Explain the energy bands in solids.

There are as several energy bands in a solid as there are energy levels in the parent atoms.

Many electrical properties of significance to engineers and scientists are associated to the upper band of energy levels and to be exact the upper two termed as the conduction band and the valance band. The valence bands have energies of similar level as those of valence electrons. Electrons in such band are in effect attached to particular atoms and thus not free to move about.

The conduction band energies are high adequate therefore electrons attaining these levels of energy are loosely attached to particular atoms or practically free such as they could easily move under the affect of an electric field. Electrons in the valence band can leave their band to join the conduction band when given adequate energy to jump the forbidden energy band (energy gap, as Eg). The size of Eg is a prime factor in determining whether a solid is a semiconductor, an insulator, or a conductor.