ECE 440
Homework VI Solutions
Spring 2006
Due: Monday, February 20, 2006
1.
(a) An ntype Si sample with N
d
= 1x10
17
/cm
3
is steadily illuminated such that g
op
= 3x10
21
EHP/cm
3
s. If
τ
n
=
τ
p
=1 μs for the excitation, calculate the separation in the quasiFermi
levels, (F
n
F
p
). Draw a band diagram such as Fig. 411.
To find the excess carrier concentration we first need to check whether we can use the low
level approximation.
Since the inequality holds, we know
.
Using equations (325) we can calculate F
n
– E
i
and E
i
– F
p
.
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(b) At t=0, the illumination in part (a) is terminated. Assuming the carrier lifetimes remain the
same, calculate the separation in the quasiFermi levels at t=4 μs. Draw a corresponding band
diagram at 300 K.
From homework #5, we know that the equations for the decay of excess carriers are
.
We can use these equations to calculate what the excess carrier concentrations are at t=4 μs.
Now we just need to repeat part (a).
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 Spring '11
 Leburton
 300 K, hole concentration, Si bar, Jpdiff

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