Modeling low-dose-rate effects in irradiated bipolar-base oxides

A physical model is developed to quantify the contribution of oxide-trapped charge to enhanced low-dose-rate gain degradation in bipolar junction transistors. Multiple-trapping simulations show that space charge limited transport is partially responsible for low-dose-rate enhancement. At low dose rates, more holes are trapped near the silicon-oxide interface than at high dose rates, resulting in larger midgap voltage shifts. The additional trapped charge near the interface causes an exponential increase in excess base current and a resultant decrease in current gain for some NPN bipolar technologies. Space charge effects also may be responsible for differences in interface trap formation at low and high dose rates.