Methods of calibrating knee-point and logarithmic slope in linear-logarithmic image sensors

Patent metadata

Field	Value
Publication number	US-9426395-B2
Application number	US-201414224264-A
Country	US
Kind code	B2
Filing date	Mar 25, 2014
Priority date	Mar 25, 2014
Publication date	Aug 23, 2016
Grant date	Aug 23, 2016

Abstract

Methods of performing two point calibration of a linear-logarithmic image sensor pixel include measuring voltages of a floating diffusion region of the pixel after establishing a plurality of unequal sub-threshold currents through a transfer transistor of the pixel. These sub-threshold currents operate to sequentially increase a voltage of the floating diffusion region to respective voltage levels that enable knee-point (KNPT) voltage and logarithmic sensitivity (LOGS) determination. The methods also include depleting a photodiode within the pixel by driving a cathode of the photodiode with a pull-up current in advance of establishing the sub-threshold currents. The method is photocurrent-independent.

First claim

That which is claimed is:

1. A method of calibrating a linear-logarithmic image sensor pixel, comprising:
at least partially depleting a photodiode within the pixel;
supplying a floating diffusion region within the pixel with a first subthreshold current by passing first photo-generated charge carriers from the at least partially depleted photodiode to the floating diffusion region; then
reading a first voltage of the floating diffusion region containing the first photo-generated charges, as first calibration data;
then, without resetting the floating diffusion region by removing the first photo-generated charges therefrom, supplying the floating diffusion region with a second subthreshold current by passing second photo-generated charge carriers from the at least partially depleted photodiode to the floating diffusion region; then
reading a second voltage of the floating diffusion region containing the first and second photo-generated charge carriers therein, as second calibration data; and
determining a knee-point voltage associated with a transition from a linear to logarithmic response of the pixel from the first and second calibration data.

2. The method of claim 1 , wherein said at least partially depleting the photodiode comprises establishing a reverse bias across anode and cathode regions of the photodiode by shorting the floating diffusion region to the cathode region.

3. The method of claim 2 , wherein said at least partially depleting the photodiode comprises shorting the floating diffusion region to the cathode region concurrently with supplying the floating diffusion region with a pull-up current.

4. The method of claim 3 , wherein said supplying the floating diffusion region with a pull-up current comprises turning on a reset transistor within the pixel, said reset transistor having a first current carrying terminal electrically connected to the floating diffusion region and a second current carrying terminal responsive to a positive reset voltage.

5. The method of claim 4 , wherein the reset transistor is an NMOS transistor having a source terminal electrically connected to the floating diffusion region and a drain terminal responsive to the positive reset voltage.

6. The method of claim 3 , wherein said supplying the floating diffusion region with a first subthreshold current is preceded by discharging the floating diffusion region; and wherein said discharging the floating diffusion region follows said at least partially depleting the photodiode.

7. The method of claim 6 , wherein said discharging the floating diffusion region comprises discharging the floating diffusion region through a reset transistor within the pixel.

8. The method of claim 1 , wherein a duration of said supplying the floating diffusion region with a second subthreshold current is greater than a duration of said supplying the floating diffusion region with the first subthreshold current.

9. The method of claim 1 , wherein a quantity of charge transferred during said supplying the floating diffusion region with a second subthreshold current is greater than a quantity of charge transferred during said supplying the floating diffusion region with the first subthreshold current.

10. A method of calibrating a linear-logarithmic image sensor pixel, comprising:
at least partially depleting a photodiode within the pixel by driving a cathode of the photodiode with a pull-up current;
supplying a floating diffusion region within the pixel with a first subthreshold current by passing first charge carriers from the at least partially depleted photodiode to the floating diffusion region; then
reading a first voltage of the floating diffusion region containing the first charge carriers, as first calibration data;
then, without adjusting a voltage of the floating diffusion region upon completion of said reading a first voltage, supplying the floating diffusion region with a second subthreshold current by passing second charge carriers from the at least partially depleted photodiode to the floating diffusion region; and then
reading a second voltage of the floating diffusion region containing the first and second charge carriers therein, as second calibration data.

11. The method of claim 10 , wherein said driving the cathode of the photodiode with the pull-up current comprises shorting the cathode of the photodiode to the floating diffusion region.

12. The method of claim 11 , wherein the pixel comprises a series arrangement of a reset transistor and a transfer transistor; wherein a source terminal of the reset transistor is electrically connected to the floating diffusion region and to a drain terminal of the transfer transistor; and wherein driving the cathode of the photodiode with the pull-up current comprises turning on the reset and transfer transistors.

13. The method of claim 12 , wherein driving the cathode of the photodiode with the pull-up current comprises pulling-up the cathode of the photodiode to a voltage equivalent to a drain terminal of the reset transistor minus a threshold voltage of the reset transistor and a threshold voltage of the transfer transistor.

14. The method of claim 13 , wherein said supplying a floating diffusion region within the pixel with a first subthreshold current is preceded by discharging the floating diffusion region by driving the drain terminal of the reset transistor high-to-low.

15. The method of claim 10 , wherein said supplying a floating diffusion region within the pixel with a first subthreshold current is preceded by discharging the floating diffusion region after said at least partially depleting the photodiode.

16. The method of claim 10 , wherein a total charge associated with the second subthreshold current is greater than a total charge associated with the first subthreshold current.

17. The method of claim 10 , wherein a duration of said supplying the floating diffusion region with the second subthreshold current is greater than a duration of said supplying the floating diffusion region with the first subthreshold current.

18. A method of calibrating a linear-logarithmic image sensor pixel, comprising:
fully depleting a photodiode within the pixel; then
discharging a floating diffusion region within the pixel;
supplying the floating diffusion region with a first subthreshold current by passing first photo-generated charges of first conductivity type from the photodiode in an at least partially depleted state to the floating diffusion region; then
reading a first voltage of the floating diffusion region as first calibration data; then
without resetting the floating diffusion region by removing the first photo-generated charges therefrom, supplying the floating diffusion region with a second subthreshold current by passing second photo-generated charges from the at least partially depleted photodiode to the floating diffusion region; then
reading a second voltage of the floating diffusion region containing the first and second photo-generated charge carriers therein, as second calibration data; and
determining a knee-point voltage associated with a transition from a linear to logarithmic response of the pixel from the first and second calibration data.

Assignees

• Samsung Electronics Co Ltd

Inventors

• Jakobson Claudio G
• Prazot Shemi
• Hizi Uzi

Classifications

• H04N25/573

  the logarithmic type · CPC title

• H04N25/583

  with different integration times · CPC title

• H04N25/76

  Addressed sensors, e.g. MOS or CMOS sensors · CPC title

• H04N25/671 Primary

  for non-uniformity detection or correction · CPC title

• H04N17/002 Primary

  for television cameras · CPC title