Valence band, the energy region of allowed electron states in a solid; at the absolute zero of temperature it is entirely filled with valence electrons (see Band theory). The electrons of the valence band contribute to the binding energy of the crystal and to its permittivity, and they determine the absorption of light in the crystal; the electrons of a filled valence band take no part in electrical conduction or in other transport processes at T≠0K. Under the influence of thermal motion (T≠0K) and of external agencies (illumination, irradiation with electrons, the introduction of impurities and the like), a usually small part of the electrons passes from the valence band into the conduction band or onto impurity levels in the forbidden band. As a result, a certain number of unfilled electron states (holes) appear in the upper part of the valence band, and the electrons of the valence band are able to take part in electrical conduction.