Direct Energy, 2018a

126 6.5 Pn Junctions
V x
-
+
Energy
q(V 0 + V x )
Conduction band
p-type
Fermi level
n-type
Valence band
position
Figure 6.13: Energy level diagram of a reversed biased pn junction.
6.12. Holes ow from the p-type to n-type region. Some of these holes
combine with electrons in the depletion layer. In an LED, photons are
emitted in this process. The energy of the emitted photon corresponds to
the energy of the energy gap. Some LEDs have an additional intrinsic,
undoped, layer at the junction, between the p-type and n-type layers to
improve the eciency of the device.
A solar cell and an optical photodetector are also essentially pn junctions.
Both of these devices convert optical electromagnetic energy to electricity.
When light shines on these devices, electron-hole pairs are created
at the junction. Due to the charge distribution across the junction, many
of the electrons and holes created are swept away from the junction before
they can recombine [9]. This ow of charges is a current, so the optical
electromagnetic energy is converted into electricity. When light shines on
a photovoltaic device, a voltage can be measured across the junction, and
this eect is called the photovoltaic eect [9, p. 212].
The vertical distance between the conduction band and the valence
band on an energy level diagram is the energy gap E g . The energy gap
of the material used to make a solar cell or photodetector determines the
properties of the device. Photons with energy greater than the energy gap
have enough energy to form electron-hole pairs while photons with less