Analysis of Temporal Noise in CMOS APS

Temporal noise sets a fundamental limit on image sensor performance, especially under low illumination and in video applications. In a CCD image sensor, temporal noise is well studied and characterized. It is primarily due to the photodetector shot noise and the thermal and 1/f noise of the output charge to voltage amplifier. In a CMOS APS several additional sources contribute to temporal noise, including the noise due to the pixel reset, follower, and access transistors. The analysis of noise is further complicated by the nonlinearity of the APS charge to voltage characteristics, which is becoming more pronounced as CMOS technology scales, and the fact that the reset transistor operates below threshold for most of the reset time. The paper presents an accurate analysis of temporal noise in APS. We analyze the noise for each stage of the sensor operation, and identify the noise contribution from each source. We analyze noise due to photodetector shot noise taking nonlinearity into consideration. We find that nonlinearity improves SNR at high illumination. Using an MOS transistor subthreshold noise model we show that the noise due to the reset transistor shot noise is at most half the commonly quoted (V2) value. Using HSPICE simulation, we find the noise due to the follower and access transistors. As expected we find that at low illumination reset noise dominates, while at high illumination photodetector shot noise dominates. Finally, we present experimental results from test structures fabricated in O.35p CMOS processes. We find that both measured peak SNR and reset noise values match well with the results of our analysis.