From the previous chapter we learned that the mobile charge carriers in a semiconductor can be categorized as electrons in the conduction band and holes in the valence band. The carriers occupy states, which define their energy and crystal momentum. One of the objectives of this chapter is to determine the density of these states in energy or momentum, as well as the densities of the carriers in real space. We begin, however, with a look at how the mobile carriers are created and destroyed.

Creation of electrons and holes

The four mechanisms by which electrons (in the conduction band) and holes can be created in the devices discussed in this book can be classed as thermal, optical, electrical, and chemical.

Thermal generation

For temperatures T > 0 K, the lattice atoms vibrate about their mean positions. Occasionally, the local amplitudes of vibration are sufficient to break a valence bond, i.e., to release an electron from the valence band. Obviously, in a perfectly periodic crystal, the energy gained by the electron must be sufficient to promote the electron into the conduction band, as there are no available states elsewhere. The situation is illustrated in Fig. 3.1, where an electron from state 〈1〉 in the valence band is promoted to state 〈1′〉 by the absorption of phonons.