Operation of silicon-germanium heterojunction bipolar transistors with different structures at deep cryogenic temperature

Abstract

Abstract In this work, the device performances of discrete and integrated SiGe heterojunction bipolar transistors (HBTs) with different device structures from 300 to 4.8\u202fK were investigated. The turn-on voltages of base-emitter and base-collector junctions increased non-linearly with temperature cooled to 4.8\u202fK. Energy bandgap engineering was taken into account for the analytical model of the turn-on voltage versus temperature. Incomplete ionization occurred in the base-collector junction because of the low doping concentration. The trap-assisted tunneling current in the forward base current was clearly observed below 20\u202fK. The ideality factor and saturation current were shown to be temperature dependence. The ideality factor was much larger than 2 below 40\u202fK, indicating that the current is not only contributed by drift, diffusion and recombination, but also by tunneling. The peak current gain of the discrete SiGe HBTs achieved the maximum value of 3,388 at 80\u202fK, while that of the integrated was 546 at 140\u202fK. The transconductance in logarithm was linearly dependent on reciprocal temperature above 50\u202fK, but flattened below 50\u202fK. Early effect was evidently observed below 77\u202fK in the fixed base current output characteristics of the discrete SiGe HBTs, and it was not obvious for the integrated SiGe HBTs.