7.4 Direct and Indirect Bandgaps
Generally semiconductors can be divided into two types. There are semiconductors with direct
bandgaps (fig. 52 (a)) and those with indirect bandgaps (fig. 52 (b)). In semiconductors with di-
rect bandgaps the top of the valence band and the bottom of the conduction band are at the same
k-value, which is not the case in semiconductors with indirect bandgaps, as fig. 52 shows.
The main differences occur in the relaxation from an excited state. After an electron is excited into
Figure 52: Top of the valence band and bottom of the conduction band in semiconductors with direct
(a) and indirect (b) bandgaps
the conduction band by a photon with an energy, which is high enough, it will occupy the lowest
possible energy state in the conduction band. The generated hole in the valence band will occupy
the highest possible energy state. If those two positions are at differentk-values, which is the case in
semiconductors with indirect bandgaps, the particle produced in the relaxation has to carry both the
difference in energy and momentum. Since photons can only carry a very little amount of momentum,
this relaxation will produce phonons. That’s the reason why light emmitting devices are built using
semiconductors with direct bandgaps like Gallium-Arsenite (fig. 54), and solar cells, which shouldn’t
emmit light, can be built using semiconductors with indirect bandgaps like Silicon (fig. 53).