Impedance Anomalies and RF Performance Limitations in AlGaN/GaN HFET's

AlGaN/GaN HFET's have demonstrated excellent RF performance. The use of these devices in WiMAX system is of particular interest due to their inherent broadband capability. However, these devices experience physical phenomena that degrade RF performance and limit the high frequency application of these devices. In particular, these devices often demonstrate Cgs variations with input power drive that are opposite to classical FET behavior. The transconductance also decreases with input RF drive. The behavior affects the frequency performance and linearity of the device. In this work these effects are investigated using the commercial ATLAS simulator. It is demonstrated that these effects result from space-charge limited current transport, and a corresponding increase in gate-source channel resistance. Simulation results are verified by comparison to experimental data, and excellent agreement is obtained. The physical operation of the device, including effects that limit RF performance and frequency response are explained and discussed.