Large-area metal -semiconductor -metal (MSM) solar-blind photodetectors with a device area of 5 × 5 mm 2 have been fabricated on Al 0.4 Ga 0.6 N/AlN/sapphire epistructure. The photodetector exhibits ultra-low dark current density of 3.2 × 10 212 A/cm 2 under 20 V bias and a corresponding breakdown voltage of up to 385 V. The solar-blind/ultraviolet rejection ratio of the photodetector is more than four orders of magnitude with a maximum quantum efficiency of 28% at 275 nm.Introduction: Wide bandgap AlGaN-based materials are especially suitable for detecting ultraviolet (UV) radiation owing to their tunable bandgap (3.4 -6.2 eV), good thermal conductivity, excellent radiation hardness and high breakdown electric field. Solar-blind AlGaN-based UV photodetectors (PDs) with cutoff wavelength smaller than 280 nm have important potential applications, including missile plume sensing, chemical/biological agent detection, solar astronomy and covert space-to-space communications [1]. For many of these applications, capability of detecting very weak UV signals is often required. Thus, the PDs are desired to have both large detector area and low dark current, which are nevertheless hard to be realised simultaneously. In past studies, small-area AlGaN-based solar-blind PDs with low dark current density and high signal-to-noise ratio have been reported [2][3][4][5]. Although there are limited reports on AlGaN-based solar-blind Schottky PDs with active area of more than 1 mm 2 , those devices suffer from high leakage current owing to material quality or uniformity-related issues [6]. In this work, by growing the AlGaN active layer on a high-temperature (HT) AlN buffer layer on sapphire, we demonstrate a high-performance AlGaN-based metal -semiconductor -metal (MSM) PD with large device area of up to 25 mm 2 , which exhibits record low dark current density and high solar-blind/ultraviolet rejection ratio.

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