PHYSICAL-MECHANISMS CONTRIBUTING TO ENHANCED BIPOLAR GAIN DEGRADATIONAT LOW-DOSE RATES

We have performed capacitance-voltage (C-V) and thermally-stimulated-c urrent (TSC) measurements on non-radiation-hard MOS capacitors simulat ing 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(Si O2)/s) exposure. Device modeling shows that such a difference in scree n-oxide N-ox is enough to account for the enhanced low-rate gain degra dation often observed in bipolar devices, due to the similar to exp (N -ox(2)) dependence of the excess base current. At the higher rates, TS C measurements reveal a similar to 10% decrease in trapped-hole densit y over low rates. Also, at high rates, up to similar to 2.5-times as m any trapped holes are compensated by electrons in border traps than at low rates for these devices and irradiation conditions. Both the redu ction in trapped-hole density and increased charge compensation reduce the high-rate midgap shift. A physical model is developed which sugge sts 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 o f this model, bipolar transistors and screen-oxide capacitors were irr adiated at 60 degrees C at 200 rad(SiO2)/s in a successful effort to m atch low-rate damage. These surprising results provide insight into en hanced low-rate bipolar gain degradation and suggest potentially promi sing new approaches to bipolar and BICMOS hardness assurance for space applications.