ABSTRACT
The electronic properties of semiconductors, which are intermediate between those of metals and insulators, are determined by the presence of both negative and positive carriers of electricity, and this implies an energy-band structure as shown in Figure 6.13, which provides the conceptual basis for describing the transport mechanism. Only a few covalent crystals are consistent with such a band structure, which must have a number of completely full bands. Typical semiconductors are pure elements, like Si and Ge, having the diamond structure, or binary AB compounds, like GaAs and GaP, most of them with the zinc blend structure. In these crystal structures, shown in Figure 3.18, each atom is surrounded by four nearest neighbours that lie at the comers of a tetrahedron. The covalent bond in solid Si and Ge, consisting of tetrahedral sp3 orbitals, has been discussed in Section 1.2. There are four valence electrons that just fill the four bonds per atom, as each is formed by an electron pair with opposite spins. In a similar manner, there are eight electrons per A-B atom pair in the binary compounds, which can be described by the structural formula A x B s~x , where X is the group number in the periodic table. The primitive cell of all these materials contains two atoms, and thus eight valence electrons. If N is the number of primitive cells in the solid,
there are SN valence electrons available, which can fill four bands, since the number of electron states accommodated in a band, including spin, is IN.
