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Unformatted text preview: ECE344 Practice Problems for Midterm Exam 2 November 15, 2005 1. Problem. An ntype silicon sample has a resistivity of 5 .cm at T = 300 K. Assume that the electron mobility at this temperature is 1,500 cm 2 /Vs and that the mobility varies as T 3 / 2 . (a) What is the donor impurity concentration (assuming that N A =0)? (b) What would the resistivity be at T = 200 K? (c) And at T = 400 K? Solution. The equation we must use is Eq. (67) of the Lecture Notes: The resistivity which is the inverse of the conductivity is expressed in terms of the mobility and carrier density as follows: = 1 = 1 e n n . (1) (a) From this equation the electron density (and so the donor concentration) can be calculated: n = N D = 1 e n = 1 1 . 6 10 19 1 . 5 10 3 5 = 8 . 33 10 14 cm 3 . (2) (b) Since the mobility scales with temperature as T 3 / 2 , we have T 3 / 2 , so ( T ) = (300) T 300 3 / 2 . (3) Thus: (200) = (300) 200 300 3 / 2 = 5 . 543 . cm = 2 . 72 . cm . (4) (c) Similarly, (400) = (300) 400 300 3 / 2 = 5 1 . 54 . cm = 7 . 68 . cm . (5) 2. Problem. A Ge p + n diode at T = 300 K has the following parameters: N D = 10 16 cm 3 and N A = 10 18 cm 3 , D n = 100 cm 2 /s, D p = 49 cm 2 /s, n = p = 5 s and the junction area A is 10 4 cm 2 . Determine the diode current for (a) a forward bias of 0.2 V and (b) a reverse bias of 0.2 V....
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