Semiconductors

A semiconductor sits between the two extremes: it has a narrow forbidden gap, $E_g<3$ eV, between its valence and conduction bands. At any finite temperature, ordinary thermal agitation is enough to break some covalent bonds and lift a modest number of electrons across this narrow gap into the conduction band, giving the material a resistivity of roughly $10^{-5}$ to $10^{6}\ \Omega\text{m}$ -- far higher than a conductor's, because only a small fraction of the valence electrons are freed, but far lower than an insulator's. The two most important elemental semiconductors are silicon (band gap 1.1 eV at room temperature) and germanium (0.7 eV). Because raising the temperature promotes MORE electrons across the gap, a semiconductor's conductivity INCREASES (its resistance decreases) with ri …