An improved direct parameter extraction method is proposed for III–V heterojunction bipolar transistor (HBT) external base resistance $R_{\rm bx}$ extraction from forward active $S$-parameters. The method is formulated taking into account the current dependence of the intrinsic base–collector capacitance found in III–V HBTs with a fully depleted collector. It is shown that the real part of $Z_{11} - Z_{12}$ reduces to the external base resistance at the collector current $I_{c} = I_{p}/(\hbox{1} - X_{0})$, where $I_{p}$ is a characteristic current and $X_{0}$ is the zero-current distribution factor given as the ratio of the emitter to the collector area. The determination of the parameters $I_{p}$ and $X_{0}$ from experimental $S$-parameters is described. The method is applied to high-speed submicrometer InP/InGaAs DHBT devices and leads to small-signal equivalent circuit models, which accurately predicts the measured $S$-parameters as well as the maximum stable power gain/maximum available power gain in the frequency range from 40 MHz to 110 GHz.