An effective gate insulator for compound semiconductors has been a challenging goal for the materials research community for nearly 40 years. Recent developments on the epitaxial deposition of complex gate oxides as gate insulators have shown promise with the demonstration of enhancement mode high electron mobility transistors (e-mode HEMTs). In this work, gate oxide epilayers deposited on III-V semiconductors for field effect transistors (III-V MOSFETs) are examined using transmission electron microscopy (TEM) to identify the structure of the oxide/semiconductor interface. The high resolution images of the cross-sectional structures for the first time reveal a crystalline nature of the interface between the oxide and the III-V semiconductor. The composition of the oxide layers are determined by Z-contrast Electron Energy Loss Spectroscopy (EELS). The surface morphology of the FET structures is investigated by atomic force microscopy (AFM) both before and after gate oxide deposition, and the structural results are related to device DC electrical characteristics. With an underlying GaN/InGaN heterojunction grown by metal-organic chemical vapor deposition (MOCVD) on sapphire, the MOSFET devices exhibit the characteristics of a substantially unpinned interface, including the capacity for significant charge accumulation and transconductance at positive gate voltages.