In this paper, the photocurrent in a CMOS compatible Si photodetector is calculated considering the effect of diffusion of photogenerated carriers from the substrate region. The parasitic effects due to the multiple diodes in lateral configuration have also been considered in the analysis. Normalized frequency response is computed for a square-area photodetector. The results on the 3 dB bandwidth are shown as a function of number of diodes, finger width, finger spacing, and the photodetector length. It is shown that some optimum choices of the device parameters exist to obtain the bandwidth maxima of the lateral Si photodetector.