GaN-based power devices have received much attention for their application in the next-generation high-frequency and high-power devices. Recently, AlGaN/GaN heterojunction field-effect transistors (HFETs) with Schottky gate structures have been implemented into modern wireless communication systems. However, their applications are restricted because of large gate leakage current through the Schottky contacts and their normally-on operation. To overcome these limitations, MOS gate stacks on GaN-based materials have been intensively studied. Moreover, normally-off vertical power MOSFETs on free-standing GaN substrates have become a hot topic in power electronics.

In contrast with the Si-based semiconductors, deposition of wide bandgap insulators on the AlGaN and GaN surfaces is thought to be a plausible method for fabricating MOS gate stacks. Among the various insulating materials, Al₂O₃ and SiO₂ exhibit a sufficiently wide bandgap and thermal stability for MOS device applications. However, it is well known that Al₂O₃ involves an essential problem of electron trapping, thus leading to significant I_d-V_g hysteresis and V_th instability. In addition, poor electrical properties of GaN-based MOS devices have been reported even for the well-established ALD-Al₂O₃ and CVD-SiO₂ films. We have investigated suitable insulating materials and deposition methods for AlGaN/GaN MOS-HFETs and GaN-MOSFETs. This paper reviews our recent findings on gate stack engineering for advanced GaN-based MOS devices and discusses a common guiding principle to improve device performance and reliability.

We implemented nitrogen incorporated Al₂O₃ (AlON) gate dielectrics into the AlGaN/GaN MOS-HFETs and successfully improved the performance and reliability of the devices. Our physical and electrical characterizations revealed superior thermal stability of the AlON/AlGaN interface and the importance of conducting dielectric deposition under a reactive nitric atmosphere to suppress intermixing at the interface. Recently, we demonstrated high current and high voltage (20A/730V) normally-off AlGaN/GaN MOS-HFETs with the AlON gate dielectrics.

For fabricating GaN-MOS structures, oxidation of the GaN surface is a fundamental method of obtaining gate insulators, but severe drawbacks, such as insufficient conduction band offset at GaO_x/GaN interfaces and rough surface morphology have been pointed out. Thus, we proposed GaN-MOS structures with thin GaO_x interlayers, in which oxide interlayers were formed by oxidation of GaN surface prior to SiO₂ deposition. We also found that high-quality SiO₂/GaO_x/GaN stacked structures were formed by optimizing conditions for plasma-enhanced CVD of SiO₂ films. Consequently, well-behaved C-V curves with negligible hysteresis and frequency dispersion were obtained. Moreover, an extremely low interface state density (< 10¹⁰ cm^-2eV^-1) and an improved reliability in terms of reproducibility and robustness against dielectric breakdown were achieved by rapid thermal annealing of SiO₂/GaO_x/GaN MOS devices.