Abstract: Aiming at the improvement of typical equivalent-circuit model's performance on precision, service band and compatibility with network analysis method, from ac I-V equations, this paper calculated SiGe HBTs' two port network parameter model described by scattering parameters. In the calculation, the influence on HBTs' ac characteristics by Si/SiGe heterojunction and base structure were emphatically analyzed, and the reject of physical quantities and the limit to frequency were avoided as much as possible to improve the model's accuracy and frequency range, which provided a wideband, high-precision quantitative approach for the work of the device design, optimization and applications.