ECE 440
Homework VI
Fall 2008
Due: Wednesday, October 08, 2008
1. Consider a silicon sample at 300 K. Assume that the electron concentration varies linearly
with distance. At x=0, the electron concentration is n(0). At x=10 μm, the electron
concentration is n(10μm)= 5x10
14
/cm
3
. If the electron diffusion coefficient, assumed
constant, is D
n
=30 cm
2
/sec, determine the electron concentration at x=0 for the following
two diffusion current densities: (a) the diffusion current density is found to be J
n
diff
= + 0.9 A/
cm
2
and (b) J
n
diff
=  0.9 A/cm
2
.
2.
The acceptor profile of a silicon sample is shown below. Assume that the majority carrier
mobility can be obtained from Fig. 323 (or the enlarged graph posted in the 440 website),
and the sample is at thermal equilibrium at 300 K.
(a) Determine the diffusion coefficients for majority carriers at points A, B and C,
respectively.
To find the diffusion coefficients, we have to first find the doping concentrations at those
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 Spring '09
 Lie
 diffusion current densities, Jndiff

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