Environmental Engineering Reference
In-Depth Information
Space charge region
p-region
n-region
Free holes
Free electrons
Diffusion
Charge distribution
Figure 4.7
Space Charge Region Formation at a p-n Junction by Diffusion of
Electrons and Holes
remain. They create a positive space charge region. Where holes have diffused
into the n-region, negatively ionized atoms remain and create a negative space
charge region.
An electric field between the n-region and p-region is thus created. It
counteracts the charge carriers and hence the diffusion cannot continue
indefinitely. Finally, a
diffusion voltage
(4.21)
is created. The charge neutrality within the boundaries
d
n
and
d
p
of the space
charge region in the n-type and p-type semiconductor region leads to:
(4.22)
The total width of the space charge region can then be calculated from:
(4.23)
For silicon with a dopant concentration of
n
D
=2
•
10
16
cm
-3
and
n
A
=1
•
10
16
cm
-3
at a temperature of
T
= 300 K, the diffusion voltage becomes to
V
d
=
0.73 V; and with
m.
When electrons are lifted from the valence band into the conduction band
and thus released from the atom in the space charge region, the electric field
will pull them into the n-region. Similarly, generated holes will move into the
p-region. This can be explained in the energy band model by band bending in
the space charge region (Figure 4.8).
As described before, the solar cell can only convert a part of the photon
energy into electrical current. For photon energies smaller than the band gap,
the energy is not sufficient to promote an electron from the valence band to
the conduction band. This is the case for wavelengths above:
ε
r
= 11.8, the widths are
d
n
= 0.13
µ
m and
d
p
= 0.25
µ
