Doping of semiconductors

Doping of semiconductors is achieved by introducing atoms with more or less electrons than the parent element. Doping is substitutional, the dopant atoms directly replace the original atoms. Suprisingly low levels of dopant are required, only 1 atom in 10⁹ of the parent atoms.

Looking at silicon; if phosphorous atoms are introduced into a silicon crystal then extra electrons will be available (one for each dopant atom introduced as P has one extra valence electron). The dopant atoms form a set of energy levels that lie in the band gap between the valence and conduction bands, but close to the conduction band. The electrons in these levels cannot move directly as there is not enough of them to form a continuous band. However, the levels themselves can act as donor levels because the electrons have enough thermal energy to get up into the conduction band where they can move freely.

Such semiconductors are known as n-type semiconductors, representing the negative charge carriers or electrons.

What if, instead of doping with phosphorous, we doped silicon with an element with one less valence electron such as gallium. Now for every dopant atom there is an electron missing, and the atoms form a narrow, empty band consisting of acceptor levels which lie just above the valence band. Electrons from the valence band may have enough thermal energy to be promoted into the acceptor levels, which are discrete levels if the concentration of gallium atoms is small. Therefore, electrons in the acceptor levels cannot contribute to the conductivity of the material. However, the positive holes in the valence band left behind by the promoted electrons are able to move.

These type of semiconductors are known as a p-type semiconductors, representing the positive holes.

Representation of p-type and n-type semiconductors is shown in (fig. 8) on the next page.