Novel Ge-profile design for high-speed SiGe HBTs: modelling and analysis

The authors investigate the optimisation of the Ge profile in SiGe HBTs, with the aim of enhancing current gain without degrading the base transit time at different ambient temperatures and points of film stability. Using a new box-triangular Ge profile, they show that a current gain enhancement of ~3 is achievable at T=300 K for y_tot =14% Ge without degrading the base transit time corresponding to that of a triangular Ge profile. The effect of an electric field on the base transit time is also studied. It is shown that for W_B=50 nm, the limiting base electric field is ~40 kV/cm, beyond which any reduction in the base transit time is offset by the electron mobility degradation