In an isolated atom, electrons occupy discrete energy states. In a solid containing many interacting atoms, these states broaden into ranges of allowed energies called bands.

The highest normally occupied band is the valence band. A higher allowed range is the conduction band. Between them may lie an energy interval with no allowed states: the band gap.

In a conductor, mobile electronic states are readily available. In an insulator, the band gap is large enough that very few carriers can reach conducting states under ordinary conditions. A semiconductor lies between these cases: its carrier population can change strongly with temperature, light or added impurities.

When an electron is promoted from the valence band into the conduction band, it leaves an unoccupied state called a hole. Conduction can therefore be described using both mobile electrons and holes.

Band structure provides the microscopic reason semiconductor conductivity can be deliberately controlled rather than being fixed like an ideal resistor.