A closed-form analytical expression for diffusion-limited photocurrent in a closely spaced photodiode array is presented with reference to modern CMOS imaging arrays. The analysis makes use of the fact that the current density at the peripheral edges of the photodiode in a closely spaced array has a maximum value near the surface and decays approximately in a linear fashion with the depth into the substrate. The approximated lateral current density at the peripheral edge is then used to derive an expression for the overall peripheral photocurrent. The final expression shows good agreement with the experimental results.