21.5 Semiconductors
		A material having electrical conductivity intermediate between a metal and an insulator is called a semiconductor. The semimetals and some of their compounds are semiconductors, as well as a few other compounds, such as ZnS. In the Interactive Student Tutorial section of Chapter 21.4, we considered the band theory of metals that are conductors. Let's next consider an insulator such as diamond, a covalent network solid. Each carbon atom is tetrahedrally bonded to four other carbon atoms. The 2s and 2p valence orbitals combine to give bands of bonding and antibonding MOs. The bonding MOs combined in the valence band are separated from the antibonding (conduction band) MOs by a large band gap. No conduction occurs because there are no vacant MOs in the valence band to which electrons can be excited by an electrical potential, and because of the large band gap. The MO energy levels for a metallic conductor and an insulator are represented in (a) and (b) below.
		The MOs of a semiconductor are shown in (c) above. The band gap in a semiconductor is smaller, and relatively few electrons have enough energy to jump the gap and occupy the higher-energy conduction band. The application of electrical potential can accelerate electrons into the partially filled bands. Table 21.3 lists the band gaps of the group 4A elements. The electrical conductivity of a semiconductor increases with increasing temperature because the number of electrons with sufficient energy to occupy the conduction band increases as the temperature rises. In contrast, the electrical conductivity of a metal decreases with increasing temperature owing to increased vibrational motion around lattice sites, disrupting the flow of electrons. Doping is a process by which the conductivity of a semiconductor can be greatly increased by adding small amounts of impurities. n-type semiconductor: Semiconductor is doped by the addition of a higher-group element. Extra electrons are available to occupy the conduction band, which increases the conductivity. The charge carriers are negative electrons. p-type semiconductor: Semiconductor is doped by the addition of a lower-group element. Bonding MOs of the valence band are partially filled and are considered positive holes.