Transport shot noise models and NFmin comparison for SiGe HBTs under different operation configurations

Triggered by the recent discoveries on the superior high-frequency performance exhibited by the common-base (CB) SiGe HBTs over the common-emitter (CE) ones, noise characteristics of the CB SiGe HBT are investigated in this paper. Accurate RF shot noise models are derived for CB SiGe HBTs and used to calculate their minimum noise figure (NF_min). The NF_min of CB SiGe HBTs are then compared with that of the CE SiGe HBTs. Simplified noise models can be obtained for both CE and CB SiGe HBTs by ignoring the base resistance effects in order to obtain an explicit expression of the NFmin for both configurations. It is found, from applying the noise models into CE and CB SiGe HBTs, that the simplified noise model can provide the same noise figure as the improved model for CE SiGe HBTs, while different results were obtained for CB SiGe HBTs. The difference indicates the critical importance of including base resistance in modelling the noise characteristics of CB SiGe HBTs. Regardless of the difference of NFmin between CE and CB SiGe HBTs, CB SiGe HBTs also exhibit low NF_min while offering their potential higher associated power gain values for promising implementation in low-noise amplifiers.