Accelerated tests for simulating low dose rate gain degradation of lateral and substrate pnp bipolar junction transistors

The effect of dose rate on radiation-induced current gain degradation at 20 krad(Si) was quantified for lateral and substrate pnp bipolar transistors over the range of 0.001 to 294 rad(Si)/s. Degradation increases monotonically with decreasing dose rate, such that, at an emitter-to-base voltage of 0.7 V, radiation-induced excess base current differs by a factor of approximately, eight at the extreme dose rates. Degradation shows little dependence on dose rate below 0.005 rad(Si)/s, suggesting that further degradation enhancement at space-like dose rates may be negligible. In addition, the effect of ambient temperature on radiation-induced gain degradation at 294 rad(Si)/s was thoroughly investigated over the range of 25 to 240 degrees C. Degradation is enhanced with increasing temperature while simultaneously being moderated by in situ annealing such that, for a given total dose, an optimum irradiation temperature for maximum degradation results. Optimum irradiation temperature decreases logarithmically with total dose and is larger and more sensitive to dose in the substrate device than in the lateral device. Based on the measurement of midgap interface trap density in the base oxide, enhancement in transistor gain degradation due to elevated temperature is explained as an increase in surface recombination velocity in the base. Maximum high dose rate degradation at elevated temperature closely approaches low dose rate degradation for both devices. Based on high-temperature irradiations, a flexible procedure for the accelerated prediction of low dose rate gain degradation at 20 krad(Si) is developed for each of the devices studied.