Modeling of Ge-Si heterojunction bipolar transistors for use in silicon monolithic millimeter-wave integrated circuits

Treating Ge-Si layers as narrow-bandgap silicon, both one-dimensional calculations and two-dimensional simulations have been carried out on heterojunction bipolar transistors (HBTs) with varying emitter widths. The results indicate that high-speed operation can be achieved with Ge-Si HBTs, although the maximum of both the unity gain frequency and the frequency at which the unilateral gain becomes unity occurred at lower current densities than expected from the simple model. Additional simulations indicate that by lowering both the base and emitter concentrations, higher cutoff frequencies can be obtained. Similarly, increasing the collector concentration would allow higher current operation, further improving device performance