + n2 1 + 3.6 8.10 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 This is negative which means that there is a 180° phase shift. The reflectance R (Equation 9.43 in the textbook), which gives the fractional reflected power, is R = (r⊥ ) = ( −0.565) = 0.319 = 31.9 % 2 2 b The light travels in GaAs crystal and becomes partially reflected at the crystal-air interface which corresponds to internal reflection. Thus n1 = 3.6 and n2 = 1. Then, r// = r⊥ = n1 − n2 3.6 − 1 = = 0.565 n1 + n2 3.6 + 1 There is no phase shift. The reflectance is again 0.319 or 31.9%. In both cases, a and b, the amount of reflected light is the same. 9.11 Antireflection coating a. Consider three dielectric media with flat and parallel boundaries with refractive indices n1, n2 and n3. Show that for normal incidence the reflection coefficient between layers 1 and 2 is the same as that between layers 2 and 3 if n2 = n1n3 . What is the significance of this? b. Consider a Si photodiode that is designed for operation at 900 nm. Given a choice of two possible antireflection coatings, SiO2 with a refractive index of 1.5 and TiO2 with a refractive index of 2.3 which would you use and what would be the thickness of the antireflection coating you chose? The refractive index of Si is 3.5. Solution For light traveling in medium 1 incident on the 1-2 interface at normal incidence, n3 n1 n − n2 n1 − n1n3 = = r12 = 1 n n1 + n2 n1 + n1n3 1+ 3 n1 1− For light traveling in medium 2 incident on the 2-3 interface at normal incidence, r23 = thus, n1 n −1 1− 3 n3 n1 = n1 n +1 1+ 3 n3 n1 n1n3 − n3 = n1n3 + n3 n2 − n3 = n2 + n3 r23 = r12 Significance? For an efficient antireflection effect, the coefficient the waves A and B (see Figure 9Q11-1) should interfere destructively and at the same time should have comparable magnitudes to cancel each other. This can be achieved by r12 = r23. n1 d n2 n3 A B Surface Antireflection Semiconductor of photovoltaic device coating 8.11 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 Figure 9Q11-1 Illustration of how an antireflection coating reduces the reflected light intensity. The best antireflection coating has to have a refractive index n2 such that n2 = (n1n3)1/2 = [(1)(3.5)]1/2 = 1.87. Given a choice of two possible antireflection coatings, SiO2 with a refractive index of 1.5 and TiO2 with a refractive index of 2.3, SiO2 both are very close. To find the thickness of the coating with a 900 nm wavelength (equation from Example 9.7 in the textbook), λ d = m 4n2 where m = 1, 3, 5, … is an odd integer. For SiO2: d= (900 nm) 4(1.5) d = 150 nm or odd multiples of d. For TiO2 (if chosen): d = (900 nm) 4(2.3) d = 97.8 nm or odd multiples of d. 9.12 Complex refractive index Spectroscopic ellipsometry measurements on a silicon crystal at a wavelength of 620 nm show that the real and imaginary parts of the complex relative permittivity are 15.2254 and 0.172, respectively. Find the complex refractive index. What is the reflectance and absorption coefficient at this wavelength? What is the phase velocity? Solution We know that εr′ = 15.2254 and that εr″ = 0.172. The real part n and the imaginary part K of the complex refractive index are solutions of the following system of equations (see Equation 9.55 in the textbook) n2 + K2 = 15.2254 and 2nK = 0.172 We can take K from the second equation and substitute for it in the first equation, 2 0.172 n2 + = 15.2254 2n This is a quadratic equation in n2 that can be easily solved to find that the four roots are: n1, 2 = ±3.902 and n3, 4 = ±0.022 Since n > 0, only the positive roots can have significance for us. The wavelength of 620 nm corresponds to photon energy of 8.12 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 −34 J s)(3 × 108 m s −1 ) hc (6.62 × 10 E= = = 3.2 × 10-19 J = 2 eV. −9 λ (620 × 10 m) This photon energy is comparable with the energy bandgap of Si (1.12 eV) and for such photon energies n should greater than one and much greater than K. Thus, we can conclude that n = 3.902. Once we know n, we can find K = 1 = 0.022. If we simply square root the real part of εr, we would find still find n = 2n 3.902, because the extinction coefficient K is very small. The reflectance of the Si crystal is given by Equation 9.57 (in the textbook) (n − 1)2 + K 2 (3.902 − 1)2 + 0.022 2 R= = = 0.35 (n + 1)2 + K 2 (3.902 + 1)2 + 0.022 2 which is the same as simply using (n - 1)2/(n + 1)2 = 0.35, because K << n. The absorption coefficient α describes the loss in the light intensity I via I = Ioexp(-αz) and by virtue of Equation 9.52 (in the textbook), α = 2 k ′′ = 2 ko K = 2 2π 2π K=2 (0.022) = 4.459×× 105 m-1 −9 λ 620 × 10 m ( ) The phase velocity is given by v= 8 −1 c (3 × 10 m s ) = 7.683 × 107 m s-1 = n (3.902) 9.13 Complex refractive index Spectroscopic ellipsometry measurements on a germanium crystal at a photon energy of 1.5 eV show that the real and imaginary parts of the complex relative permittivity are 21.56 and 2.772 respectively. Find the complex refractive index. What is the reflectance and absorption coefficient at this wavelength? How do your calculations match with the experimental values of n = 4.653 and K = 0.298, R = 0.419 and α = 4.53 × 106 m-1 ? a. Show that the attenuation coefficient a due to free carrier absorption is given by ω ε ″ 1 σ α = r = Free carrier absorption c n cε o n where ω is the angular frequency of the EM radiation, εr″ is the imaginary part of the relative permittivity, n is the refractive index and σ is the conductivity due to free carriers in the sample. b. Intrinsic germanium has a conductivity of about 2.1 Ω-1 m-1. Calculate the imaginary part εr″ of the relative permittivity at a wavelength of 20 µm. Find the attenuation coefficient α due to free carrier absorption. The refractive index of germanium at the specified wavelength is n = 4. Solution 8.13 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) a Chapter 9 We know that εr′ = 21.56 and that εr″ = 2.772. Thus, from Equation 9.55 (in the textbook), we have n2 + K2 = 21.56 and 2nK = 2.772 We can take K from the second equation and substitute for it in the first equation, 2 2.772 n2 + = 21.56 2n This is a quadratic equation in n2 that can be easily solved to find, that the four roots are: n1, 2 = ±4.634 and n3, 4 = ±0.299 × 10 −3 Since n and K should be positive, and for photon energies of about 1.5 eV, n should be greater than one and greater than K, n = 4.634 is the only root having physical significance. Knowing n, we can find K = 1 = 0.299. Both values compare very well with the experimental 2n results. From Equations 9.55 and 9.52 (in the textbook), we can calculate the reflectance R and the absorption coefficient α, respectively R= (n − 1)2 + K 2 ( 4.634 − 1)2 + 0.2992 = = 0.418 (n + 1)2 + K 2 ( 4.634 + 1)2 + 0.2992 α = 2 k ′′ = 2 ko K = 2 =2 2πE ph 2π K=2 K= λ hc 2π (1.5 eV) (0.299) = 4.540 × 106 m-1 (4.135 × 10 eV s)(3 × 108 m s−1 ) −15 Since n and K were in good agreement with the experiment, α and R are also very close to their experimental values. b The absorption coefficient is defined through Equation 9.52 (in the textbook) as α = 2k ′′ . Further, using Equation 9.53 (in the textbook), we can easily show that α = 2ko K (1) 2π , or expressed through the angular frequency ω λ ω ko = c From Equation 9.55 (in the textbook), By definition ko = ε r′′ 2n Combining (1), (2) and (3) we receive (2) K= (3) ω ε ′′ α = r , c n (4) 8.14 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 which is exactly the relation we had to prove. Taking into account the Conduction loss equation (Equation 9.49 in the textbook) we can easily transform (4) to 1 σ α = cε o n (5) c Equation 9.49 (in the textbook) relates the imaginary part of relative permittivity ε r′′ and the conductivity due to free carriers in the sample: ε r′′ = σ ε oω The angular frequency can be trivially calculated from the wavelength λ 2πc λ Thus, for ε r′′ , we receive ω= ε r′′ = (20 × 10 −6 m)(2.1 Ω−1 m −1 ) λσ = 0.002518 = 2πcε o 2π (3 × 108 m s −1 )(8.85 × 10 −12 F m −1 ) Since we are given that for a wavelength of 20 µm, the refraction index of germanium crystal is 4, we can calculate the attenuation coefficient α from (5) 2.1 Ω −1 m −1 ) 1 σ ( 1 = 197.74 m-1 α = = ( 4) cε o n (3 × 108 m s −1 )(8.85 × 10 −12 F m −1 ) 9.14 Evanescent wave Total internal reflection (TIR) of light from a boundary between a more dense medium n1 and a less dense medium n2 is accompanied by an evanescent wave propagating in medium 2 near the boundary. Find the functional form of this wave for the component normal to the plane of incidence and discuss how its magnitude varies with the distance y into medium 2. (See Figure 9Q14-1) Et,// y t z x into paper Transmitted wave kt t Et, ki Ei,// i Ei, Incident wave r Er,// Ei,// Er, Ei, kr Reflected wave i Incident wave = 90 r n2 Et, Evanescent wave n1 > n2 Er, Er,// Reflected wave (b) i > c then the incident wave suffers total internal reflection. However, there is an evanescent wave at the surface of the medium (a) i < c then some of the wave is transmitted into the less dense medium. Some of the wave is reflected. 8.15 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 Figure 9Q14-1 Light wave traveling in a more dense medium strikes a less dense medium. The plane of incidence is the plane of the paper and is perpendicular to the flat interface between the two media. The electric field is normal to the direction of propagation. It can be resolved into perpendicular ( ⊥ ) and parallel (||) components. Solution The transmitted wave has the general form Et,⊥ = t⊥Eio,⊥expj(ωt - kt⋅ r) where t⊥ is the transmission coefficient. The dot product, examining Figure 9Q14-2, is kt⋅ r = yktcosθt + zkt sinθt. y Direction of propagation k E(r, t) r r z O Figure 9Q14-2 A traveling plane EM wave along a direction k. However, from Snell's law, when θi > θc, sinθt = (n1/n2)sinθi > 1 and cosθt = √[1 - sin2θt] = ±jA2 is a purely imaginary number. Thus, taking cosθt = -jA2 Et,⊥ = t⊥Eio,⊥expj(ωt – zktsinθt + jyktA2) = t⊥Eio,⊥exp(-yktA2)expj(ωt – zktsinθt) which has an amplitude that decays along y as exp(–α2y) where α2 = ktA2. Note that +jA2 is ignored because it implies a light wave in medium 2 whose amplitude and hence intensity grows. Consider, the traveling wave part, expj(ωt – zktsinθt). Here, ktsinθt = kisinθi (by virtue of Snell's law). But kisinθi = kiz which is the wavevector along z, that is, along the boundary. Thus the evanescent wave propagates along z at the same speed as the incident and reflected waves along z. Furthermore, for TIR we need sinθi > n2/n1. This means that the transmission coefficient, t⊥ = ni cosθ i n 2 2 cosθ i + 2 − sin θ i n1 12 = t⊥0 exp (j ψ ⊥ ) must be a complex number as indicated by t⊥0exp(jψ⊥) where t⊥0 is a real number and ψ⊥ is a phase change. Note that t⊥ does not however change the general behavior of propagation along z and the penetration along y. 8.16 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 9.15 Quartz half-wave plate What are the possible thicknesses of a half-wave quartz plate for a wavelength λ ≈ 1.01 µm given the extraordinary and ordinary refractive indices are no = 1.534 and ne = 1.543? Solution Equation 9.69 (in the textbook) describes the relative phase difference through a retarder plate 2π φ= (ne − no ) L λ Halfwave retardation is a phase difference of π but the effect will be the same if the phase difference is φ = (2 m + 1)π , m = 0,1, 2,... All possible thicknesses of halfwave plate retarder are then (2 m + 1)λ L= , m = 0,1, 2,...... 2(ne − no ) When m = 0 we receive the smallest possible value 1.01 × 10 −6 ) ( λ L= = = 56.1 µm 2(ne − no ) 2(1.543 − 1.534) For m = 1 we have L = 168 µm, for m = 2 we have L = 280.6 µm, for m = 3 we have L = 392.8 µm and so on. 9.16 Pockels Cell Modulator What should be the aspect ratio d/L for the transverse LiNiO3 phase modulator in Figure 9Q16-1 that will operate at a free-space wavelength of 1.3 µm and will provide a phase shift ∆φ of π (half wavelength) between the two field components propagating through the crystal for an applied voltage of 20 V? The Pockels coefficient r22 is 3.2 × 10-12 m/V and no = 2.2. Ey V 45° y d Input light Ea x Ey ∆φ Ex z Output light Ex z L Figure 9Q16-1 Transverse Pockels cell phase modulator. A linearly polarized input light into an electrooptic crystal emerges as a circularly polarized light. Solution From Equation 9.75 (in the textbook), putting ∆φ = π for the phase difference between the field components Ex and Ey in Figure 9Q16-1 (in the textbook) gives, 2π 3 L no r22 Vλ / 2 = π λ d d 1 2π 3 1 2π no r22Vλ / 2 = ⋅ = ⋅ (2.2)3 (3.2 × 10 −12 m V −1 )(20 V) L ∆φ λ π (1.3 × 10 −6 m ) ∆φ = or giving d/L = 1.048 × 10-3 8.17 Solutions to Principles of Electronic Materials and Devices: 2nd Edition (Summer 2001) Chapter 9 This particular transverse phase modulator has the field applied along the y-direction and light traveling along the z-direction as in Figure 9Q16-1. If we were to use the transverse arrangement in which the field is applied along the z-axis, and the light travels along the y-axis, the relevant Pockels coefficients would be greater and the corresponding aspect ratio d/L would be ~ 5 × 10-2. We cannot arbitrarily set d/L to any ratio we like for the simple reason that when d becomes too small, the light will suffer diffraction effects that will prevent it from passing through the device. d/L ratios 10-3 - 10-2 in practice can be implemented by fabricating an integrated optical device. “I know you believe you understand what you think I said, but I am not sure you realize that what you heard is not what I meant” Alan Greenspan Chairman of US Federal Reserve Board 8.18