Optimal design of high frequency power transistors

Recent requirements for high power transistors in the UHF region have necessitated certain new design concepts. This paper treats these concepts and their application to a device which has recently delivered 1.0 watt with 10 db gain and 3.0 watts with 5 db gain as an amplifier at 500Mc. High frequency power gain requires that the ratio of the emitter edge to emitter area be maximized, leading to the conventional linear design. Gain considerations also tend to require that the ratio of the emitter area to the collector area be maximized. These considerations lead to the unbalanced linear design in which the base surrounding the emitter is only that required to support the transistor action over the entire emitter length. The linear design has been successfully utilized in an interdigitated pattern at low power levels, but experience with this and other re-entrant patterns has indicated thermal problems at higher power levels. The annular pattern employing linear design has been quite successful at the higher power levels, but is wasteful of crystal area. A spiral based on the annular design provides the most efficient usage of crystal area for a continuous device. This pattern in conjunction with new breakdown voltage and space-charge current limitation considerations has produced a more satisfactory thermal design for power transistors in this UHF region.