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ts3201f08 - EMSE 201 Introduction to Materials Science...

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EMSE 201 — Introduction to Materials Science & Engineering 3 December 2008 Name: SOLUTION Department of Materials Science and Engineering 1 of 5 Case Western Reserve University Test #3 — 75 minutes; 100 points; 6 questions; 5 pages; 10% of course grade Partial credit will be given for correct set-ups and reasoning. Give units on numerical answers where appropriate. Please write all answers on these pages; use the backs if needed. Constants : Wiedemann-Franz constant, £ = π 2 k B 2 /(3q e 2 ) = 2.445 × 10 -8 W K -2 Boltzmann’s constant, k B = 1.381 × 10 -23 J K -1 = 8.620 × 10 -5 eV K -1 gas constant, R : 8.314 J mol -1 K -1 Charge on an electron, q e = –1.602 × 10 -19 C Faraday’s constant, F = 96,500 C mol –1 1) State whether each of the following elements will act as a donor or acceptor when added to the indicated semiconducting material. Briefly justify your answer. Assume that the added elements act as substitutional impurites. a) (2 points) B added to Si Boron is in column IIIA, whereas Si is in column IVA. With one fewer valence electron to contribute to the covalent bonding of the Si crystal, an empty state (hole) will result in the valence band ( 1 pt ), making boron an acceptor dopant ( 1 pt ) in silicon. b) (4 points) Zn added to GaAs Zinc will most likely substitute for Ga, being just 1 lower in atomic number than Ga ( 1.5 pt ). Zinc sits in column IIB, whereas Ga is in column IIIA. With one fewer valence electron than Ga to contribute to the covalent bonding of the GaAs crystal, an empty state (hole) will result in the band structure ( 1.5 pt ), making Zn an acceptor dopant ( 1 pt ) in GaAs. c) (4 points) In added to CdTe Indium will most likely substitute for Cd, being just 1 higher in atomic number than Cd and being more cationic/metallic in character than anionic ( 1.5 pt ). Indium sits in column IIIA, whereas Cd is in column IIB. With one more valence electron than Cd to contribute to the covalent bonding of the CdTe crystal, an extra electron will result in the conduction band ( 1.5 pt ), making In a donor dopant ( 1 pt ) in CdTe. 2) (8 points) The room-temperature electrical conductivity of a silicon specimen is 5.93 × 10 –3 ( Ω –1 m –1 ). The hole concentration is known to be 7.0 × 10 17 m –3 . Given that the electron and hole mobilities are 0.14 and 0.050 m 2 V –1 s –1 respectively, compute the electron concentration. State whether the specimen is intrinsic, n-type, or p-type. Justify your answer. To solve for n , use Equation 12.13, which, after rearrangement, gives: n = σ pq e μ h q e μ e ( 3 pts ) = 1.4 × 10 16 m –3 ( 2 pt answer + 1 pt units) This material is p-type ( 1 pt ) since p (7.0 × 10 17 m –3 ) is greater than n ( 1 pt ).
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EMSE 201 — Introduction to Materials Science & Engineering 3 December 2008 Department of Materials Science and Engineering 2 of 5 Case Western Reserve University 2) Human teeth consist of a shiny, hard outer layer (enamel) covering a more porous inner material (dentine). Properties of enamel, dentine, Pyrex® glass, and polycrystalline alumina (Al 2 O 3 ) ceramic are listed below.
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