Conduction of Electricity in Semiconductors

In a semiconductor the gap between the valence band and the conduction band is small (Fig. 1.33c), so a few electrons can jump into the conduction band and give some conductivity. Because more electrons cross the gap as the temperature rises, the conductivity of semiconductors increases with temperature. Pure silicon and germanium behave this way and are called intrinsic semiconductors.

The conductivity of intrinsic semiconductors is too low for practical use. It is boosted by adding a small amount of a suitable impurity — a process called doping — using an impurity that is either electron-rich or electron-deficient relative to Si or Ge.

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(a) Electron-rich impurities → n-type

Silicon and germanium are group 14 elements with four valence electrons, and in their crystals each atom forms four covalent bonds (Fig. 1.34a). Doping with a group 15 element like P or As (five valence electrons) puts the dopant on some lattice sites (Fig. 1.34b). Four of its electrons form the four bonds to neighbouring atoms, but the fifth is extra and becomes delocalised, raising the conductivity. Because the extra charge carriers are negative electrons, this is an n-type semiconductor.

(b) Electron-deficient impurities → p-type

Doping instead with a group 13 element like B, Al or Ga (only three valence electrons) leaves the site of the missing fourth bond as an electron hole (electron vacancy) (Fig. 1.34c). A neighbouring electron can jump in to fill the hole, but this leaves a new hole behind, so the hole appears to move in the direction opposite to the electrons. Under an electric field the holes behave like positive charge carriers drifting to the negative plate, giving a p-type semiconductor.

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Applications and related compounds

Combinations of n- and p-type semiconductors make many electronic components. A diode (an n–p junction) acts as a rectifier; transistors (npn or pnp, made by sandwiching one type between two layers of the other) detect or amplify radio or audio signals; and the solar cell is an efficient photo-diode that converts light into electrical energy. …