I. Introduction
In recent years, with the rapid development of wireless communication and phased array radar, W-band applications are pressingly needed. Large breakdown voltage and large current density of the devices are required to ensure high gain and high output power for W-band applications. Owing to its wide bandgap of 3.4 eV, GaN has a large critical breakdown electrical field, which ensures a high operating voltage. High density 2DEG with high electron mobility generated from AlGaN/GaN heterojunction provides a high output current density. High electron saturated velocity of GaN contributes to its high operating frequency. These remarkable properties of GaN make it one of the most ideal candidates for W-band applications.