EPFL Proposes 5T Pixel with 0.32e- Noise and Enhanced DR

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IEEE Electron Device Letters gives an early access to EPFL paper "A CMOS Image Sensor Pixel Combining Deep Sub-electron Noise with Wide Dynamic Range" by Assim Boukhayma, Antonino Caizzone, and Christian Enz.

"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."

"Thanks to the high CG of 250 µV/e− and optimized PMOS SF, the read noise is as low as 0.32 e− RMS. This result is confirmed by Fig. 5 obtained by plotting the histogram of 1500 pixel outputs while the chip is exposed to very low input light. The histogram features peaks and valleys where each peak corresponds to a charge quantum."

"The reset phase consists in three steps. First, the RST switch is closed connecting IN to VRST. While VRST is set to VDD, the potential barrier between IN and SN is lowered by setting TX2 to a voltage VTX2H1 in order to dump the charge from the SN as depicted in Fig. 2(a). TX2 is set back to 0 in order to split the IN and SN and freeze the SN voltage at its maximum level.

VRST is then switched to a lower voltage VRSTL between the pin voltage of the PPD Vpin and VSN,max. After this step, the reset switch is opened again to freeze the IN voltage at a value VIN as depicted in Fig. 2(b). The last step of the reset phase consists in setting TX2 to a voltage VTX2H2 making the barrier between the IN and SN equal or slightly higher than VIN as shown in Fig. 2(c). In this way, any excess charge transferred to IN would diffuse towards the SN.

After lowering back TX2, the SN reset voltage VSN,rst is sensed. Transferring the charge integrated in the PPD to the SN takes place by pulsing both TX1 and TX2 as depicted in Fig. 2(d). TX1 is pulsed to a value VTX1H in order to set the voltage under the TG between the PPD pin voltage Vpin and the intermediate node voltage VIN while TX2 is pulsed again to transfer this charge to the SN. The signal corresponds to the difference between the SN
voltage after reset VSN,rst and the one sensed after the transfer VSN,transfer.

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