We propose a new photodiode structure to increase the charge handling capability of solid-state image sensors. This photodiode includes an N+ sidewall that surrounds the charge storage region of a conventional diode with a vertical overflow drain structure. The operation and performance of the diode were analyzed using three-dimensional numerical analyses. We found that an abrupt potential increase, produced around the boundary of the channel stop and the N+ sidewall, create a flat potential profile that spreads all over the storage region, at the beginning of the charge storage. This profile suppresses electron overflow into the substrate that occurs due to small number of signal electrons concentrated around the center of the storage region. Therefore, the N+ sidewall increases the storable charge. For example, the charge handling capability of a conventional 3.0-μm diameter photo diode was improved by more than a factor four by adding the N+ sidewall. The validity of the analyses was confirmed from the results of one-dimensional analyses on the relationship between incident light intensity and stored charge and on the potential barrier height at signal charge saturation.
