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.