A 2D heterostructure photodetector composed of WSe₂/Ta₂NiSe₅ is reported, which simultaneously enhances photodetection gain and response speed, overcoming the trade-off between responsivity and speed. The device utilizes photogating-assisted tunneling, which synergistically enables photocarrier multiplication and carrier acceleration through tunneling under an electric field. The photogating effect in the device features low power consumption (in the order of nW) and depends on the polarization states of incident light, which can be further tuned by source-drain voltages, enabling wavelength discrimination with a two-electrode planar structure. The device demonstrates high responsivity (above 10³ A/W), fast response time (down to -1 μs), and polarization sensitivity due to the intrinsic anisotropy of Ta₂NiSe₅. The anisotropic photoresponse ratio is bias-tunable and wavelength-dependent, offering a promising platform for wavelength discrimination. The device also shows broad working range from visible to infrared and low power consumption, making it suitable for applications in miniaturized spectroscopy, spectral imaging, and object and threat identification. The device's performance is superior to existing 2D photogating devices in imaging applications, with a responsivity of 2.2 × 10⁴ A/W at 1 V bias. The device's polarization-sensitive photodetection capability is enhanced under positive bias due to the in-plane anisotropic crystal structure of Ta₂NiSe₅. The device's polarization-dependent photocurrent is described by a general formula, and the polarization ratio and anisotropic ratio are extracted as figures of merit. The device's performance is further validated by its ability to discriminate different wavelengths by sweeping the bias. The device's response time is estimated to be -1.8 μs, with a responsivity of 7.3 × 10³ A/W at 1 V bias. The device's performance is comparable to or lower than commercial photodetectors based on silicon, germanium, and indium gallium arsenide. The device's tunable photoresponse by switching source-drain voltages provides insight into the spectral information of incident light, enabling future spectral learning procedures for reconstructing spectra from photoresponse vectors. The device's ability to address the responsivity-speed trade-off and broadband polarized photodetection with wavelength discrimination opens possibilities for novel on-chip optoelectronic applications, such as polarization imaging, high contrast polarizers, and miniaturized spectrometers.A 2D heterostructure photodetector composed of WSe₂/Ta₂NiSe₅ is reported, which simultaneously enhances photodetection gain and response speed, overcoming the trade-off between responsivity and speed. The device utilizes photogating-assisted tunneling, which synergistically enables photocarrier multiplication and carrier acceleration through tunneling under an electric field. The photogating effect in the device features low power consumption (in the order of nW) and depends on the polarization states of incident light, which can be further tuned by source-drain voltages, enabling wavelength discrimination with a two-electrode planar structure. The device demonstrates high responsivity (above 10³ A/W), fast response time (down to -1 μs), and polarization sensitivity due to the intrinsic anisotropy of Ta₂NiSe₅. The anisotropic photoresponse ratio is bias-tunable and wavelength-dependent, offering a promising platform for wavelength discrimination. The device also shows broad working range from visible to infrared and low power consumption, making it suitable for applications in miniaturized spectroscopy, spectral imaging, and object and threat identification. The device's performance is superior to existing 2D photogating devices in imaging applications, with a responsivity of 2.2 × 10⁴ A/W at 1 V bias. The device's polarization-sensitive photodetection capability is enhanced under positive bias due to the in-plane anisotropic crystal structure of Ta₂NiSe₅. The device's polarization-dependent photocurrent is described by a general formula, and the polarization ratio and anisotropic ratio are extracted as figures of merit. The device's performance is further validated by its ability to discriminate different wavelengths by sweeping the bias. The device's response time is estimated to be -1.8 μs, with a responsivity of 7.3 × 10³ A/W at 1 V bias. The device's performance is comparable to or lower than commercial photodetectors based on silicon, germanium, and indium gallium arsenide. The device's tunable photoresponse by switching source-drain voltages provides insight into the spectral information of incident light, enabling future spectral learning procedures for reconstructing spectra from photoresponse vectors. The device's ability to address the responsivity-speed trade-off and broadband polarized photodetection with wavelength discrimination opens possibilities for novel on-chip optoelectronic applications, such as polarization imaging, high contrast polarizers, and miniaturized spectrometers.