Band theory describes how electrons behave in solids. In isolated atoms, electrons occupy discrete energy levels. When atoms come together to form a solid, these energy levels split into bands that contain closely spaced energy levels.
The key bands for electronic behavior are:
The size of the band gap determines whether a material is a conductor, semiconductor, or insulator:
Metals have overlapping bands or partially filled bands, allowing electrons to move freely without additional energy.
Semiconductors have a small band gap. Thermal energy can excite electrons from the valence to conduction band.
Insulators have a large band gap. Normal temperatures don't provide enough energy for electrons to jump to the conduction band.