Photodiode Peripheral Utilization Effect on CMOS APS Pixel Performance

A photodiode (PD)-type CMOS active pixel sensor (APS) pixel is comprised of a reverse-biased p-n-junction diode (PD) for photon conversion and charge storage, and a number of MOS transistors. Junction capacitance of the PD has two major components; bottom plate (area) and side wall (periphery). Both play important roles in the electro-optical performance of PD-APS pixels. This paper reports PD peripheral junction utilization effects on the pixel's electro-optical performance, full-well capacity and spectral response for an 18times 18 mum CMOS PD-APS pixel were improved by opening multiple circular holes in the PD diffusion layer. A prototype CMOS APS imager was designed, fabricated, and tested in 0.5-mum, 5 V, 2P3M CMOS process, containing a 424 times 424 pixel array with smaller sub-arrays for multiple pixel designs. Four test pixels with 7, 11, 14, and 17 circular, 1.6-mum-diameter holes were placed on one pixel array, with one control pixel for reference. Pixel characteristics, dark current, PD capacitance, quantum efficiency, sensitivity, and pixel full-well capacity were measured. It was found that increased PD junction peripheral would potentially help to improve total capacitance of the PD, with the expense of higher dark current. We also found that increased PD peripheral capacitance improves spectral response up to 12% of the PD-APS pixel, especially at short wavelengths.