ECE 440
HW5 Solutions
Summer 2009
Tue, Jul 07, 2009
1
1.
Consider a silicon sample at 300 K. Assume that the hole concentration varies linearly
with distance. At x=0, the hole concentration is p(0). At x=10 µm, the hole concentration
is p(10 µm)= 5x10
14
/cm
3
. If the hole diffusion coefficient, assumed constant, is D
p
=14
cm
2
/sec, determine the hole concentration at x=0 for the following two diffusion current
densities: (a) the diffusion current density is found to be J
p
diff
= + 0.19 A/cm
2
and (b)
J
p
diff
=  0.19 A/cm
2
.
The expression for the diffusion current is in 422b.
We can replace the derivative with the
slopes of the hole concentration.
Thus,
2.
The donor 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 online), and the
sample is at thermal equilibrium at 300 K.
X (µm)
0
10
17
/cm
3
N
d
10
16
/cm
3
1
2
3
4
5
6
7
8
A
B
C
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HW5 Solutions
Summer 2009
Tue, Jul 07, 2009
2
(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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 Summer '09
 Lie
 Condensed matter physics, builtin electric field, diffusion current densities

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