Advances in the design of microwave power transistors

A comparative study of the techniques employed by various manufacturers to optimize their transistor designs has been completed. Sufficient transistor theory is outlined to exhibit the relationship between performance parameters such as beta, cut-off frequency and power gain and the physical characteristics of the device, both vertically and horizontally. Current crowding is discussed and the consequent figure of merit, the emitter perimeter/base area defined. Attempts to maximize this aspect ratio have led to a number of geometries: interdigitated, ladder or overlay, emitter mesh and diamond. Such design innovation as multiple cells, emitter bias resistors and internal impedance matching are discussed. The combination of fine geometry, high currents and high temperature make inordinate demands on the metallization system of a microwave power transistor. The advantages and limitations of different metallization schemes are discussed.