| Band Theory for Solids (623 words) |
 | In insulators the electrons in the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors there is a small enough gap between the valence and conduction bands that thermal or other excitations can bridge the gap. |
| Most solid substances are insulators, and in terms of the band theory of solids this implies that there is a large forbidden gap between the energies of the valence electrons and the energy at which the electrons can move freely through the material (the conduction band). |
| The increase in conductivity with temperature can be modeled in terms of the Fermi function, which allows one to calculate the population of the conduction band. |
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