A unified model of carrier diffusion and sampling aperture effects on MTF in solid-state image sensors

A unified model is used for calculating the modulation-transfer function (MTF) of a front-side-illuminated, solid-state image sensor including the effects of lateral diffusion of charge carriers within the substrate and less than full pixel sampling apertures. Moreover, it is found that, in general, these two effects cannot be treated independently and then combined, using the convolution theorem to arrive at the total sensor MTF as has often been done in the past. It is found that the actual MTF is higher than predicted by simply multiplying the diffusion and aperture MTFs together. This discrepancy is seen to increase as pixel-to-pixel crosstalk increases for a given aperture size due to an increase in diffusion length, a decrease in depletion depth, or a decrease in the substrate´s absorption (coefficient) and/or as the pixel´s aperture decreases with respect to the pixel size