This preview shows pages 1–2. Sign up to view the full content.
EE2
HW3
Solutions
1.
Mobility of electrons,
μ
n
= 1200 cm
2
/Vsec = 0.12 m
2
/Vsec
Given concentration of electrons, n = 10
16
cm
3
Resistivity,
ρ
= (qn
μ
n
+ qp
μ
p
)
1
For ntype Si, p << n,
Therefore,
ρ
= (qn
μ
n
)
1
= (1.6 x 10
19
x 10
16
x 1200)
1
= 0.52
Ω
cm
Effective mass, m
c
*
= 0.26m
0
Average scattering time,
sec
10
775
.
1
10
6
.
1
12
.
0
10
1
.
9
26
.
0
13
19
31
*
−
−
−
×
=
×
×
×
×
=
=
q
m
n
c
m
μ
τ
= 0.1775 psec
2.
Hole density varies as p(x) = A exp(x/L
p
) i.e. hole density has a gradient, which
obviously gives rise to diffusion
Hole diffusion current density,
)
/
exp(
))
/
exp(
(
)
(
,
p
p
p
p
p
p
diff
p
L
x
L
A
qD
dx
L
x
A
d
qD
dx
x
dp
qD
J
−
=
−
−
=
−
=
The current varies exponentially with distance. This is because the gradient of the hole
density varies with distance, hence the diffusion flux varies too and the current is
proportional to the diffusion flux. The hole density decreases with x because there is
recombination with electrons. There are more electrons for larger x in a forward biased p
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview. Sign up
to
access the rest of the document.
 Spring '10
 Liu

Click to edit the document details