The band gap in diamond is 580 kJ/mol-so large that electrons are trapped in Giled valence band and cannot make the transition to the conduction band, even at elevated temperatures. Thus, it is not possible to create positive "holes," and dia- mond is an insulator, a nonconductor. Silicon and germanium have much smaller band gaps, 106 kJ/mol for silicon and 68 kJ/mol for germanium. As a result they are semiconductors. These elements can conduct a small current because thermal energy is sufficient to promote a few electrons from the valence band across the band gap to the conduction band (Figure 12.17). Conduction then occurs when the electrons in the conduction band migrate in one direction and the positive holes in the valence band migrate in the opposite direction. Pure silicon and germanium are called intrinsic semiconductors, with the name referring to the fact that this is an intrinsic or naturally occurring property of the pure material. In intrinsic semiconductors, the number of electrons in the conduc- tion band is determined by the temperature and the magnitude of the band gap. The smaller the band gap, the smaller the energy required to promote a significant num- ber of electrons. As the temperature increases, more electrons are promoted into the conduction band, and a higher conductivity results. There are also extrinsic semiconductors. The conductivity of these materials is controlled by adding small numbers of different atoms (typically 1 in 106 to 1 in 108) called dopants. That is, the characteristics of semiconductors can be changed by altering their chemical makeup. Suppose a few silicon atoms in the silicon lattice are replaced by aluminum atoms (or atoms of some other Group 3A element). Aluminum has only three valence elec- trons, whereas silicon has four. Four Si-Al bonds are created per aluminum atom in the lattice, but these bonds must be deficient in electrons. According to band theory, the Si-Al bonds form a discrete but empty band at an energy level higher than the valence band but lower than the conduction band. This level is referred to as an ac- ceptor level because it can accept electrons from the valence band, The gap between the valence band and the acceptor level is usually quite small, so electrons can be promoted readily to the acceptor level. The positive holes created in the valence band are able to move under the influence of an electric potential, so current results from the hole mobility. Because positive holes are created in an aluminum-doped semicon- ductor, this is called a p-type semiconductor (Figure 12.17b, left).