Microwave signal-to-noise performance of CdSe bulk photoconductive detectors

The microwave signal and noise response of CdSe bulk photoconductive detectors have been measured and compared with that of a high-speed junction photodiode. The signal measurements were made with CW intensity modulated light at 3000 Mc. Excellent quantitative agreement was obtained between the simple photoconductivity theory of direct light demodulation and experiment. At low optical intensities, the observed noise can be accounted for within the framework of existing theories of generation-recombination (GR) noise when the effects of electron retrapping are accounted for. At high optical intensities, nonlinear deviations are observed which can be accounted for only qualitatively. The results demonstrate clearly that such a photoconductor, when used as a simple quantum counter, will always be thermal rather than GR noise limited. Measurements show that the photoconductor as it is usually used in a nonoptimum configuration is some 50 db less sensitive than a good photodiode.