Small-Signal Modeling of SiGe HBTs Using Direct Parameter-Extraction Method

A simple and accurate parameter-extraction method of a high-frequency small-signal SiGe heterojunction bipolar transistor model is proposed in this paper. It was found that, without taking the intrinsic circuit elements into account, the conductance of the substrate network will be underestimated, while the susceptance of the substrate network will be overestimated. Therefore, a new extraction technique of the substrate-network parameters was developed, which has taken the intrinsic circuit elements into consideration. Transforming the intrinsic equivalent circuit into its common-collector configuration, all the intrinsic circuit elements are extracted directly from the measured S-parameters without any numerical optimization. Two formulas used to determine the intrinsic base resistance are presented, which is followed by an accuracy-improvement procedure to achieve a better accuracy of the extraction results. Simplified formulas to determine the base-emitter resistance, base-emitter capacitance, transconductance, and excess phase delay are also presented. The proposed method is validated with SiGe HBTs fabricated with a 0.35-mum BiCMOS technology from 1 to 30 GHz. The agreements between the measured and modeled data are excellent in the desired frequency range over a wide range of bias points