a photoconductor has an active layer of gallium nitride having approximately 10 15 to 5 × 10 15 net donor sites per cubic centimeter and is sensitive to uv radiation . this photoconductor has at least one of a sheet resistance in the approximate range of 10 4 to 5 × 10 6 ohms / unit area and a relatively low level of photoluminescence in the range from about 430 - 450 nm when excited with light of energy higher than the bandgap energy of 3 . 4 ev . these criteria tend to define similar semiconductor materials which can form the active layer of an ultraviolet photodetector having the improved characteristics of a relatively low dark resistance , high sensitivity over at least a range of uv radiation intensity , and decreasing gain with increasing uv radiation .