Band-gap theory

Conduction is the flow of electrons or ions in a material, which can be understood through band-gap theory. Band-gap theory says that in all materials there exists a valence band and conduction band. These bands represent energy levels. In a conductive material such as metal, there is no energy difference between the CB and CB, and the bands will overlap. In an insulating material, the energy gap is too large for electrons to pass and thus is "forbidden". In a semi-conductor, the energy gap exists, but electrons can move from the valence band to the conduction band in the aid of temperature, pressure or doping.

Fig 1. Conductors vs. insulators via band-gap theory.