Sweepout and Dielectric Relaxation in Compensated Extrinsic Photoconductors

Experimental evidence is presented concerning sweepout of majority carriers in p-type doped germanium photoconductors under high-resistivity conditions where the dielectric relaxation time τ_ρ exceeds the recombination time. This evidence demonstrates sweepout at signal modulation frequencies lower than the inverse dielectric relaxation time for the case of dc photoconductive gain greater than unity and gain saturation for signal modulation frequencies greater than the inverse dielectric relaxation time. These experimental findings are shown to be in general agreement with the predictions of a theoretical approach to semiconductor transport in this regime based upon the frequency dependence of the Debye length in compensated extrinsic photoconductors. This approach calculates the spatial dependence of the photoinduced hole concentration in the sample for an assumed boundary condition of Δp=0 at the anode. This dependence is then used to predict an approximate maximum gain-bandwidth product in the form GB≅1τ_ρ. Lack of exact agreement between theoretical curves and experimental results can be explained by postulating an inhomogeneity in sample resistivity.