Analysis of GaN HBT structures for high power, high efficiency microwave amplifiers

GaN has become increasingly important for microwave applications up to K_a band as a result of its wide band-gap, which provides a high critical breakdown field and good thermal stability, yielding excellent potential for high power and high voltage operation. It is of major interest to understand the device structures that will lead to high efficiency, high power microwave amplifiers. In this paper, we investigate by simulation the microwave performance of InGaN/GaN Heterojunction Bipolar Transistors (HBTs), with proper device geometry to account for the effects of current crowding. We provide an analysis of both emitter-up and collector-up InGaN/GaN HBT structures, based on a distributed HBT model implemented in ADS. We simulate their performance in a fully matched Class B high power amplifier. The results show that an important issue for high power operation is the mismatch between optimum load conditions for peak output power and peak gain. To increase the output impedance, it is important to decrease base-collector capacitance, which can be achieved with a collector-up structure. Furthermore, current crowding in the emitter caused by the sheet resistance in the base becomes more important at higher power operation, and is therefore a key design consideration.