Direct Energy, 2018a

122 6.5 Pn Junctions
E
Indirect Semiconductor
Figure 6.9: Two possible mechanisms of photon absorption in an indirect
gap semiconductor.
| −→ k |
What happens when we shine light on a direct semiconductor?A photon
of sucient energy can excite an electron from the valence band to the
conduction band to create an electron-hole pair. What happens in an indirect
semiconductor?Figure 6.9 illustrates two possibilities. As illustrated
by the longer arrow, an electron can be excited directly from the valence to
conduction band. However, this requires a photon of more energy than the
vertical distance between the top of the valence band and the bottom of
the conduction band [25, p. 200]. Alternatively, as illustrated by the other
two arrows, excitation from the top of the valence band to the bottom of
the conduction band may involve a photon and a phonon. Both energy and
momentum must be conserved, so a change in crystal momentum is needed
to excite an electron in this case. Solar cells and photodetectors may be
made from either direct or indirect semiconductors.
6.5 Pn Junctions
Many devices, including photovoltaic devices, LEDs, photodiodes, semiconductor
lasers, and thermoelectric devices are essentially made from pn
junctions. To understand photovoltaic devices and these other energy conversion
devices, we need to understand pn junctions. Consider a semiconductor
crystal composed of an n-type material (with excess electrons) on
one side and a p-type material (lacking electrons, in other words, with an
excess of holes) on the other side. The junction of the p-type and n-type
materials is called a pn junction. Assume the junction is abrupt and is at
thermal equilibrium.
Some pn junctions are made from elemental semiconductors like Si, and
other pn junctions are made from compound semiconductors like GaAs.