This work characterizes a GaN:Mg on silicon ultraviolet photodetector with a cutoff at 3.3 eV and a responsivity of 12 A/W at 4 V bias for optical intensities on the order of 1 W/m2 and below. A weak photovoltaic response is also reported. The photocurrent is nearly linear versus optical intensity for up to 10 W/m2. The responsivity increases nearly linearly with applied voltage up to 8 V, then the increase slows toward saturation. To explain this high responsivity in a direct gap semiconductor, it is hypothesized that holes are captured at either compensated Mg deep acceptor sites or Mg-related trap/recombination centers, resulting in a greatly prolonged electron free-carrier lifetime.