Exact noise analysis of a CMOS BDJ APS

At low illumination levels, the performance of image sensors is severely limited by their intrinsic electrical noise. In the buried double junction (BDJ) active pixel sensor (APS), as in any CMOS image sensor, noise is primarily due to the photodetector and to the in-pixel transistors. In this paper, we present an exact noise analysis of CMOS BDJ APS in charge storage mode during both reset and integration phases. Classical frequency-domain analysis is performed for the reset interval and a kT/C noise can be put in evidence. During the integration phase, as the stationary state condition is never fulfilled, noise analysis must be carried out in the time-domain. The novelty of this approach consists in taking into account the non-linearity of junction capacitances, which yields more realistic results.