Leakage Current Phenomena in Irradiated SOS Devices

A detailed study of ionizing radiation effects on SOS devices has been performed with emphasis placed on determining the mechanisms of back-channel leakage current phenomena. Behavior for n-channel transistors fabricated with both wet and dry gate oxides is compared and differences in radiation response are attributed to a larger density of hole traps in the sapphire for dry-oxide devices. It is observed that reduction of radiation-induced leakage current to very near its preirradiation value can be readily accomplished by continuing to irradiate a device under a condition of zero drain bias. Studies with low-energy electrons (6 - 45 keV) from a scanning electron microscope reveal that energy must be deposited in the sapphire in order to increase the leakage current and that energy deposition in the sapphire bulk is unimportant in terms of leakage current production. For the present devices, the region which is effective in producing such current extends into the sapphire a distance on the order of 2 ¿m from the Si-Al2O3 interface. If hole traps are spatially distributed, then the dominant mechanism for reducing leakage current is shown to be injection of electrons from Si into Al2O3 where they neutralize trapped holes. A model for radiation-induced production and reduction of leakage current in SOS devices is described.