Mixed conduction in doped semiconductor structures related to quasi-metallic conduction in the impurity band

Mixed conduction due to simultaneous contributions from allowed states in the valence band and extended impurity (acceptor) states, which occur in the impurity band at high impurity concentrations because of the Anderson transition, is observed in a series of GaAs/AlGaAs structures. Mixed conduction manifests itself in the existence of a minimum in the temperature dependence of the carrier concentration and a noticeable bend in the temperature dependences of the conductivity. Expressions for the mixed conductivity on which the calculations are based were derived taking into account the spectrum of impurity states in the quantum wells (the upper and lower Hubbard impurity bands), their occupancies, and the sign of charge carriers in the valence and the impurity bands; the important assumption was made that the width of impurity bands is much smaller than the spacing from the valence band. The calculation results agree well with the experiment and were used to determine the binding energies for the upper and lower Hubbard bands, the acceptor concentration, and the degree of compensation. It is shown that formulas commonly used for the calculation of mixed conductivity need significant corrections in the case of narrow impurity bands.