SUMMARYGaN and its heterojunction structures are attracting considerable attention as materials for high-voltage, highfrequency power device applications. This paper gives an overview of the basic device structure, process technology, and recent device performance achieved with AlGaN/GaN heterojunction FET structures. The emphasis is on the performance of power amplifiers developed for use in cellular base station transmitters and pseudo-millimeter-wave wireless access systems. Future research topics and prospects of nitride-based transistors are also discussed.