Depth defined optoelectronic modulation spectroscopy

It is shown that if the gate depletion region of a MESFET is maintained with a constant width, channel current optoelectronic modulation spectroscopy reveals a spectrum of responses arising predominantly from charge variations in the depletion region at the interface between the active layer and the substrate. Optically induced charge variations as small as 2x10 super(9) electrons/cm super(2)/eV have been detected. A refinement of the basic method is described which should allow the responses of electron states in a region of selected depth within the active layer states to be seen. The method will be of particular value in observing states that have a role in back-gating.