Ionizing-radiation-induced gain degradation in lateral PNP bipolar-junction transistors is due to an increase in base current as a result of recombination at the surface of the device. A qualitative model is presented which identifies the physical mechanism responsible for excess base current. The increase in surface recombination velocity due to interface traps results in an increase in excess base current and the positive oxide charge moderates the increase in excess base current and changes the slope of the current-voltage characteristics. Analytical and empirical models have been developed to quantitatively describe the excess base current response to ionizing radiation. It is shown that the surface recombination velocity dominates the excess base current response to total dose.

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 radiationinduced gain degradation at 294 rad(Si)/s was thoroughly investigated over the range of 25 to 240 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 hightemperature 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.

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.

The dose-rate dependence of gain degradation in lateral PNP transistors is even stronger than the dependence previously reported for NPN BJTs. In this work, several hardness-assurance approaches are examined and compared to experimental results. obtained at low dose rates. The approaches considered include irradiation at high dose rates while at elevated temperature and high-dose-rate irradiation followed by annealing. The lateral PNP transistors continue to degrade during post-irradiation annealing, in sharp contrast to NPN devices studied previously. High-temperature conditions significantly increase the degradation during high-dose-rate irradiation, with the amount of degradation continuing to increase with temperature throughout the range studied here (up to 125 degrees C). The high-temperature degradation is nearly as great as that observed at very low dose rates, and is even greater when differences between Co-60 and x-ray irradiation are accounted for. Since high-temperature irradiation has previously been shown to enhance the degradation in NPN transistors, this appears to be a promising hardness-assurance approach for bipolar integrated circuits. Based on these results, preliminary testing recommendations are discussed.

A comparison is presented of ionizing-radiation-induced gain degradation in lateral, substrate, and vertical PNPs. The dose-rate dependence of current gain degradation in lateral PNP BJTs is even stronger than the dependence previously reported for NPN BJTs. Various mechanisms are presented and their relative significance for gain degradation in the lateral, substrate, and vertical PNPs is discussed. A detailed comparison of the lateral and substrate PNP devices is given. The specific lateral and substrate devices considered here are fabricated in the same process and possess identical emitters. Even though these devices have identical emitters and undergo the same processing steps, the lateral devices degrade significantly more than the substrate devices.

Process and device simulation software tools are used to produce an animated visualization of the mechanisms involved in the ionizing-radiation and hot-carrier stress responses of BJTs. A physically-based model is presented, which compares ionizing-radiation response with hot-carrier response in poly-silicon-emitter BJTs. During ionizing radiation, positive charge accumulates along the oxide-silicon interface. The accumulated charge causes excess base current to flow, characterized by an ideality factor between one and two for low total doses of ionizing radiation, and an ideality factor of two for high total doses of ionizing radiation. During hot-carrier stress, the oxide damage is localized near the emitter-base junction, and the excess base current has an ideality factor of two.<>