An empty energy level in the valence band of a semiconductor due to an electron being lost from the band by thermal excitation (see electron-hole pair) or being trapped by an acceptor impurity (see semiconductor). The total current resulting from electrons in a filled valence band is zero:
where j is the current density, n and e the electron density and charge, and v the average velocity of electrons in the valence band. If the jth electron is excited to the conduction band or trapped by an impurity then
The net effect of the mobile electrons occupying all but the jth energy level is therefore equivalent to the effect of a single positive electronic charge ‘occupying’ the vacant level and hence termed a hole. The hole velocity is equal to the velocity of an electron in the same energy level.
Under the influence of an electric field holes can drift through the material by a process of continuous exchange with adjacent electrons (see hole conduction). They therefore act as mobile positive charge carriers in the valence band and are the majority carriers in p-type semiconductors.