Effect of compensation and correlation on conduction near the metal non-metal transition

This paper reviews the experimental evidence which demonstrates that the critical impurity concentration N c for the metal non-metal transition increases when the concentration n of charge carriers is made less than N c by means of compensation. Mott's criterion N c 1/3 a*H = constant, where a*H is the effective Bohr radius of the impurity centre, does not account for the compensation dependence of N c. The statistics of correlated electrons occupying impurity states is discussed. It is shown that the common use of an upper D − band may lead to errors in describing impurity band conduction in the so-called ε2 range at intermediate concentrations unless the proper statistical probability functions are used.