Photoresponse nonlinearity of solid-state image sensors with antiblooming protection

The physical mechanism for photoresponse nonlinearity in solid-state image sensors with antiblooming protection is described and analyzed. This mechanism is the premature turn-on of the antiblooming structure, and can be characterized by its nonideality factor. It is shown that a lower nonideality factor results in a more linear response. Electrostatic modeling results and measurements show that devices with low-overflow drain (LOD) antiblooming structures can achieve nonideality factors very close to unity and, therefore, offer superior photoresponse linearity. This is especially true for devices with large pixels having low charge capacity, since the maximum voltage swing across the detector is small. Conversely, devices with vertical-overflow drain (VOD) structures are seen to have nonideality factors at best around two and in some other cases even higher, resulting in severe response nonlinearity.>