Apparatus and method for reducing edge effect in an image sensor
View Patent ↗A method and apparatus for forming dummy pixels exhibiting electrical characteristics virtually identical to the clear pixels of the imaging array. Arrays of such dummy pixels are used to form regions that isolate the main imaging array and sub-arrays of optical black pixels while preventing edge effects. The dummy pixels are preferably clear but can also be covered with optical black. By setting quiescent operation in soft reset, the dummy pixels exhibit the diode ideality and R o A product that are typical of any of the pixels in the entire array.
1. A dummy pixel comprising:
a photodiode; and
a reset transistor connected to the photodiode, wherein the reset transistor is biased in a soft reset mode;
wherein the reset transistor has a gate and drain connected to a reset line, and wherein the reset line is set to a first predetermined voltage.
2. The dummy pixel of claim 1 , wherein the reset line is not connected to a reset bus.
3. The dummy pixel of claim 2 , further comprising an amplifier transistor having a gate connected to the photodiode and a source of the reset transistor, and a drain connected to the reset line.
4. The dummy pixel of claim 3 , further comprising a readout transistor having a drain connected to a source of the amplifier transistor, and having a gate connected to a second predetermined voltage.
5. The dummy pixel of claim 4 , wherein the first predetermined voltage is a circuit supply voltage.
6. The dummy pixel of claim 5 , wherein the second predetermined voltage is zero volts.
7. The dummy pixel of claim 1 , wherein the photodiode forms a current sink for stray electrons.
8. The dummy pixel of claim 1 , wherein the dummy pixel is formed as a clear pixel.
9. The dummy pixel of claim 1 , wherein the dummy pixel is formed as an optical black pixel.
10. A method for reducing dark current in an image sensor, the method comprising:
forming at least one row or column of dummy pixels in an image sensor;
wherein each dummy pixel comprises a pixel having a reset transistor permanently biased in a soft reset mode; and
wherein a photodiode in each dummy pixel sinks current for stray electrons.
11. The method of claim 10 , wherein the pixel is formed as a clear pixel.
12. The method of claim 10 , wherein the pixel is formed as an optical black pixel.
13. The method of claim 10 , wherein the at least one row or column of dummy pixels is formed on a perimeter region of the image sensor to reduce edge effect.
14. The method of claim 10 , wherein the at least one row or column of dummy pixels is formed between sub-arrays of the image sensor to isolate the sub-arrays from each other.
15. An image sensor comprising:
an array of clear pixels;
a plurality of sub-arrays of optical black pixels; and
a plurality of sub-arrays of dummy pixels located between the array of clear pixels and the sub-arrays of optical black pixels, wherein each dummy pixel comprises:
a photodiode; and
a reset transistor connected to the photodiode, wherein the reset transistor is biased in a soft reset mode, and wherein the reset transistor has a gate and drain connected to a reset line, and wherein the reset line is set to a first predetermined voltage; and wherein the reset line is not connected to a reset bus.
16. The image sensor of claim 15 , further comprising sub-arrays of dummy pixels located on perimeter of the image sensor to reduce edge effect.
17. The image sensor of claim 15 , wherein the dummy pixels are formed as clear pixels.
18. The image sensor of claim 15 , wherein the dummy pixels are formed as optical black pixels.