A group of novel device phenomena are reported in state-of-the-art SiGe HBTs operating at cryogenic temperatures. Both negative-differential-resistance (NDR) and an unusual "hysteresis" behavior are observed in the forced-I-B output characteristics of 350 GHz SiGe HBTs at cryogenic temperatures. Unlike the NDR effects in resonance-tunneling-diodes and III-V HBTs, the phenomena demonstrated in this paper are correlated to SiGe HBT high-injection effects and modulated by bias level. This unusual cryogenic behavior have been systematically investigated, and the results are compared to 50 GHz, 120 GHz, and 200 GHz SiGe HBT technology generations. An advanced Shockley-Read-Hall (SRH) recombination model including tunneling effects is introduced and used to explain the underlying NDR and "hysteresis" mechanisms in these cooled SiGe HBTs. Implications for potential novel device and circuit designs are suggested. (c) 2006 Elsevier Ltd. All rights reserved.