We show that silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) are naturally suited for space applications requiring operation of electronics at cryogenic temperatures, and present the first comprehensive investigation of the effects of gamma irradiation on the characteristics of SiGe HBTs operating at liquid nitrogen temperature (77 K). We find that exposure to 1 Mrad total dose at 77 K produces significantly less base current degradation than exposure at 300 K, and hence the total dose tolerance of SiGe HBTs, which is already excellent at room temperature without intentional hardening, improves with cooling. We compare 77 and 300 K radiation results in order to better understand the damage mechanisms.
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