In
solid-state physics, a
band gap, also called an
energy gap or
bandgap, is an energy range in a
solid where no
electron states can exist. In graphs of the
electronic band structure of solids, the band gap generally refers to the energy difference (in
electron volts) between the top of the
valence band and the bottom of the
conduction band in
insulators and
semiconductors. It is closely related to the
HOMO/LUMO gap in chemistry. If the valence band is completely full and the conduction band is completely empty, then electrons cannot move in the solid; however, if some electrons transfer from the valence to the conduction band, then current
can flow (see
carrier generation and recombination). Therefore, the band gap is a major factor determining the
electrical conductivity of a solid. Substances with large band gaps are generally
insulators, those with smaller band gaps are
semiconductors, while
conductors either have very small band gaps or none, because the valence and conduction bands overlap.