SiGe, GaAs, and InP Heterojunction Bipolar Transistors

CHAPTER FIVE

HBT Modeling

Device models are commonly used in circuit simulation for the design of integrated circuits. Understanding of physical equations in the derivation of device models can also help process improvement, diagnosis, and new device design. Generally speaking, there are two types of device models: the physics-based model and the empirical model. The physics-based model is derived from device physics. This model is able to account for bandgap discontinuities and other physical effects in the transport equations of the HBT. It can predict device behavior for given device doping profiles and dimensions. The physics-based model could be used for the evaluation of process variation on device behavior. The physics-based model is comprehensive but computationally intensive. The accuracy of physical parameters is important for the determination of terminal currents. The empirical model is developed based on measured I–V and C–V characteristics. The goal of empirical modeling is to obtain analytic formulas that approximate existing data with good accuracy and minimal complexity. Empirical models provide efficient approximation and interpolation. They do not provide insight, predictive capabilities, or encapsulation of theoretical knowledge. The empirical model, however, is compact and suitable for circuit simulation with a large number of transistors on integrated circuits.

5.1   SILICON–GERMANIUM HBT MODELS

In this section we present collector and base current equations ...