Intermodulation in heterojunction bipolar transistors

The modeling of small-signal intermodulation distortion (IMD) in heterojunction bipolar transistors (HBTs) is examined. The authors show that IMD current generated in the exponential junction is partially canceled by IMD current generated in the junction capacitance, and that this phenomenon is largely responsible for the unusually good IMD performance of these devices. Thus, a nonlinear model of the HBT must characterize both nonlinearities accurately. Finally, the authors propose a nonlinear HBT model suitable for IDM calculations, show how to measure its parameters, and verify its accuracy experimentally