Circuit Analysis of Photosensitive Capacitance in Semi-Insulating GaAs

We describe a circuit model for photosensitive capacitance in bulk semi-insulating GaAs toward tunable resonant applications. Capacitances from two separate regions are considered to interpret experimental results. A smaller valued capacitance exists between the depletion edges within the bulk material. Photodoping in this region progressively shorts out the bulk capacitance, leaving only the higher valued depletion capacitance. The depletion capacitance also increases with illumination, and numerical simulation is used to aid interpretation. Thus, the series combination of capacitance can be optically varied over orders of magnitude. Our results indicate that capacitance is nearly independent of applied voltage over a usable parameter space, making the concept attractive for linear application.