The Effects of Feedback Capacitance on Thermally Shunted Heterojunction Bipolar Transistor ’ s Linearity

The effects of feedback capacitance on linearity of thermally shunted heterojunction bipolar transistors (HBTs) were experimentally determined through power load pull measurements. The results show state-of-the-art linearity performance of devices biased and matched for power operation at X-band. Gain is easily traded for increased linearity by increasing the feedback capacitance in the device design. The data shows that a self-aligned process is necessary to maximize power performance (gain and efficiency), but an advantage of a re-aligned process is the ability to trade gain for linearity by increasing the emitterto-base metal separation. With this additional design flexibility in a fixed technology, circuit performance can be optimized through layout variations only. For example, one HBT technology would be suitable for a Transmit/Receive module and each block could be easily optimized for different performance specifications.