An Improved Small-Signal Model for SiGe HBT Under OFF-State, Derived From Distributed Network and Corresponding Model Parameter Extraction

An improved high-frequency small-signal model for SiGe HBTs under the off-state is presented in this paper. The proposed model takes into account the distribution characteristics of the intrinsic transistor, link base region under spacer, and extrinsic base–collector junction. The equivalent circuit for each region is separately derived using the transmission line equation with reasonable approximations. Being different from previous models, the intrinsic base resistance in the proposed model is pushed inside the internal base node and added to the components of collector and emitter resistance. To extract all the parameters for the proposed model, a novel extraction technique based on rational function fitting over the whole range of frequencies is developed. After the rational function fitting to related admittance parameters, a number of coefficients are accurately obtained and then all the model parameters are directly extracted without any special test structure or numerical optimization. The proposed model and extraction technique are validated with a series of sized SiGe HBTs from 100 MHz to 20.89 GHz at a wide range of bias points. An excellent agreement is obtained between the measured and simulated S-parameters.