A 0.7e−rms-temporal-readout-noise CMOS image sensor for low-light-level imaging

For low-light-level imaging, the performance of a CMOS image sensor (CIS) is usually limited by the temporal readout noise (TRN) generated from its analog readout circuit chain. Although a sub-electron TRN level can be achieved with a high-gain pixel-level amplifier, the pixel uniformity is highly impaired up to a few percent by its open-loop amplifier structure [1]. The TRN can be suppressed without this penalty by employing either a high-gain column-level amplifier [2] or a correlated multiple sampling (CMS) technique [3-5]. However, only 1-to-2 electron TRN level has been reported with the individual use of these approaches [2-5], and the low-frequency noise of the in-pixel source follower i.e. 1/f and RTS noise is a further limitation. Therefore, by implementing a high-gain column-level amplifier and CMS technique together with an in-pixel buried-channel source follower (BSF) [6], the TRN level can be reduced even further.