A variety of solid-state devices are being developed for use in systems where they are required to produce pulses of microwave power at specified duty cycles. The design of these devices is interesting as they will be smaller and have less thermal capacity than an equivalent continuous-wave source generating the same microwave power. This size reduction is important as it can result in improved electrical efficiency and a higher frequency of operation. The paper describes the application of transmission line matrix diffusion modelling to the transient thermal design of a transistor structure and compares the technique with finite difference and finite element approaches.