Embodiments of mechanisms for forming an image-sensor device are provided. The image-sensor device includes a substrate having a front surface and a back surface. The image-sensor device also includes a radiation-sensing region formed in the substrate. The radiation-sensing region is operable to detect incident radiation that enters the substrate through the back surface. The radiation-sensing region further includes an epitaxial isolation feature formed in the substrate and adjacent to the radiation-sensing region. The radiation-sensing region and the epitaxial isolation feature have different doping polarities.