CMOS APS MTF modeling

In this paper, a unified model, based on a thorough analysis of experimental data, is developed for the overall modulation transfer function (MTF) estimation for CMOS image sensors. The model covers the physical diffusion effect together with the influence of the pixel active area geometrical shape. Comparison of both our predicted results and the MTF calculated from the point spread function (PSF) measurements of an actual pixel array gives excellent agreement. This confirms the hypothesis that the active area shape together with the photocarrier diffusion effect are the determining factors of the overall CMOS active pixel sensor (APS) MTF behavior, and allows us to extract the minority-carrier diffusion length. The results indicate that for any potential active area shape, a reliable estimate of image performance is possible, so the tradeoff between the conflicting requirements, such as signal-to-noise ratio (SNR) and MTF could be compared per pixel design.