Source of slow transient effects in GaAs photoconductors

Photoconductive GaAs devices made from a low-doped epitaxial layer on a semi-insulating substrate show transients in their dark-current DC characteristics that persist for up to ~2000 s. The authors studied these transients for a range of devices both experimentally and by numerical simulation. They solve simultaneously the Poisson and continuity equations with a model that allows for deep-level traps in the active layer, on the surface of the device, and in the substrate. Results from the model suggest that transients of such long duration arise from deep traps at the surface of the active layer rather than in its bulk. This is shown qualitatively through a series of two-dimensional simulations. Chemical etching of the devices confirms experimentally that conduction near the surface is very important