Dose-Rate Effects in the Permanent Threshold Voltage Shifts of MOS Transistors

Data has been collected that shows the permanent threshold voltage shift occurring in MOS transistors exposed to the same total dose of gamma radiation can be greater in a high dose-rate environment than in a low dose-rate environment. This dose-rate effect is ascribed to a "Photovoltaic" bias generation in the substrate of a device which results in an effective gate bias change (positive for P-channel and negative for N-channel transistors). The bias change ranges from 0 to ±1 volt during the radiation burst. Thus, in a high dose-rate ionizing environment, the permanent gate threshold voltage shift of an MOS device, which is known to be a function of the gate bias during irradiation, will exhibit an indirect dose-rate dependence which is caused by an internal change in instantaneous gate bias.