Physical mechanisms contributing to enhanced bipolar gain degradation at low dose rates

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Abstract
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 O-V irradiation at /spl sim/25/spl deg/C, the net trapped-positive-charge density (N/sub ox/) inferred from midgap C-V shifts is /spl sim/25-40% greater for low-dose-rate (100 rad(SiO/sub 2/)/s) exposure. Device modeling shows that such a difference in screen-oxide N/sub ox/ is enough to account for the enhanced low-rate gain degradation often observed in bipolar devices, due to the /spl sim/exp(N/sub ox//sup 2/) dependence of the excess base current. At the higher rates, TSC measurements reveal a /spl sim/10% decrease in trapped-hole density over low rates. Also, at high rates, up to /spl sim/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/sub /spl delta//' centers). On the basis of this model, bipolar transistors and screen-oxide capacitors were irradiated at 60/spl deg/C at 200 rad(SiO/sub 2/)/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.