METHODS AND APPARATUS FOR HIGH DYNAMIC OPERATION OF A PIXEL CELL

• US 20100231771A1
• Filed: 03/12/2009
• Published: 09/16/2010
• Est. Priority Date: 03/12/2009
• Status: Active Grant

• Current Assignee: Micron Technology, Inc.
• Original Assignee: Micron Technology, Inc.
• Inventors: Yaghmai, Sohrab
• Granted Patent: US 8,184,188 B2
• Time in Patent Office: Days
• US Class Current: 348/308
• CPC Class Codes: H04N 25/589 with different integration times, e.g. short and long exposures
• CPC Class Codes: H04N 25/75 Circuitry for providing, modifying or processing image signals from the pixel array
• CPC Class Codes: H04N 25/771 comprising storage means other than floating diffusion
• CPC Class Codes: H04N 25/778 comprising amplifiers shared between a plurality of pixels, i.e. at least one part of the amplifier must be on the sensor array itself
• CPC Class Codes: H04N 25/78 Readout circuits for addressed sensors, e.g. output amplifiers or A/D converters

Abstract

A pixel circuit providing high dynamic operation, and methods of operating the pixel circuit providing for high dynamic operation. Methods include a lateral overflow and a double exposure mode, and a pixel output signal is determined according to whether a photosensor of the pixel circuit is saturated.

Claims

1. A method of operating a pixel circuit comprising:

generating a first photo-generated charge in at least one photoconversion region during a first exposure period;

transferring at least a portion of said first photo-generated charge to a storage region;

generating a second photo-generated charge in said at least one photoconversion region during a second exposure period;

sampling said at least a portion of said first photo-generated charge during said second exposure period;

transferring said second photo-generated charge to said storage region; and

sampling said second photo-generated charge.

2. The method of claim 1, wherein said first exposure time is longer than said second exposure time.

3. The method of claim 1, further comprising:

prior to transferring said at least a portion of said first photo-generated charge to said storage region, transferring an excess portion of said first photo-generated charge above a threshold voltage to an opposite-charged region.

4. The method of claim 1, further comprising:

determining an output pixel signal with a processor according to whether said photoconversion region is saturated during said first exposure period.

5. The method of claim 4, wherein said output pixel signal is a sampled pixel signal corresponding to said first photo-generated charge when said photoconversion region is not saturated during said first exposure period.

6. The method of claim 4, wherein said output pixel signal is a normalized pixel signal corresponding to said second photo-generated charge when said photoconversion region is saturated during said first exposure period.

7. The method of claim 4, wherein said output pixel signal is a normalized pixel signal corresponding to said second photo-generated charge and said at least a portion of said first photo-generated charge when said photoconversion region is saturated during said first exposure period.

8. The method of claim 3, further comprising:

determining an output pixel signal with a processor according to whether said photoconversion region is saturated during said first exposure period, wherein said processors determines said output pixel signal according to Equation 1;

$\begin{array}{cc}\left[\begin{array}{c}\mathrm{If}<br><br><br><br>V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}<<br><br>V_{\mathrm{TH\_MIN}}\to <br><br>V_{\mathrm{OPIX}}=V_{\mathrm{OPIX\_DIF}<br><br><br><br>1};<br><br>\\ \mathrm{If}<br><br><br><br>V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}\ge <br><br>V_{\mathrm{TH\_MIN}},\\ \mathrm{and}<br><br><br><br>N*V_{\mathrm{OPIX\_DIF}<br><br><br><br>2}<<br><br>V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}\to <br><br>V_{\mathrm{OPIX}}=V_{\mathrm{OPIX\_DIF}<br><br><br><br>1};<br><br>\\ \mathrm{If}<br><br><br><br>V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}\ge <br><br>V_{\mathrm{TH\_MIN}},\\ \mathrm{and}<br><br><br><br>N*V_{\mathrm{OPIX\_DIF}<br><br><br><br>2}\ge <br><br>V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}\to <br><br>V_{\mathrm{OPIX}}=N*\left(V_{\mathrm{OPIX\_DIF}<br><br><br><br>2}-V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}\right),\end{array}\right]& \mathrm{Equation}<br><br><br><br>1\end{array}$

wherein V_OPIX is said output pixel signal, V<sub>OPIX—

DIF1</sub> is said sampled at least a portion of said first photo-generated charge, V<sub>TH—

MIN</sub> is a minimum threshold voltage of said pixel circuit, V<sub>OPIX—

DIF2</sub> is said sampled second photo-generated charge, and N is the ratio of said first exposure period and said second exposure period.

9. The method of claim 1, further comprising:

determining an output pixel signal with a processor according to whether said photoconversion region is saturated during said first exposure period, wherein said processor determines said output pixel signal according to Equation 2;

$\begin{array}{cc}\left[\begin{array}{c}\mathrm{If}<br><br><br><br>V_1<<br><br>V_{\mathrm{TH\_MIN}}\to <br><br>V_{\mathrm{OPIX}}=V_{\mathrm{OPIX\_DIF}<br><br><br><br>1};<br><br>\\ \mathrm{If}<br><br><br><br>V_1\ge <br><br>V_{\mathrm{TH\_MIN}},\\ \mathrm{and}<br><br><br><br>N*V_2<<br><br>V_1\to <br><br>V_{\mathrm{OPIX}}=V_{\mathrm{OPIX\_DIF}<br><br><br><br>1};<br><br>\\ \mathrm{If}<br><br><br><br>V_1\ge <br><br>V_{\mathrm{TH\_MIN}},\\ \mathrm{and}<br><br><br><br>N*V_2\ge <br><br>V_1\to <br><br>V_{\mathrm{OPIX}}=N*\left(V_{\mathrm{OPIX\_DIF}<br><br><br><br>2}\right),\end{array}\right]& \mathrm{Equation}<br><br><br><br>2\end{array}$

wherein V_OPIX is said output pixel signal, V<sub>OPIX—

DIF1</sub> is said sampled at least a portion of said first photo-generated charge, V<sub>TH—

MIN</sub> is a minimum threshold voltage of said pixel circuit, V<sub>OPIX—

DIF2</sub> is said sampled second photo-generated charge, and N is the ratio of said first exposure period and said second exposure period.

10. The method of claim 1, wherein sampling said first and second photo-generated charges further comprises performing correlated double sampling of said at least a portion of said first photo-generated charge and said second photo-generated charges.

11. A pixel circuit comprising:

a photoconversion region for generating a first photo-generated charge during a first exposure period and a second photo-generated charge during a second exposure period;

a storage transistor connecting said photoconversion region to a first storage region;

a transfer transistor connecting said first storage region to a second storage region; and

readout circuitry configured to readout a first voltage corresponding to at least a portion of said first photo-generated charge and a second voltage corresponding to said second photo-generated charge,

wherein;

said storage transistor is configured to transfer the at least a portion of said first photo-generated charge to said first storage region after said first exposure period;

said transfer transistor is configured to transfer the at least a portion of said first photo-generated charge to said second storage region;

said storage transistor is configured to transfer said second photo-generated charge to said first storage region after said second exposure period;

said transfer transistor is configured to transfer said second photo-generated charge to said second storage region; and

said readout circuitry is configured to perform correlated double sampling of said first and second photo-generated charges.

12. The pixel circuit of claim 11, further comprising:

an anti-blooming transistor connecting said photoconversion region to a drain region, said anti-blooming transistor being configured to transfer an excess portion of said first photo-generated charge after said first exposure period.

13. The pixel circuit of claim 12, wherein said anti-blooming transistor is configured to partially open transfer said excess portion.

14. The pixel circuit of claim 13, wherein said anti-blooming transistor is configured to retain substantially all of said first photo-generated charge in said photoconversion region when partially open if said first photo-generated charge is less than or equal to a threshold voltage.

15. The pixel circuit of claim 13, wherein said anti-blooming transistor is configured to retain a first portion of said first photo-generated charge that is approximately equal to a threshold voltage, and transfer a second portion of said first photo-generated charge that is approximately equal to an amount of said first photo-generated charge in excess of said threshold voltage to said opposite-charged drain region if said first photo-generated charge is greater than a threshold voltage.

16. The pixel circuit of claim 14, wherein said threshold voltage corresponds to a level of an anti-blooming signal received at a gate of said anti-blooming transistor.

17. The pixel circuit of claim 11, wherein said readout circuitry is configured to readout said first photo-generated charge during said second integration period.

18. The pixel circuit of claim 11, wherein said pixel circuit is configured to operate according to a double exposure mode.

19. The pixel circuit of claim 12, wherein said pixel circuit is configured to operate according to a lateral overflow mode.

20. An imager comprising:

a pixel circuit including;

a photoconversion region for generating a first photo-generated charge during a first exposure period and a second photo-generated charge during a second exposure period;

a storage transistor connecting said photoconversion region to a first storage region;

a transfer transistor connecting said first storage region to a second storage region; and

readout circuitry configured to readout a first voltage corresponding to at least a portion of said first photo-generated charge and a second voltage corresponding to said second photo-generated charge;

a processor configured to determine and output a pixel signal corresponding to said first and second photo-generated charges; and

a control circuit electrically connected to said pixel circuit, said control circuit configured to provide;

a first storage signal to a gate of said storage transistor to transfer the at least a portion of said first photo-generated charge to said first storage region after said first exposure period;

a first transfer signal to a gate of said transfer transistor to transfer the at least a portion of said first photo-generated charge to said second storage region;

first readout signals to said readout circuitry providing correlated double sampling of the at least a portion of said first photo-generated charge;

a second storage signal to said gate of said storage transistor to transfer said second photo-generated charge to said first storage region after said second exposure period;

a second transfer signal to said gate of said transfer transistor to transfer said second photo-generated charge to said second storage region; and

second readout signals to said readout circuitry providing correlated double sampling of said second photo-generated charge.

21. The imager of claim 20, wherein said processor is configured to output a pixel signal according to said first photo-generated charge when said photoconversion region is not saturated during said first exposure period.

22. The imager of claim 20, wherein said processor is configured to output a normalized pixel signal according to said second photo-generated charge when said photoconversion region is saturated during said first exposure period.

23. The imager of claim 20, wherein said processor is configured to output a normalized pixel signal according to said first and second photo-generated charges when said photoconversion region is saturated during said first exposure period.

24. The imager of claim 20, wherein said processor is configured to output a pixel signal according to Equation 2:

$\begin{array}{cc}\left[\begin{array}{c}\mathrm{If}<br><br><br><br>V_1<<br><br>V_{\mathrm{TH\_MIN}}\to <br><br>V_{\mathrm{OPIX}}=V_{\mathrm{OPIX\_DIF}<br><br><br><br>1};<br><br>\\ \mathrm{If}<br><br><br><br>V_1\ge <br><br>V_{\mathrm{TH\_MIN}},\\ \mathrm{and}<br><br><br><br>N*V_2<<br><br>V_1\to <br><br>V_{\mathrm{OPIX}}=V_{\mathrm{OPIX\_DIF}<br><br><br><br>1};<br><br>\\ \mathrm{If}<br><br><br><br>V_1\ge <br><br>V_{\mathrm{TH\_MIN}},\\ \mathrm{and}<br><br><br><br>N*V_2\ge <br><br>V_1\to <br><br>V_{\mathrm{OPIX}}=N*\left(V_{\mathrm{OPIX\_DIF}<br><br><br><br>2}\right),\end{array}\right]& \mathrm{Equation}<br><br><br><br>2\end{array}$

wherein V_OPIX is said output pixel signal, V<sub>OPIX—

DIF1</sub> is the sampled at least a portion of said first photo-generated charge, V<sub>TH—

MIN</sub> is a minimum threshold voltage of said pixel circuit, V<sub>OPIX—

DIF2</sub> is the sampled second photo-generated charge, and N is the ratio of said first exposure period and said second exposure period

25. The imager of claim 20, wherein said pixel circuit further comprises an anti-blooming transistor connecting said photoconversion region to a drain region, said control circuit further configured to provide an anti-blooming signal to a gate of said anti-blooming transistor to transfer an excess portion of said first photo-generated charge after said first exposure period, and

said processor is configured to output a pixel signal according to Equation 1:

$\begin{array}{cc}\left[\begin{array}{c}\mathrm{If}<br><br><br><br>V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}<<br><br>V_{\mathrm{TH\_MIN}}\to <br><br>V_{\mathrm{OPIX}}=V_{\mathrm{OPIX\_DIF}<br><br><br><br>1};<br><br>\\ \mathrm{If}<br><br><br><br>V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}\ge <br><br>V_{\mathrm{TH\_MIN}},\\ \mathrm{and}<br><br><br><br>N*V_{\mathrm{OPIX\_DIF}<br><br><br><br>2}<<br><br>V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}\to <br><br>V_{\mathrm{OPIX}}=V_{\mathrm{OPIX\_DIF}<br><br><br><br>1};<br><br>\\ \mathrm{If}<br><br><br><br>V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}\ge <br><br>V_{\mathrm{TH\_MIN}},\\ \mathrm{and}<br><br><br><br>N*V_{\mathrm{OPIX\_DIF}<br><br><br><br>2}\ge <br><br>V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}\to <br><br>V_{\mathrm{OPIX}}=N*\left(V_{\mathrm{OPIX\_DIF}<br><br><br><br>2}-V_{\mathrm{OPIX\_DIF}<br><br><br><br>1}\right),\end{array}\right]& \mathrm{Equation}<br><br><br><br>1\end{array}$

wherein V_OPIX is said output pixel signal, V<sub>OPIX—

DIF1</sub> is the sampled at least a portion of said first photo-generated charge, V<sub>TH—

MIN</sub> is a minimum threshold voltage of said pixel circuit, V<sub>OPIX—

DIF2</sub> is the sampled second photo-generated charge, and N is the ratio of said first exposure period and said second exposure period.

26. A method of operating an imager in a lateral-overflow mode, the method comprising:

accumulating a first photo-generated charge in a photosensor during a first exposure period;

transferring an excess portion of said first photo-generated charge to a drain area of an anti-blooming transistor if said first photo-generated charge exceeds a threshold voltage;

transferring a remaining portion of said first photo-generated charge to a first storage region;

accumulating a second photo-generated charge in said photosensor during a second exposure period;

sampling said remaining portion of said first photo-generated charge during said second exposure period;

transferring said second photo-generated charge to said first storage region; and

sampling said second photo-generated charge.

27. The method of claim 26, wherein said remaining portion is substantially equal to said threshold voltage.

28. The method of claim 26, wherein said remaining portion is substantially all of said first photo-generated charge.

29. The method of claim 26, wherein said first exposure time is longer than said second exposure time.

30. The method of claim 29, wherein said first exposure time is approximately twice as long as said second exposure time.

31. The method of claim 26, further comprising:

determining and outputting a first sampled voltage corresponding to said remaining portion of said first photo-generated charge if said photoconversion region is not saturated during said first exposure period.

32. The method of claim 26, further comprising:

determining and outputting a normalized voltage if said photoconversion region is saturated during said first exposure period.

33. The method of claim 32, wherein determining said normalized voltage comprises:

calculating a difference between a second sampled voltage corresponding to said sampled second photo-generated charge and said sampled remaining portion; and

multiplying said difference by a ratio of durations of said first exposure period and said second exposure time.

34. A method of operating an imager in a double exposure mode, the method comprising:

accumulating a first photo-generated charge in a photosensor during a first exposure period;

transferring said first photo-generated charge to a first storage region;

accumulating a second photo-generated charge in said photosensor during a second exposure period;

sampling said first photo-generated charge during said second exposure period;

transferring said second photo-generated charge to said first storage region; and

sampling said second photo-generated charge.

35. The method of claim 34, wherein said first exposure time is longer than said second exposure time.

36. The method of claim 34, wherein said first exposure time is approximately twice as long as said second exposure time.

37. The method of claim 34, further comprising:

outputting a first sampled voltage corresponding to said first photo-generated charge if said photoconversion region is not saturated during said first exposure period.

38. The method of claim 34, further comprising:

determining and outputting a normalized voltage if said photoconversion region is saturated during said first exposure period.

39. The method of claim 38, wherein determining said normalized voltage comprises:

multiplying a second sampled voltage corresponding to said second photo-generated charge by a ratio of durations of said first exposure period and said second exposure period.