A PHYSICS-BASED, ANALYTICAL HETEROJUNCTION BIPOLAR-TRANSISTOR MODEL INCLUDING THERMAL AND HIGH-CURRENT EFFECTS

We present a detailed, analytical model to predict the dc and high-frequency performance of AlGaAs/GaAs graded heterojunction bipolar transistors (HBT's). The model is developed based on the relevant device physics such as current-induced base pushout and thermal effect. The current gain, cutoff frequency, and maximum frequency versus the collector current density, which is a function of the applied voltage as well as the corresponding temperature in the HBT, are calculated. Our results suggest that the conventional HBT model, which assumes the HBT temperature is the same as that of the ambient, can overestimate the three figures of merit considerably when the collector current density is high. Furthermore, it is shown that the present model correctly explains the experimentally observed HBT high-current behavior like the rapid fall-off of the current gain and cutoff frequency. The model predictions compare favorably with the results obtained from a model which solves numerically the Poisson and continuity equations coupled with the lattice heat equation.