Transient Radiation-Induced Response of MOS Field Effect Transistors

It was found that the radiation-induced leakage current across the gate-to-substrate and the drain-to-substrate semiconductor junction dominates the behavior of MOS field effect transistors for small gate impedances. The smaller radiation-induced gate leakage current consists, to a large extent, of components due to secondary emission and air ionization effects. For large gate impedances, the gate leakage current can be important because of a modulation of the gate bias. The voltage dependence of this leakage current is also discussed. Based upon these findings and the physical structure of the device, an equivalent circuit model has been developed to predict the behavior of devices of this type in a transient radiation environment. The model consists of a standard nonradiation equivalent circuit modified by the inclusion of elements to describe the substrate junctions and the addition of current generators in parallel with the gate-to-substrate and the drain-to-substrate capacitances to account for transient radiation effects. A voltage dependent gate-to-channel current generator can also be included to describe the gate leakage current. The utility of the model was confirmed by comparing the predicted behavior of radiation-induced responses with responses observed at a flash X-ray. The equivalent circuit model developed is suitable for use with existing automated circuit prediction techniques and contains parameters measurable with standard instrumentation procedures.