Analysis of noise in CMOS image sensor

CMOS image sensors based on active pixel sensors (APS) are now the preferred technology for most imaging applications. With advanced technology, reduced channel size, novel designs extending the more established pixels based on three transistors (3T design) into four transistors (4T design employing pinned photodiodes), the performance keeps improving [1-2]. Noise sets a limit on image sensor performance, mainly under conditions of low illumination. Analysis of noise in CMOS APS has been reported by several authors. In this paper, we present an analysis of noise due to thermal, 1/f and shot noise sources in two types of APS, known as the 3T and 4T pixel design, based on a unified time-dependent approach, using the separation of the system into two parts: time-invariant part (the APS without switching) and a time-variant part (taking into consideration the switching process).system. To calculate explicit noise expressions for noise performance we, therefore, resort to time dependent circuit models and perform time-domain noise analysis, taking into account the stationary nature of the various noise processes. The main advantage of the present method is that it is mathematically correct and avoids a basic flaw underlining the widely-used conventional methods (that freely use time- invariant methods for time-dependent systems).