Band theory of semiconductors
Figure 5.12d illustrates a situation in which a fully occupied
band is separated from an unoccupied band by a small band
gap. This property characterizes a semiconductor. In this
case, electrical conductivity depends upon there being sufficient
energy available for thermal population of the upper
band, and it follows that the conductivity increases as the
temperature is raised. In the next section, we look more
closely at the types and properties of semiconductors.