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