Abstract: GaN-based hydrogen sensors fabricated with a Pd–SiO2 mixture as-deposited and wet-etched were investigated. Scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and secondary ion mass spectrometer (SIMS) methods were employed to study the mixtures as-deposited and wet-etched. The results show that Pd nanoparticles and an interfacial layer with oxygen were formed, which contribute to improved dissociation rate, diffusion rate, and storage capability. The sensor fabricated with the mixture wet-etched responds well to various hydrogen-containing gases and produces a barrier-height variation over 275 mV at 2.13 ppm H2/N2. Comparisons to the sensor with the mixture as-deposited also evidence that hydrogen atoms can possibly be trapped inside the mixture.