A photodiodes may be formed on a substrate such as an imager substrate. The photodiode may include first and second layers in the substrate that form a p-n junction. The first layer may have a first doping type such as p-type doping, whereas the second layer may have a second, opposite doping type such as n-type doping. A counter-doping implant region may be provided that only partially overlaps with the second layer of the photodiode. The counter-doping implant region may have an opposite doping type to the second layer and may have a dopant concentration that is less than the dopant concentration of the second layer. The counter-doping implant region may extend into a third layer of the substrate that may have the same doping type of the second layer but at a lower concentration than the counter-doping implant region.