RF Performance of GaN-Based Graded-Channel HEMTs

GaN-based HEMTs are attractive devices for RF, microwave, and millimeter-wave applications, with excellent power and noise figure performance having been demonstrated. Recently, graded-channel structures have been explored for their potential to improve linearity through channel engineering. However, due to the longitudinal optical (LO) phonon coupling in III-N structures, graded channel devices are also seen to exhibit improved carrier saturation velocity over a wider range of device bias conditions. In turn, this may enable RF performance improvements. We report the first study of bias dependence of millimeter-wave RF performance for graded-channel GaN-based HEMTs. In contrast to conventional abrupt AlGaN/GaN heterostructure HEMTs, the higher carrier velocity leads to high ft and fmax over a wide range of bias conditions. This has implications for the design of high-linearity and power-efficient amplifiers.