A new extraction technique for the complete small-signal equivalent-circuit model of InGaP/GaAs HBT including base contact impedance and AC current crowding effect

In this paper, both ac current crowding and base contact impedance are considered and included in the T-type small-signal equivalent circuit of InGaP/GaAs heterojunction bipolar transistors. The ac current crowding effect and base contact impedance are modeled as a parallel RC circuit, respectively. Devices parameters of the,equivalent circuit are obtained by a new parameters extraction technique. The technique is to directly analyze the two-port parameters of multibias conditions (cutoff-bias, open-collector, and active-bias modes). The parallel capacitances (C-B and C-bi), base resistances (R-B and R-bi), and base inductance (L-B) are especially determined under the active-bias mode without numerical optimization. In addition, the small-signal equivalent circuits of cutoff-bias and open-collector modes are directly derived from the active-bias mode circuit for consistency. By considering base contact impedance and intrinsic base impedance effects in the presented small-signal equivalent circuit, the calculated S-parameters agree well with the measured S-parameters. The observed difference in the slope for the unilateral power gain (U) versus frequency at high frequency is mainly attributed to the ac emitter current crowding effect and well modeled in this study.