Group III-V Nitride semiconductors, especially gallium nitride (GaN), are very promising materials for high-power and high-frequency applications due to its outstanding properties such as a high breakdown field in wide-band gap semiconductor materials and a high saturation electron velocity. A number of GaN-based devices such as light-emitting diodes [LEDs], laser diodes [LDs], Solar-blind MSM-photodetectors, metal-semiconductor field effect transistor [MESFETs], heterostructure field-effect transistors [HFETs] and high electron mobility transistors [HEMTs] have been reported. However, larger leakage current through Schottky rectifiers adversely affect operation, power consumption, noise and reliability of devices. Reduction in the leakage current can be realized by employing an insulated layer metal-semiconductor technique. For the development of metal-insulator-semiconductor (MIS) devices, detailed electrical characterizations of metal/insulator/GaN MIS interfaces have to be investigated. This book deals with the high quality GaN-based MIS structures, specially on electrical properties, morphological and current transport mechanisms for microelectronic device applications.