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In this letter, we report on the fabrication of near-ultraviolet photodetectors based on gallium nitride (GaN) layers grown on a Si(111) substrate. Optoelectronic characterization was performed using front-side and backside illumination, the latter possible by locally etching the Si substrate under the detectors using reactive ion etching. The dark current after removal of the Si substrate decreased by two orders of magnitude to around 20 fA at -1 V for a 300-mum-diameter Schottky photodiode. Responsivity at the cutoff wavelength (370 nm) was equal to 35 mA/W for the backside illumination. Detection at smaller wavelengths was not possible due to a nonoptimized layer stack. These first results do however illustrate the potential of backside-illuminated GaN-on-Si Schottky photodiodes in 2-D UV imagers.