There has been a growing interest in the development of solar-blind ultraviolet photodetectors for a variety of applications, including detection of missiles, precision guidance, biochemical analysis, lithography aligners, secure space-to-space communications, and ozone monitor. Al_xGa_1−xN has emerged as the most promising material system for such devices. Al_xGa_1−xN-based solar-blind photodetectors with different structures had been successfully fabricated, and their working principle has been demonstrated. However, the material quality and the device process of AlGaN devices still need to be further improved to enhance performance, including the development of ohmic metal contacts to both n-type and p-type Al_xGa_1−xN films with high Al component.