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Unformatted text preview: ECE344 Midterm Exam November 18, 2005 1. Problem. Consider a sample of silicon at T = 300 K. Assume that the electron concentration varies linearly from n (0) at x = 0 to 5 × 10 14 cm 3 at x = 0.01 cm. The diffusion current density is measured to be j n = 0 . 19 A/cm 2 . Knowing the electron diffusion constant, D n = 25 cm 2 /s, determine the electron concentration n (0) at x =0. Solution. The electron diffusion current density is given by Eq. (89) of the Notes: j n,diff = eD n dn dx . (1) In our case: e 25 dn dx = e 25 5 × 10 14 n (0) . 01 = 0 . 19 . (2) Solving for n (0): n (0) = 5 × 10 14 . 19 × . 01 1 . 6 × 10 19 × 25 = 2 . 5 × 10 13 cm 3 . (3) 2. Problem. A p type sample of GaAs is doped with N A = 10 16 cm 3 and N D = 0. Assume for the recombination lifetime of both electrons and holes τ n = τ p = 2 × 10 7 s. Assume also that the sample is under illumination resulting in a constant and spatially uniform generation rate of electronhole pairs G = 2 × 10 21 cm 3 /s. Finally, assume T = 300 K. (a) Calculate the steadystate electron density. (b) Assuming for the electron and hole mobilities μ n = 8,500 cm 2 /Vs and μ p = 400 cm 2 /Vs, respectively, calculate the change in conductivity due to the illumination....
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This note was uploaded on 09/13/2010 for the course ECE ECE344 taught by Professor Polizinni during the Spring '10 term at University of Massachusetts Boston.
 Spring '10
 Polizinni

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