Semiconductor

A semiconductor is a substance with electrical conductivity intermediate between metals and insulators. Most commonly these materials are solids.

Density of states

(CC) Image: John R. Brews
Calculated density of states for crystalline silicon.

In liquid and solid materials where atoms are in close proximity to one another, the energy levels available to electrons fall into bands separated by energy gaps. The density of energy levels per unit energy as a function of energy might look as in the figure. This density of states relate to the semiconducting behavior of this material as follows:

1. Bands of allowed energy are separated by an energy gap. Those above the gap are called conduction band energy levels, and those below are valence band energy levels.
2. At low temperatures, electrons occupy all the energy levels below the gap, and none above the gap.
3. The energy gap is small enough that at normal temperatures some electrons can acquire thermal energy sufficient to occupy a few conduction band energy levels above the gap, leaving vacant levels (holes) in the valence band energy levels below the gap.
4. The conduction band energy levels, and possibly the hole energy levels too, correspond to states spatially extended through large regions of the material and are not localized. That means that electrons (and possibly holes too) can travel easily (conduct) by moving between adjacent levels in response to an applied field.