Optimization of The Ultra-Low Dark Current Complementary MOS Image Sensor Cell Using n+ Ring Reset

Abstract

An ultra low dark current pixel has been developed for embedded active-pixel complementary metal oxide semiconductor (CMOS) image sensors using a standard CMOS logic process. Conventional CMOS image sensors suffer from high dark current as a result of the high interface state density at the field oxide edge. In the proposed novel pixel, the photo-sensing diode is surrounded by a ring-shaped poly-silicon reset gate which isolates the photo-sensing area from the field oxide edge. Hence, the dark current level can be effectively reduced. However, in the novel pixel, the large overlap capacitance between the reset poly-gate ring and the sensing node can severely affect the output swing. To optimize this novel pixel, the poly-silicon gate is limited to isolate the photo-sensing area from the field oxide edge. Furthermore, a constant bias voltage applied to the poly-silicon gate provides an additional advantage of extended dynamic range. The effects of technology scaling on the pixel performance are investigated as well.