GaN can be expected to play a central rule in semiconductor electrical device materials as Si in the near future. To expand an application area of GaN transistors, high-quality dielectric films are indispensable. In this study, we focus attention on III-oxides as the strong candidates. We will fabricate and analyze ideal oxide/nitride composite structures by getting rid of external factors in terms of continuous film formation from nitride semiconductors to oxides in an ultra high vacuum chamber. Not only improvements in device characteristics but also evolutions of GaN transistors will be the target of this research.