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 tran sistors. Multiple-trapping simulations show that space charge limited trans port is partially responsible for low-dose-rate enhancement. At low dose ra tes, more holes are trapped near the silicon-oxide interface than at high d ose rates, resulting in larger midgap voltage shifts. The additional trappe d charge near the interface causes an exponential increase in excess base c urrent and a resultant decrease in current gain for some NPN bipolar techno logies. Space charge effects also may be responsible for differences in int erface trap formation at low and high dose rates.