Analysis of temporal noise in CMOS photodiode active pixel sensor

Temporal noise sets the fundamental limit on image sensor performance, espe cially under low illumination and in video applications. In a CCD image sen sor, temporal noise is primarily due to the photodetector shot noise and th e output amplifier thermal and 1/f noise. CMOS image sensors suffer from hi gher noise than CCDs due to the additional pixel and column amplifier trans istor thermal and 1/f noise. Noise analysis is further complicated by the t ime-varying circuit models, the fact that the reset transistor operates in subthreshold during reset, and the nonlinearity of the charge to voltage co nversion, which is becoming more pronounced as CMOS technology scales. The paper presents a detailed and rigorous analysis of temporal noise due to th ermal and shot noise sources in CMOS active pixel sensor (APS) that takes i nto consideration these complicating factors. Performing time-domain analys is, instead of the more traditional frequency-domain analysis, ne find that the reset noise power due to thermal noise is at most half of its commonly quoted kT/C value. This result is corroborated by several published experi mental data including data presented in this paper. The lower reset noise, however, comes at the expense of image lag. We find that alternative reset methods such as overdriving the reset transistor gate or using a pMOS trans istor can alleviate lag, but at the expense of doubling the reset noise pow er. We propose a new reset method that alleviates lag without increasing re set noise.