A semiconductor device includes a device layer, a semiconductor layer, a sensor element, a dielectric layer, a color filter layer, and a micro-lens. The semiconductor layer is over the device layer. The semiconductor layer has a plurality of microstructures thereon. Each of the microstructures has a substantially triangular cross-section. The sensor element is under the microstructures of the semiconductor layer and is configured to sense incident light. The dielectric layer is over the microstructures of the semiconductor layer. The color filter layer is over the dielectric layer. The micro-lens is over the color filter layer.