Al0.5Ga0.5N-based metal-semiconductor-metal photodetectors (PDs) with a large device area of 5 × 5 mm2 are fabricated on a sapphire substrate,which are tested for vacuum ultraviolet light detection by using a synchrotron radiation source.The PD exhibits low dark current of less than 1 pA under 30 V bias and a spectral cutoff around 260 nm,corresponding to the energy bandgap of Al0.5 Ga0.5N.A peak photo-responsivity of 14.68 mA/W at 250nm with a rejection ratio (250/360nm) of more than four orders of magnitude is obtained under 30 V bias.For wavelength less than 170 nm,the photoresponsivity of the PD is found to increase as wavelength decreases,which is likely caused by the enhanced photoemission effect.