Review of sub-electron noise CMOS image sensors

Low-light imaging applications, like astronomy or microscopy, require low-noise image sensors with a noise level far below 1 e - RMS . As such, single-photon detection can be achieved, which is the pinnacle of image sensing. Existing imaging technologies capable of detecting and resolving individual photons, like Photomultiplier Tubes (PMT), Single Photon Avalanche Diodes (SPAD), and Electron Multiplication Charge-Coupled Devices (EMCCD), employ internal multiplication of the primary electron. Although these have noise far below 1 e - RMS , these methods suffer from two performance-limiting properties: dark counts and excess noise. This paper surveys solutions for photo charge integrating active pixel sensors (APS) and classic CMOS image sensor (CIS) technology. For integrating image sensor pixels, the readout noise at the pixel frontend must be substantially less than 0.3 e - RMS to enable quantum-limited photon detection. APS pixels separate the photo charge sensing and the charge-to-voltage conversion node. This results in a high in-pixel conversion gain and the cancellation of the kTC noise with correlated double sampling (CDS). The pinned photodiode in APS has benefits regarding dark current and rivals the charge-coupled devices (CCD) performance. Continuous improvement of CIS over the last two decades has led to the ubiquitous replacement of CCD by CIS. In this survey, we review the publications that report a noise figure close to 0.3 e - RMS or lower and explore the device and circuit techniques used and breakthroughs reached.