A Multiregion Approach to Modeling the Base-Collector Junction Capacitance

The junction capacitance of heterojunction bipolar transistors (HBTs) is commonly modeled based on the theory of the p-n-homojunction with constant doping levels, made more flexible by the introduction of adjustable model parameters. In III-V HBTs, however, the low-doped collector is often not uniform, but contains both material and doping steps used to suppress the collector current blocking and to improve the linearity. Under these circumstances, the classical formulation is not sufficient to model the capacitance. Similarly, the exponential spatial doping dependence in the collector of modern high-speed HBTs may require a more in-depth approach. In this brief, a smooth expression for modeling the junction capacitance of a multiregion layer stack is presented. The accuracy of the new model is demonstrated based on both simulated and measured data.