<title>Abstract</title>
A recent study has shown that anomalous positive fixed charge is generated at SiO₂/GaN interfaces by forming gas annealing (FGA). Here, we conducted systematic physical and electrical characterizations of GaN-based metal-oxide-semiconductor (MOS) structures to gain insight into the charge generation mechanism and to design optimal interface structures. A distinct correlation between the amount of FGA-induced fixed charge and interface oxide growth indicated the physical origins of the fixed charge to be defect formation driven by the reduction of the Ga-oxide (GaO<italic>
_x
</italic>) interlayer. This finding implies that, although post-deposition annealing in oxygen compensates for oxygen deficiencies and FGA passivates defect in GaN MOS structures, excessive interlayer GaO<italic>
_x
</italic> growth leads to instability in the subsequent FGA treatment. On the basis of this knowledge, SiO₂/GaO<italic>
_x
</italic>/GaN MOS devices with improved electrical properties were fabricated by precisely controlling the interfacial oxide growth while taking advantage of defect passivation with FGA.