ABSTRACT
As is known from solid state physics (Ashcroft and Mermin, 1979a and 1979b; Davydov, 1976; Kittel, 1978), the solids can be classified according to the nature of electrical conductivity as electrical conductors (usually metals), semiconductors and insulators (dielectrics). Appreciable conductivity is typical of conductors and semiconductors. In terms of the band theory of solids1, semiconductors differ from metals in that their electrons are located in the valence band, separated from free conductionband by the forbidden band (energy gap), the width of which can range from several tenths of eV to several eV. In conductors (metal) there is no forbidden gap between the valence and conduction bands. Another significant difference between semiconductors and metals is the strong direct dependence of conductivity on temperature, whereas in metals it is the opposite. In addition, the conductivity of semiconductors is strongly dependent on the purity of the material, i.e. the presence of impurities.
