We have utilized the high-efficiency heterojunction with intrinsic thin-layer (HIT) solar-cell structure for applications in photodetectors. In this research, we have investigated the relationship between responsivities and thicknesses of active layers, including top intrinsic amorphous Si and bulk crystalline Si. The thickness of a HIT photodetector should depend on the detection wavelength desired. For a typical HIT structure with a 5-nm-thick top intrinsic amorphous Si layer, the responsivities at 450, 650, and 850 nm wavelengths could be 0.511, 0.529, and 0.641 A/W, respectively. The responsivity at the 450 nm wavelength can reach 0.591 A/W for a 20-nm-thick top intrinsic amorphous Si layer even when the bulk Si is only 1.5 µm thick.