Impedance properties and broad-band operation of GaAs photoconductive detectors

The impedance properties of GaAs ion-implanted photoconductive detectors reveal an interesting effect. The reflection coefficient S11of the photoconductors shows the detector impedance to be a constant resistance with no reactive component over a bandwidth of more than 1.5 GHz at a particular bias voltage. Broad-band matching to the detector load and transit time limited detector response (no RC time constant) are possible due to the absence of reactive components. A model is presented that explains this effect, as well as the superlinear increase in output signal with bias observed with these detectors. Also, no evidence of the long low response tails present in these and similar photoconductors in seen in the S11measurement, lending experimental support to the common theory that these tails are due to the trapping of excess photogenerated minority carries.