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 approximately 2000 s. These hamper the dc characterization of a device, degrade its performance in applications such as crossbar switches, and imply that the off-state resistance and isolation vary with time. We have studied these transients for a range of devices both experimentally and by numerical simulation. We 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. We show this qualitatively through a series of two-dimensional simulations. Chemical etching of the devices confirms experimentally that conduction near the surface is very important.