Rigorous treatment of the electro-thermal behaviour of multi-finger microwave power heterojunction bipolar transistors

A thermo-electrical model for single and multi-finger Heterojunction Bipolar Transistors (HBTs) was developed. The electrical part is based on a thermionic emission-diffusion theory. The thermal part solves the nonlinear thermal boundary problem using an orthogonal function approximation. The high frequency performance of the whole transistor is determined, calculating the admittance matrix of each individual emitter cell. The influence of the base bandgap, the emitter spacing and metal airbridges on the RF performance of multi-finger HBTs is determined numerically and analytically. The impact of the emitter contact as a thermal shunt is demonstrated.