A back side illuminated image sensor includes a pixel formed by three doped photosensitive regions that are superposed vertically in a semiconductor substrate. Each photosensitive region is laterally framed by a respective vertical annular gate. The vertical annular gates are biased by a control circuit during an integration phase so as to generate an electrostatic potential comprising potential wells in the central portion of the volume of each doped photosensitive region and a potential barrier at each interface between two neighboring doped photosensitive regions.