A challenge facing specific classes of wide bandgap semiconductor devices is the development of a robust dielectric insulating layer that is chemically and thermally compatible with the semiconductor. Requirements for gate dielectrics include sufficient band offsets, low leakage currents, and low interface state densities. Magnesium oxide holds promise for integration with gallium nitride (GaN), as it has been shown to meet many of these requirements in epitaxial films synthesized by molecular-beam epitaxy (MBE), pulsed laser deposition (PLD), and atomic layer deposition (ALD). Large area growth with low impurity content and smooth growth surfaces are the remaining obstacles. This study presents the results of the epitaxial growth of magnesium oxide by radio frequency (RF) magnetron sputtering on n-type GaN. An epitaxial intermixing layer between the GaN top layer and the oxide was found. Electrical characterization shows explicit depletion and accumulation of charge in metal–oxide–semiconductor devices up to 500 °C. The density of interface states at room temperature was determined using photo-assisted capacitance–voltage measurements to be as low as <inline-formula> <tex-math notation="LaTeX">$2.9\times 10^{{11}}$ </tex-math></inline-formula> cm<inline-formula> <tex-math notation="LaTeX">$^{-{2}}$ </tex-math></inline-formula> eV<inline-formula> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula>, showing that the interfacial phase is not detrimental to the interfacial electronic properties of the metal–oxide–semiconductor capacitor (MOSCap) devices.