Backside illuminated global shutter imaging array

1. An apparatus comprising a plurality of pixel sensors comprising an ordered array of pixel sensors, each pixel sensor being characterized by a position in said ordered array of pixel sensors and comprising:

a main photodiode; and

a correction photodiode;

said apparatus further comprising a controller that

resets all of said main photodiodes at a first time that is the same for all of said plurality of pixel sensors;

resets all of said correction photodiodes at a second time that is the same for all of said pixel sensors after said first time; and

sequentially reads out said pixel sensors, each of said plurality of pixel sensors being read out at a third time that is different for different ones of said pixel sensors and depends on said position of said pixel sensor in said ordered array of pixel sensors, said read out of each of said plurality of pixel sensors comprising:

measuring a correction charge that accumulated on said correction photodiode in said pixel sensor between said third time and said second time;

measuring a total charge that accumulated on said main photodiode in said pixel sensor at said third time; and

computing a pixel sensor exposure value representing a charge that had accumulated on said main photodiode at said second time.

2. The apparatus of claim 1 wherein said correction photodiode is characterized by a first light conversion efficiency and said main photodiode is characterized by a second light conversion efficiency, said first light conversion efficiency being less than said second light conversion efficiency.

3. The apparatus of claim 2 wherein said main photodiode is characterized by a first area of silicon that converts light to photoelectrons and said correction photodiode is characterized by a second area of silicon that converts light to photoelectrons, said first area of silicon being greater than said second area of silicon.

4. The apparatus of claim 1 wherein each pixel sensor comprises:

a floating diffusion node;

a first transfer gate that connects said main photodiode to said floating diffusion node in response to a first transfer signal; and

a second transfer gate that connects said correction photodiode to said floating diffusion node.

5. The apparatus of claim 4 further comprising a bit line, each pixel sensor comprises:

an amplifier that amplifies a voltage on said floating diffusion node to produce a signal on an amplifier output;

a bit line gate that connects said amplifier output to said bit line in response to a row select signal; and

a reset gate that connects said floating diffusion node to a first reset voltage source in response to a reset signal.

6. The apparatus of claim 4 wherein said main photodiode and said correction photodiode are connected in parallel with respect to said floating diffusion node by said first transfer gate and said second transfer gate, respectively.

7. The apparatus of claim 4 wherein said main photodiode and said correction photodiode are connected in series with respect to said floating diffusion node, said main photodiode being connected to said correction photodiode by said first transfer gate and said correction photodiode connected to floating diffusion node by said second transfer gate.

8. The apparatus of claim 4 wherein said controller measures said total charge by resetting said floating diffusion node and measuring a first voltage on said floating diffusion node after resetting said floating diffusion node; measuring a second voltage on said floating diffusion node after connecting said correction photodiode to said floating diffusion node at said third time, and measuring a third voltage on said floating diffusion node after said main photodiode is connected to said floating diffusion node.

9. A method for operating an imaging system comprising an ordered array of pixel sensors, each pixel sensor being characterized by a position in said ordered array of pixel sensors, each pixel sensor comprising a main photodiode that receives light during an image exposure and a correction photodiode, said method comprising:

resetting all of said main photodiodes at a first time;

defining a second time that marks an end of said image exposure;

sequentially measuring a total charge that accumulated on said main photodiode in that pixel sensor at a third time that depends on said position of said pixel sensor in said ordered array of pixel sensors, said third time being different for different ones of said plurality of pixel sensors;

correcting said total charge for charge that accumulated on said main photodiode in a time period between said second and third times to obtain an estimate of a global shutter charge that accumulated on said main photodiode at said second time;

resetting said correction photodiode at said second time;

measuring a correction charge on said correction photodiode at said third time; and

correcting said total charge based on said correction charge to obtain said global shutter charge.

10. The method of claim 9 wherein each pixel sensor further comprises a floating diffusion node and wherein obtaining an estimate of said global shutter charge for one of said plurality of pixel sensors comprises:

resetting said floating diffusion node of that one of said pixel sensors at said third time and measuring a first voltage on said floating diffusion node of that one of said plurality of pixel sensors;

connecting said correction photodiode of that one of said plurality of pixel sensors to said floating diffusion node of that one of said plurality of pixel sensors and measuring a second voltage on said floating diffusion node of that one of said plurality of pixel sensors;

connecting said main photodiode of that one of said plurality of pixel sensors to said floating diffusion node of that one of said plurality of pixel sensors; and measuring a third voltage on said floating diffusion node of that one of said plurality of pixel sensors; and

determining said estimate of said global shutter charge of that one of said plurality of pixel sensors from said first, second, and third voltages.