8
(
)
3
11
3
14
23
12
2
K400 ,i
K400
cm 106.1
cm 10
cm 104
n
n
p
−
−
−
⋅
=
⋅
=
=
.
The thermal limit for this example, according to
is approximately 220°C (490K).
Above this temperature, the intrinsic charge carrier concentration n
i
reaches the values
of thebasic dopingof the n

regionof 10
14
cm
3
and thereforebecomes dominant.
1.1.3 Charge carriermovement in the semiconductor
There are twomain types of charge carriermovement in a semiconductor:
•
Carrier drift
•
Diffusion
The type of movement known as carrier drift occurs because of the presence of an
electrical field E in the semiconductor at point x, which drives the charge carriers at an
average speed v. Considering the electrons and holes within a particular semiconductor
volume, thedensities for the charge carriers are:
n
drift ,n
vnq
J
⋅ ⋅ =
Eq. 1.11
p
drift ,p
vpq
J
⋅ ⋅ =
Eq. 1.12
J:Charge carrier density
2
cm
A
q:Elementary charge
C 10 602 .1
19
−
⋅
v:Average speedof the electrons or holes
s
cm
The average drift velocity v
n
or v
p
, created by the electrical field E, can also be
described as the product of the electrical fieldwith the specific charge carrier mobility µ
n
or µ
p
.
)x(E nq
J
n
drift ,n
⋅µ⋅ ⋅ =
Eq. 1.13
)x(E pq
J
p
drift ,p
⋅µ⋅ ⋅ =
Eq. 1.14
µ: Specific charge carriermobility
⋅
sV
cm
2
E(x): Electric field at the location x
cm
V
For silicon, the electron (hole) mobility µ
n
(µ
p
) at 300K is approximately
sV
cm 1500
2
⋅
⋅
sV
cm 450
2
.