Study of non-diffusive electron transport in thin-base cryogenic SiGe HBTs using the Boltzmann equation and lower limits on noise figure

Silicon-germanium heterojunction bipolar transistors (HBTs) are attractive as low-noise microwave amplifiers due to their low cost and availability as a BiCMOS technology. Their fundamental noise performance limits are thus of interest, and recent studies report that tunneling and non-equilibrium transport phenomena in the base play a role in setting these limits at cryogenic temperatures. Here, we use a semi-analytic solution of the BTE with ab-initio scattering inputs to study non-diffusive base transport at cryogenic temperatures. We examine the impact of non-diffusive transport on transconductance and transit time. Our work helps to identify the lower limits for the noise figure of cryogenic low-noise HBT amplifiers.