Semiconductors and Insulators

Abstract

The most important aspect of semiconductor1 bandstructuremay be summarised as follows; at absolute zero the highest completely filled band (the valence band) is separated from the lowest empty band (the conduction band) by an energy gap or band gap Eg of forbidden states. Therefore the material does not conduct electricity at T = 0. At finite temperatures a variety of processes enable electrons to be excited into the conduction band and empty states to occur in the valence band, thus allowing electrical conduction. However, we shall see that the presence of the energy gap still dominates the properties of the semiconductor. Most of the technologically-important semiconductors, such as Ge, Si, GaAs and (Hg,Cd)Te, have a face-centred-cubic lattice with a two-atom basis; this is illustrated in Figure 6.1. In the case of Si and Ge, the atoms on the A and B sites in Figure 6.1 are identical (the so-called diamond structure). In the case of binary semiconductors such as (for example) GaAs the A sites will have Ga atoms and the B sites As atoms; the crystal is said to have the zinc blende structure. As stated above, in each case, the underlying lattice is face-centred cubic and there is a two atom basis. Thus, both the group IV elements and many of the binary semiconductors such as GaAs have the same shape of first Brillouin zone, which is shown in Figure 6.1 and which will be useful for understanding the band diagrams in the following discussion.