A low-cost and reliable SiO2 insulating layer was successfully deposited onto GaN by liquid-phase deposition (LPD) using supersaturated H2SiF6 and H3BO3 solutions. The interface-trap density, Dit, was estimated to be 1.2×1012 cm-2 eV-1 for the as-grown, not annealed LPD-SiO2 layers. It was found that the leakage current density was 2.06×10-5 A/cm2 at a negative bias of 10 V for the as-grown Al/20 nm LPD-SiO2/GaN metal–insulator–semiconductor (MIS) capacitors. It was also found that the LPD-SiO2 layer could be used to suppress the dark current of nitride-based photodetectors. A large photocurrent to dark-current contrast ratio higher than four orders of magnitude and a maximum responsivity of 0.65 A/W were observed from the fabricated indium–tin-oxide (ITO)/LPD-SiO2/GaN MIS UV photodetectors. These results could be explained by defect-assisted tunneling.