Dependence of total ionizing dose effect of nMOS transistors on the on/off duty ratio of a gate voltage

The dependence of the total ionizing dose effect (TID effect) of continuously operating nMOS transistors, irradiated with gamma-ray from a Co60 source, on the on/off duty ratio of a gate voltage was studied. The transistors were manufactured by a 180 nm process. It was found that the maximum leak current in the case of 10% duty ratio was by a factor of about 100 less than the case of 90% duty ratio. To explain the observed result, a two-step reaction model was adopted, which describes the increase in the density of positively charged taps in an oxide layer and negatively charged interface traps. It was shown that a set of rate equations based on the proposed model can explain the observed dependence of the leak current on the duty ratio.