The advent of GaN based HEMTs has attracted enormous interest due to excellent material and device performance in the field of high-power and high-frequency amplifiers,as well as high voltage power switches. However, the commercialization of GaN based HEMTs requires detailed and comprehensive understanding on the material properties, device operation mechanisms, degradation behaviors and reliability properties. This book has a comprehensive study on the important issues regarding the device operation of GaN based HEMTs, which includes the DC, microwave, and noise properties with special focus on the device degradation study. The device degradation mechanisms as wells the role of traps have also been well explained.