There is a tradeoff between the quantum efficiency and the spectral response linewidth in resonant-cavity-enhanced (RCE) photodetectors. Recently a new type of photodetector structure has been proposed, which utilizes an inclined mirror in a three-mirror cavity. It decouples the quantum efficiency and the spectral response linewidth and has high response speed, high quantum efficiency, and narrow linewidth simultaneously. The analysis result indicates that the linewidth of 0.8 nm and quantum efficiency higher than 70% can be achieved on this device. For a 30*30 μm2 device, the maximum bandwidth approaches 23 GHz.