Transmission line matrix modelling applied to thermal diffusion problems [microwave HBT]

There is currently interest in the use of solid state devices, particularly heterojunction bipolar transistors, (HBTs) for producing pulses of high frequency microwave radiation. The optimal design of such devices necessitates equal consideration of the electrical layout as well as the thermal behaviour. The thermal design process requires the simulation of a repetitive power pulse to ascertain the peak temperature. This may involve simulation over a relatively large time period and use considerable computer time. To try and develop a thermal simulator which was more realistic in terms of its computing requirements it was decided to investigate the transmission line matrix method. The paper outlines some of the features of the TLM simulator which was developed and indicates some aspects of the method which needed to be implemented to develop a potentially useful piece of software. The previous work using finite difference methods had already established that the simulation needed to be three dimensional