Exam1Sp03

Exam1Sp03 - Name Section: (check one) Melloch Schwartz EE...

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Unformatted text preview: Name Section: (check one) Melloch Schwartz EE 305 Exam 1 Spring 2003 1) Open book—no extra pages in the book, no inserts. 2) Allowed a calculator and one 8.5x11 crib sheet 3) Be sure you have 9 test pages including the cover and score sheet. 4) Write only on the question sheets. Show all your work. If you need more room for a particular problem, use the reverse side of the same page. 5) Write neatly, if your writing is illegible then print. 6) This exam is worth 100 points. Problem (8 pts) 1. A unit cell is shown for the cubic system. (a) What are the Miller indices of the shaded plane shown for the cubic system? (b) What are the Miller indices of the vector A ? (4 pts) 2. For the alloy GaAsojPOj, A) 50% of the atoms are Ga, 25% are As, and 25% are P B) 30% of the atoms are Ga, 30% are As, and 30% are P C) 100% of the atoms are Ga, 50% are As, and 50% are P D) 50% of the atoms are Ga, 50% are As, and 50% are P E) none of the above (6 pts) 3. Germanium, is from column IV of the periodic table. 5x1016 cm'3 of the germanium atoms are replaced by aluminum atoms from column III of the periodic table and 1x1016 cm-3 of the germanium atoms are replaced by arsenic atoms from column V of the periodic table. If ni = 2x1013 cm"3, what are the equilibrium electron and hole concentrations? 4 (12 pts) 4. Shown are the energy band diagram and an electron and hole at x = O. The electron and hole travel from x = 0 to x = 3 [Mn while maintaining the same total energy. Graph the electron’s and hole’s potential and kinetic energy as a function of position. Label your curves. Electron Energy (eV) 1.0 ——-— ———————— E l— | | " 1.5 —__.o_._L.____I____I Hole I I I I I I I Energy (eV) 0 1 2 3 Electron Energy (eV) 1.5 1.0 0.5 o xoum) _ 0.5 1 2 3 -1.0 -1.5 Hole Energy (eV) 1.5 1.0 0.5 o x(pm) - 0.5 1 2 3 -1.0 -1.5 5 (5 pts) 5. As the photon energy increases above the bandgap of a semiconductor, the absorption coefficient increases because of A) an increase in the number of phonons B) an increase in the density of states C) the absorption coefficient does not change with photon energy D) the absorption coefficient actually decreases with increase in photon energy E) none of the above (8 pts) 6. A semiconductor is 4 gm long in the x—direction. The energy it takes to break a bond in this semiconductor is 2 eV. There is a uniform electric field of value 1 V/[Lm in the +x direction. Sketch the energy bands as a function of x. x(pm) (5 pts) 7. In equilibrium, A) current flow due to electrons balances current flow due to holes. B) The separation between the electron and hole quasi—Fermi levels remains constant as a function of position. C) Current flow due to drift of electrons is balanced by current flow due to diffusion of holes. D) Current flow due to drift of electrons is balanced by current flow due to diffusion of electrons. (5 pts) 8. Light of energy sufficient to break the silicon-silicon bond is shining on a piece of silicon. The light intensity is breaking 1018 bonds per sec per cm3. A) Eventually the silicon will fall apart as all the bonds are broken by the light. B) The generated electron—hole pairs will start absorbing the light and preventing the silicon from falling apart. C) The light cannot break these bonds in silicon because silicon is an indirect band—gap semiconductor. D) The recombination rate of electron—hole pairs because of mid—gap traps will increase until it balances the photo generation. E) None of the above. (5 pts) 9. A piece of silicon is doped with acceptors to a concentration of 1.1xlO15 cm'3 and donors to a concentration of 1x1014 cm'S. If u" = 1358 cm2 /Vs and ,up = 458 cmZ/Vs,the conductivity of this silicon is A) 0.0733 (£26m)—1 B) 0.2173 (Qcm)‘1 C) 0.1023 mm)“1 D) 0.2463 (9.0m)—1 (5 pts) 10. The electron diffusion coefficient A) represents the ease with which electrons diffuse in a semiconductor B) is a measure of how much the electrons in the conduction band act like a gas C) determines how rapidly excess electron-hole pairs recombine D) none of the above. 7 “(10 pts) 11. Shown are the energy bands for a semiconductor (3 pts) (21) The semiconductor is A) n—type B) p-type C) can not tell from the diagram (3 pts) (b) The semiconductor is A) in low—level injection B) in high—level injection C) can not tell from the diagram. (4 pts) (0) Assuming T = 300 K and mi = 109 cm'3, find the electron and hole concentrations 8 3 _.(5 pts) 12. A semiconductor has equilibrium carrier concentrations of no 2 1017 cm' and p0 = 10 ems. At a temperature of 300 K, and with an applied electric field of 100 V/cm, the electrons attain a velocity of 105 cm/s. What is the electron mobility in this semiconductor? (10 pts) 13. A piece of silicon is doped with donors to a concentration of 1017 ems. The temperature is 300 K and the minority carrier lifetime is 1 ,us. If the sample is uniformly illuminated for at least 1 s with . 16 -3 -1 . a photogeneration rate of 10 cm s , What are the electron and hole concentrations? 9 (12 pts) 14. Shown is a piece of silicon with equilibrium electron concentration of 1017 cm'3. The silicon is very short compared to a diffusion length. At x=0 excess holes are injected at a concentration of Apn(0) =1014 cm“3. Any holes that reach x = 0.01 pm are instantly extracted, i.e. Apn (0.01pm) 2 0. The hole profile inside the bar is shown. The hole minority carrier lifetime is 1 ,us, the hole diffusion coefficient is 12 cm2 /s, and the electric field is zero everywhere. Ap(cm'3) 1014 x (pm) 0 0.01 pm (2 pts) Do low level injection conditions prevail throughout the silicon? (3 pts) Approximately how much recombination of the excess holes is going on in the bar? (7 pts) What is the hole diffusion current density in the silicon? What is its direction? ...
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This note was uploaded on 03/25/2011 for the course ECE 305 taught by Professor Melloch during the Spring '08 term at Purdue.

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Exam1Sp03 - Name Section: (check one) Melloch Schwartz EE...

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