PHYSICAL-MECHANISMS CONTRIBUTING TO ENHANCED BIPOLAR GAIN DEGRADATION AT LOW-DOSE RATES

We have performed capacitance-voltage (C-V) and thermally-stimulated-current (TSC) measurements on non-radiation-hard MOS capacitors simulating screen oxides of modern bipolar technologies. For 0-V irradiation at similar to 25 degrees C, the net trapped-positive-charge density (N-ox) inferred from midgap C-V shifts is similar to 25-40% greater for low-dose-rate (< 10 rad(SiO2)/s) than for high-dose-rate (> 100 rad(SiO2)/s) exposure. Device modeling shows that such a difference in screen-oxide N-ox is enough to account for the enhanced low-rate gain degradation often observed in bipolar devices, due to the similar to exp (N-ox(2)) dependence of the excess base current. At the higher rates, TSC measurements reveal a similar to 10% decrease in trapped-hole density over low rates. Also, at high rates, up to similar to 2.5-times as many trapped holes are compensated by electrons in border traps than at low rates for these devices and irradiation conditions. Both the reduction in trapped-hole density and increased charge compensation reduce the high-rate midgap shift. A physical model is developed which suggests that both effects are caused by time-dependent space charge in the bulk of these soft oxides associated with slowly transporting and/or metastably trapped holes (e. g., in E(delta)' centers). On the basis of this model, bipolar transistors and screen-oxide capacitors were irradiated at 60 degrees C at 200 rad(SiO2)/s in a successful effort to match low-rate damage. These surprising results provide insight into enhanced low-rate bipolar gain degradation and suggest potentially promising new approaches to bipolar and BICMOS hardness assurance for space applications.