Improved External Base Resistance Extraction for Submicrometer InP/InGaAs DHBT Models

An improved direct parameter extraction method is proposed for III-V heterojunction bipolar transistor (HBT) external base resistance R_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 Z₁₁ - Z₁₂ reduces to the external base resistance at the collector current Ic = Ip/(1 - X₀), where Ip is a characteristic current and X₀ is the zero-current distribution factor given as the ratio of the emitter to the collector area. The determination of the parameters Ip and X₀ 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.