A photodetector (100, 200, 300) comprising a gallium nitride substrate, at least one active layer (104, 302) disposed on the substrate (102, 202, 306), and a conductive contact structure (106, 210, 308) affixed to the active layer (104, 302) and, in some embodiments, the substrate (102, 202, 306). The invention includes photodetectors (100, 200, 300) having metal-semiconductor-metal structures, P-i-N structures, and Schottky-barrier structures. The active layers (104, 302) may comprise Ga1-x-yAlxInyN1-z-w PzAsw, or, preferably, Ga1-xAlxN. The gallium nitride substrate comprises a single crystal gallium nitride wafer and has a dislocation density of less than about 105 cm−2. A method of making the photodetector (100, 200, 300) is also disclosed.