EE115A
Frank Chang
Winter10
Homework #1
Due Mon, Jan 18th, 2010 noon
Problem 1.
The intrinsic carrier concentration of germanium (GE) is expressed as
15
3/ 2
3
1.38
10
exp
,
2
g
i
E
n
T
cm
kT
−
−
=
×
where E
g
= 0.66eV.
(a) Calculate n
i
at 290K.
(b) Assuming
2
3900
/(
. )
n
cm
V s
µ
=
,
2
1900
/(
. )
p
cm
V s
µ
=
and a piece of germanium with
length of 0.1 μm and a cross section area of 0.1 μm by 0.1 μm sustains a voltage
difference of 1V what is the total current at 290K if it is doped with P at a density of 10
17
cm
-3
.
Problem 2.
Due to a manufacturing error, the p-side of a pn junction has not been doped.
If
3
16
10
5
−
×
=
cm
N
D
, calculate the built-in potential at T=300K.
Problem 3.
Two identical pn junctions are placed in series.
(a) Prove that this combination can be viewed as a single two-terminal device having an
exponential characteristic.
(b) For a tenfold change in the current, how much voltage change does such a device
require?
Problem 4.
Figure 1.

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