Novel collector current phenomenon in advanced bipolar transistors operated at deep cryogenic temperatures

A unique phenomenon that we have observed in all advanced bipolar transistors operating below liquid-nitrogen temperature (LNT=77.3K) is the development of a nonideal collector current at very low injection levels. A trap-assisted tunneling of carriers from the emitter to the collector through the base potential barrier is suggested as the mechanism responsible for this observed phenomenon. The differences in the temperature dependence of the collector leakage current between SiGe HBTs and Si BJTs is explained by modelling the electron tunneling transition probability from the emitter region to traps located in the neutral-base using SCORPIO, a calibrated 1-D simulator for low-temperature Si/SiGe bipolar transistors. A self-aligned, shallow- and deep-trench isolated, epitaxial-base transistor with a polysilicon emitter contact was used in the present investigation.