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AlGaN-based back-illuminated solar-blind ultraviolet (UV) p-i-n photodetectors (PDs) with high quantum efficiency are fabricated on sapphire substrates.To improve the overall performance of the PD,a series of structural design considerations and growth procedures are implemented in the epitaxy process.A distinct wavelengthselective photo-response peak of the PD is obtained in the solar-blind region.When operating in photovoltaic mode,the PD exhibits a solar-blind/UV rejection ratio of up to 4 orders of magnitude and a peak responsivity of ～ 113.5 mA/W at 270 nm,which corresponds to an exteal quantum efficiency of ～52％.Under a reverse bias of -5 V,the PD shows a low dark current of ～1.8pA and an enhanced peak quantum efficiency of ～64％.The thermal noise limited detectivity is estimated to be ～ 3.3 × 1013 cm.Hz1/2 W-1.