An analytic expression of fmax for HBTs

An analytic expression of f_max is derived based on a practical HBT circuit model where collector junction capacitance is split up into internal and external parts. It shows that for advanced HBTs, f_max is well characterized by the product of the base resistance and internal collector capacitance. In other words, the external collector capacitance has only a small impact on the determination of f_max. Good agreement between experimental results and numerical circuit simulation confirms the validity of the HBT circuit model. The influence of the external collector capacitance on maximum available gain is also described