Simulating enhanced photo carrier collection in the multifinger photogate active pixel sensors

Classic photo gate APS uses a MOS capacitor that can capture incident illumination with a potential well created under the photogate. The major drawback of such a technology is the absorption of shorter wavelength by the polysilicon gate resulting in a higher sensitivity in the red visible spectrum than in the blue range. To reduce this we previously had experimentally shown that a multifinger photo gate APS designs with 0.72 mu m fingers implemented in the 0.18 mu m CMOS technology have a significant increase in sensitivity of 1.7 times the standard photo gate APS. Using advanced 2-dimensional device simulations had shown that the fringing fields form the these fingers would create a potential well shape that approached that of the standard fully covered photo gate, but with large open areas which would have less optical absorption. Reducing the gate widths resulted in higher efficiency of photo carriers generated in the larger open areas while keeping the potential well shape desired. In this work, we use optical simulation package on the 2D device simulation tools to simulate the multi finger photo gate designs with white light illumination. Sensitivity of the pixel is calculated as the count of total number of photocarriers that are collected by the potential well for a given exposure cycle. All the multifinger designs achieved a significant increase in efficiency with respect to the standard photogate APS design, with the peak sensitivity of 550% by the 7finger design with a gate width of 0.25 mu m.