We demonstrate a wavelength-selective photodetector that combines a Fabry-Perot filtering cavity (FPC) with a taper absorption cavity (TAC). The taper cavity shows a nonresonant effect but exhibits an absorption enhancement effect, so that high speed, high quantum efficiency, wide tuning range, and an ultranarrow spectral linewidth can be achieved simultaneously. Device performance was theoretically investigated by including key factors such as taper angle, finite-size diffracting-beam input, and lateral walk-off in the taper cavity. The device was fabricated by bonding a GaAs-based FPC, which can be tuned via thermal-optic effect, with an InP-based TAC. An integrated device with a spectral linewidth of 0.6 nm (FWHM), a wavelength tuning range of 10.2 nm(1518.0-1528.2 nm), a 3 dB bandwidth of 12 GHz, and a quantum efficiency of approximately 70% was demonstrated, and the absorption layer thickness is only 0.3 microm.