All materials can be classified conductively into the groups conductors, semiconductors and non-conductors. The basic model for this classification is the band model, in which the electrons and their energetic movement are characterized.
This model represents the conductivity of the materials and consists of the valence band and the conduction band. The valence band contains free electrons, and the conduction band has no electrons in the ground state. There is an energetic difference between these two bands, which is the band gap that determines the conductivity of the materials. This band gap is an energetic measure, expressed in electron volts( eV), that describes the potential required for electrons to change from the valence band to the conduction band.
For semiconductor materials made of gallium and indium compounds such as gallium arsenide( GaAs), gallium arsenide phosphide (GaAsP), gallium indium phosphide (GaInP), indium gallium arsenide( InGaAs) and indium phosphide( InP) used in light-emitting devices such as light-emittingdiodes, semiconductor lasers or electroluminescent films, the band gap determines the color of the light emission.
In semiconductors, the band gap can be overcome with relatively low energy. The bridgeable potential is between 0.4 eV and 3.6 eV. Semiconductors with a bandgap between 1 eV and 1.5 eV are called non-wide bandgap semiconductors. If the bandgap values are above 3 eV, then they are wide bandgap semiconductors(WBG). These include silicon carbide( SiC) and gallium nitride( GaN).
For non-conductors, this threshold is extremely high, with conductivity ranging from `10^-15` to `10^-18` S/cm (Siemens per centimeter).
Conductors have no band gap. Electrons can move from the valence band to the conduction band without energy input. Both bands touch each other; the electrons can move freely between the bands.