A Charge Transfer Model for CMOS Image Sensors

Gong

IEEE J. Electron Devices Soc.

et al. 2020

The output features of pixels in CMOS image sensors (CISs) are influenced by different exposure conditions. This paper presents an analytical model to describe the output characteristics of the exposure process in pinned photodiode (PPD) CMOS image sensors with the accumulation of three charge sources: photogenerated charge, p-n junction-generated charge, and emission charge. In the proposed model, the difference between the time-based and light intensity-based photo response process is illustrated. The model also reveals the relationship between photodiode potential and light intensity at different exposure values. At the low exposure values, the PD potential increases with light intensity, and a contrary trend is observed at the high exposure values. This concludes that a larger linear output range can be obtained in high light intensity conditions. Furthermore, the improved model provides development analysis in terms of light intensity influence for long exposure time noise. The models were verified with technology computer-aided design simulation and the test devices were fabricated using a 0.18-μm CIS process. The model demonstrates good consistency with simulation and measured results.

Section: Measurement Resultsmentioning

confidence: 99%

Section: B Modeling Of Exposure Processmentioning

confidence: 99%

Analytical Modeling of Exposure Process in Pinned Photodiode CMOS Image Sensors

Gong

IEEE J. Electron Devices Soc.

et al. 2020

IEEE Electron Device Lett.

“…The 5T pixel exhibits higher lag due to the lower voltage applied on TX 1 and the additional diffusion path separating the IN from the SN. However this lag increase is not dramatic since the charge transfer in standard CIS 4T pixels is already limited either by internal diffusion or the presence of a potential barrier underneath the TG [9], [10]. Regarding the photo-response non-uniformity (PRNU), a measurement in HCG mode have been performed on a subset of 1500 pixels.…”

Section: Discussionmentioning

confidence: 99%

A CMOS Image Sensor Pixel Combining Deep Sub-Electron Noise With Wide Dynamic Range

Boukhayma

Caizzone

Enz

2020

This letter introduces a 5-transistors (5T) implementation of CMOS Image Sensors (CIS) pixels enabling the combination of deep sub-electron noise performance with wide dynamic range (DR). The 5T pixel presents a new technique to reduce the sense node capacitance without any process refinements or voltage level increase and features adjustable conversion gain (CG) to enable wide dynamic imaging. The implementation of the proposed 5T pixel in a standard 180 nm CIS process demonstrates the combination of a measured high CG of 250 µV/e − and low CG of 115 µV/e − with a saturation level of about 6500 e − offering photo-electron counting capability without compromising the DR and readout speed.

IEEE Trans. Electron Devices

“…In (2), I 0,diff is the diffusion current at equilibrium. This equation shows that the diffusion current is nonnegligible at small reverse bias (it is permitted by thermionic emission over the p-n junction potential barrier [14]). Once electrons have been emitted from the hot pixel PPD to the epitaxy, they can diffuse toward surrounding pixels.…”

Section: B Effect Of Temperaturementioning

confidence: 99%

Dark Current Blooming in Pinned Photodiode CMOS Image Sensors

Belloir

Lincelles

Pelamatti

et al. 2017

International audienceThis paper demonstrates the existence of dark current blooming in pinned photodiode CMOS image sensors with the support of both experimental measurements and TCAD simulations. It is usually assumed that blooming can appear only under illumination, when the charge collected by a pixel exceeds the full well capacity (i.e. when the photodiode becomes forward biased). In this work, it is shown that blooming can also appear in the dark by dark current leakage from hot pixels in reverse bias (i.e. below the full well capacity). The dark current blooming is observed to propagate up to nine pixels away in the experimental images and can impact hundreds of pixels around each hot pixel. Hence, it can be a major image quality issue for state-of-the-art pinned photodiode CMOS Image Sensors used in dark current limited applications such as low-light optical imaging and should be taken into account in the dark current subtraction process. This work also demonstrates that one of the key parameter for dark current optimization, the transfer gate bias during integration, has to be carefully chosen depending on the application because the optimum bias for dark current reduction leads to the largest dark current blooming effects