Low temperature operation of Si and SiGe bipolar transistors

The operation of Si and SiGe epitaxial-base bipolar transistors at liquid nitrogen temperature is demonstrated to be comparable or superior to that at room temperature. Tunneling leakage at the emitter-base junction is suppressed at all temperatures as a result of a lightly doped spacer. The maximum current gain of both the Si and the SiGe devices shows a very small temperature dependence. Monte Carlo simulations of a Si-base transistor at 77 K indicate the presence of velocity overshoot, which is supported by the higher current density at the onset of base pushout. The peak cutoff frequency at 85 K is 57 GHz for the Si transistor and 94 GHz for the SiGe transistor, respectively 8% and 25% higher than at 298 K.<>