Many-particle Systems, 10

Intrinsic semiconductors Intrinsic semiconductors have negligible concentrations of impurity atoms. Their electrical conductivity arises primarily from electrons excited into the otherwise empty conduction band from the otherwise filled valence band—usually by absorbing sufficient energy from phonons at finite temperature. Exciting an electron into the conduction band leaves a vacant state in the valence band. An electron at lower energy in the valence band can fill this vacant state. That, in turn, makes available a possible state for yet another valence band electron to fill. In other words, the excitation of the electron provides a mobile charge in the conduction band as well as a mobile unoccupied state in the valence band. When an external electric field is turned on, the conduction band electron accelerates in the opposite direction while the valence band state accelerates in the same direction as the field. For this reason, the state is considered to be a mobile, effective positive charge. Such a positive, effective charge is called a “hole.