A novel composite ultraviolet (UV)/blue photodetector is proposed in this paper. Lateral ring-shaped PN junction is used to separate photogenerated carriers and inject the non-equilibrium excess carriers to the bulk, changing the bulk potential and shifting the threshold voltage of the metal-oxide-semiconductor field-effect transistor (MOSFET) as well as the drain current. Numerical simulation is carried out, and the simulation results show that the composite photodetector has the enhanced responsivity for UV/blue spectrum. It exhibits very high sensitivity to weak and especially ultra-weak light. A responsivity of 7000 A/W is obtained when the photodetector is illuminated under incident optical power of 0.01 μW. As a result, this proposed combined photodetector has great potential for UV/blue and ultra- weak light applications.