Operation of SiGe heterojunction bipolar transistors in the liquid-helium temperature regime

We present the first dc measurements of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) operating in the liquid-helium temperature (LHeT=4.2 K) regime. The current gain of the self-aligned, UHV/CVD-grown SiGe HBT increases monotonically from 110 at 300 K to 1045 at 5.84 K, although parasitic base current leakage limits the useful operating current to above about 1.0 μA at 5.84 K. An aggressively designed base profile (peak N/sub AB//spl ap/8×10/sup 18/ cm/sup -3/) is used to suppress base freeze-out at LHeT (R/sub bi/=18.3 k/spl Omega///spl square/ at 4.48 K). We have also identified a non-ideal minority carrier transport mechanism in the collector current at temperatures below 77 K (I/sub C/ is not proportional to exp(qV/sub BE//kT)) which is unaccounted for in conventional device theory. Preliminary calculations suggest that this phenomenon is due to trap-assisted carrier tunneling from the emitter to the collector through the base potential barrier.