GaN has very high breakdown electric field of ~3.3 MV/cm and high electron velocity of ~2.5 x 107 cm/s. GaN-based high-electron mobility transistors (HEMTs), such as AlGaN/GaN, InAlN/GaN, have shown to be the prime candidates for coming generations high power and high frequency applications. As a result, GaN-based HEMTs will play significant roles in both future high power switching devices (power supplies, AC-DC invertors, DC-DC convertors, hybrid vehicles) and high frequency power amplifier (base transceiver station, satellite communication, radar).