This study examined the effect of GaN grown by metalorganic vapor phase epitaxy at low temperature (LT-GaN) on the surface of Al0.25Ga0.75N/GaN heterostructures. Depositing a high-resistivity LT-GaN surface layer on the Al0.25Ga0.75N/GaN heterostructures increased both the sheet carrier concentrations and the electron mobility. This enhancement changed when the LT-GaN high-resistivity layer was removed by high-density-plasma etching. These observations are attributable to the passivation effect, implying that the LT-GaN behaves like a dielectric film, such as silicon dioxide, to passivate the surface states, yielding a different, maybe lower, electronic density of states than that of the Al0.25Ga0.75N free surface. Hall-effect measurement and gate lag measurement were performed on the field-effect transistor devices to clarify the effect of LT-GaN cap layer on Al0.25Ga0.75N/GaN heterostructures.