Analytical Model of the Vertical Pinned Photodiode

Abstract

This article presents a simple analytical model of the vertical pinned photodiode (PPD). In the existing pixels with relatively large sizes, the photodiode is formed by p(+)-n-p doping in a planar manner, and thus the vertical electric field determines the potential of the photodiode. However, as the pixel size becomes smaller, the size of the photodiode also decreases. Accordingly, the influence of the doping concentration in the periphery becomes larger and the pinning voltage is eventually predominantly determined by the horizontal electric field in the submicrometer region. In this case, the analytical model describing the conventional photodiode structure is no longer applicable. Therefore, in this article, a simple analytical model applicable to a small pixel size is provided and verified through TCAD simulation. It is thought that the proposed simple model helps understand the potential of small pixel size and provides a major starting point for design.