LOW-FREQUENCY TRANSPORT IN SEMIINSULATING GAAS

By extending to the frequency domain the analysis of transport in semi-insulating GaAs two-terminal structures, in which one terminal injects only electrons and the other may inject holes, closed forms were obtained for ac charge and potential distribution under conditions of low-level injection. The presence of deep traps results in frequency dependence of both the excess free and trapped carriers. At low frequencies free electrons move so as to neutralize injected holes, but at higher frequencies charge separation of free carriers due to zero local recombination, typical of relaxation semiconductors, occurs. The corresponding admittance can be represented by an equivalent circuit consisting of a frequency-dependent conductance in parallel with a frequency-dependent capacitance. At very low frequencies the conductance decreases with increasing hole injection. At higher frequencies it increases and then saturates with frequency. At low frequencies the capacitance increases with increasing hole injection and is a strongly decreasing function of frequency. At higher frequencies the admittance depends only weakly on the hole injection ratio. Although we focus on semi-insulating GaAs, the equations presented are in a general form, which is applicable to the frequency-dependent transport in a variety of other semiconductors under conditions of low-level injection.