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High drain-leakage currents have been observed in n-channel SOS transistors after exposure to ionizing radiation. These currents could not be explained by the change in gate threshold voltage or by other radiation effects in the gate insulator. Two mechanisms were identified as being responsible for the radiation-induced leakage current. Charge trapping in the sapphire and the consequent formation of a channel at the silicon-sapphire interface is the major cause of the leakage current. Under some conditions, a smaller component of leakage current can also result from charge trapping in the silicon dioxide at island edges.