Overshoot in the response of a photoconductor excited by subpicosecond pulses

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Abstract
Experiments designed to observe velocity overshoot in GaAs photoconductors excited by 2.0 eV photons are discussed. Monte Carlo transient velocity computations are presented which indicate that, for 620 nm (2.0 eV) excitation of GaAs, velocity overshoot will not occur for fields less than 20 kV/cm. Photoconductive device model equations are solved numerically for the case of subpicosecond-pulse excitation. The computed photoconductor response is observed to have an overshoot on the picosecond time scale resulting from charge-separation effects. The overshoot behavior is very similar to that observed in measurements of subpicosecond photoconductor response and previously interpreted in terms of velocity overshoot. We conclude that experimentally observed overshoot response at 620 nm is the result of charge-separation effects and not the result of velocity overshoot.