Anomalous dose rate effects in gamma irradiated SiGe heterojunction bipolar transistors
Low dose rate (LDR) cobalt-60 (0.1 rad(Si)/s) gamma irradiated Silicon Germanium (SiGe) Heterojunction Bipolar Transistors (HBTs) were studied. Comparisons were made with devices irradiated with 300 rad(Si)/s gamma radiation to verify if LDR radiation is a serious radiation hardness assurance (RHA) issue. Almost no LDR degradation was observed in this technology up to 50 krad(Si). The assumption of the presence of two competing mechanisms is justified by experimental results. At low total dose (<20 krad), an anomalous base current decrease was observed which is attributed to self-annealing of deep-level traps to shallower levels. An increase in base current at larger total doses is attributed to radiation induced generation-recombination (G/R) center generation. Experiments on gate-assisted lateral PNP transistors and 2D numerical simulations using MEDICI were used to confirm these assertions.