A key issue in the design of power microwave HBTs for high efficiency is the gain roll-off as device size is increased. Careful design is required to maintain adequate gain in order that the high power-added efficiency potential of these devices may be realized. This paper reviews the options for finger combination and the mechanisms controlling the gain scaling are quantified by modelling. This insight is then used to optimize the device layout of an improved common-emitter power device structure. Experimental devices have been fabricated and measured. Excellent gain scaling is demonstrated which verifies the design approach.

The GaAs based heterojunction bipolar transistor (HBT) continues to show improvements in power, efficiency and bandwidth making it very desirable for power amplifier applications. Among these are military phased array radar, secure communications, mobile phones, wireless networks and satellite communications. The foundation for the HBT is firmly rooted in its material base. The early candidate for HBT devices GaAlAs/GaAs has in many instances now been replaced by GaInP/GaAs due to its superior electrical and physical properties. While the inclusion of GaInP does offer numerous benefits for HBT devices it is comparatively immature and is not without its own difficulties. In this paper the technology and problems associated with the practical realisation of power HBT devices based on the material system GaInP/GaAs are discussed.