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-fre quency performance of AlGaAs/GaAs graded heterojunction bipolar transi stors (HBT's). The model is developed based on the relevant device phy sics such as current-induced base pushout and thermal effect. The curr ent gain, cutoff frequency, and maximum frequency versus the collector current density, which is a function of the applied voltage as well a s the corresponding temperature in the HBT, are calculated. Our result s suggest that the conventional HBT model, which assumes the HBT tempe rature is the same as that of the ambient, can overestimate the three figures of merit considerably when the collector current density is hi gh. 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 predictio ns compare favorably with the results obtained from a model which solv es numerically the Poisson and continuity equations coupled with the l attice heat equation.