Current gain degradation in bipolar junction transistors due to radiation, electrical and mechanical stresses

Abstract

The current gain of bipolar junction transistors is reduced due to ionizing radiation exposure or hot-carrier stressing. Radiation-induced degradation is particularly severe at the low dose rates encountered in space. In this work, the dose rate effect in lateral and substrate pnp bipolar transistors is rigorously quantified over the range of 0.001 to 294 rad(Si)/s. Gain degradation shows little dependence on dose rate below 0.005 rad(Si)/s, suggesting that degradation enhancement comparable to that expected from space-like dose rates was achieved. In addition, the effect of ambient temperature on radiation-induced gain degradation at 294 rad(Si)/s is 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. Maximum high dose rate degradation at elevated temperature closely approaches low dose rate degradation in both of the devices. A flexible hardness assurance methodology based on accelerated irradiations at elevated temperatures is described. The influence of mechanical stress on the radiation hardness of single-crystalline emitter transistors is investigated using x-ray diffraction. Correlation of device radiation sensitivity and mechanical stress in the base supports previously reported observations that Si-SiO₂ interfaces exhibit increased susceptibility to radiation damage under tensile Si stress. Relaxation of processing-induced stress in the base oxide due to ionizing radiation is smaller than the stress induced by emitter contact metallization followed by a post-metallization anneal. Possible mechanisms for radiation-induced stress relaxation and their effect on the radiation sensitivity of bipolar transistors are discussed. The combined effects of ionizing radiation and hot-carrier stress on the current gain of npn transistors are investigated. The hot-carrier response of the transistors is improved by radiation damage, whereas hot-carrier damage has little effect on subsequent radiation stress. Characterization of the temporal progression of hot-carrier effects reveals that hot-carrier stress acts initially to reduce excess base current and improve current gain in irradiated transistors. Numerical simulations show that the magnitude of the peak electric-field within the emitter-base depletion region is reduced significantly by net positive oxide charges induced by radiation. The interaction of the two stress types is explained in a physical model based on the probability of hot-carrier injection and the neutralization and compensation of radiation damage in the base oxide. The results of this work further the understanding of stress-induced gain degradation in bipolar transistors and provide important insight for the use of bipolar transistors in stress environments.