A semiconductor device is provided, which includes a substrate and at least one nanostructure. The substrate has sensing pixels, and each of the sensing pixels has a photo sensing region for absorbing incident light. The nanostructure is directly on the photo sensing region. The nanostructure of each of the sensing pixels has a projected portion on an upper surface of the substrate, and a circle equivalent diameter of the projected portion of the nanostructure of each of the sensing pixels is substantially within a wavelength range of 100 nm to 1900 nm of the incident light configured to enter the substrate through the nanostructure.