In an embodiment, an image sensor includes a semiconductor region, a first doped region disposed over the semiconductor region, a ring shaped well disposed over the first doped region and surrounding parts of the first doped region, a second doped region formed within the ring shaped well and disposed over the first doped region, and a third doped region disposed over the second doped region. The ring shaped well is defined by a conductor surrounded by an insulator. The conductor is connected to a voltage terminal. The third doped region is more heavily doped than the second doped region, which is more heavily doped than the first region, and are all of the same doping type. The first doped region and the second doped region within the ring shaped well, form a potential barrier for controlling transfer of charge carriers from the first doped region to the third doped region.