At 0K, all valence electrons are engaged in covalent bonds and are, therefore, unavailable as carriers—that is, as transporters of electric charge. No current can flow through the crystal; it is an insulator.
However, if a bond is disrupted (by thermal agitation of the lattice or through the impact of a photon or a high-speed free electron), then one of the valence electrons is ejected from the bond and becomes free to carry electricity, leaving behind an incomplete bond, one in which a hole exists into which an electron from a neighboring bond can fall. This causes the hole to move to a new place. Thus, the disruption of a bond creates a pair of carriers—an electron and a hole imparting some degree of conductivity to the material.
It is clear from our picture, that, in this particular case, the number of free (conduction) electrons is exactly equal to the number of holes. Such materials are called intrinsic.
One can now understand why semiconductors usually have an electric conductivity that increases when the temperature increases: the warmer the material, the greater the number of carriers.
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