We have fabricated the AlGaN solar-blind ultraviolet metal–semiconductor–metal (MSM) photodetectors (PDs) with an Al composition of 0.55. The surface roughness and dislocations of the high-Al-content Al0.55Ga0.45N epitaxial layer are analyzed by atomic force microscopy and transmission electron microscopy, respectively. The device exhibits high spectral responsivity and external quantum efficiency due to the photoconductive gain effect. The current reveals a strong dependence on high temperatures in the range of 4–10 V. Moreover, the Poole–Frenkel emission model and changing space charge regions are employed to explain the carrier transport and photoconductive gain mechanisms for the AlGaN PD, respectively.