OPERATION OF SIGE HETEROJUNCTION BIPOLAR-TRANSISTORS IN THE LIQUID-HELIUM TEMPERATURE REGIME

We present the first de 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 mu A at 5.84 K. An aggressively designed base profile (peak N-AB approximate to 8 X 10(18) Cm-3) is used to suppress base freeze-out at LHeT (R(bi) = 18.3 k Omega/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-C is not proportional to exp(qV(BE)/ET)) 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.