Imaging array with improved dynamic range utilizing parasitic photodiodes within floating diffusion nodes of pixels
A pixel sensor having a main photodetector and a parasitic photodiode and a method for reading out that pixel sensor are disclosed. The pixel sensor is read by reading a first potential on a floating diffusion node in the pixel sensor while the floating diffusion node is isolated from the main photodiode. The pixel sensor is then exposed to light such that the floating diffusion node and the photodetector are both exposed to the light. A second potential on the floating diffusion node is then readout while the floating diffusion node is isolated from the main photodiode. After the first and second potentials are readout, a third potential on the floating diffusion node is readout. The main photodiode is then connected to the floating diffusion node, and a fourth potential on the floating diffusion node is readout. First and second light intensities are determined from the readout potentials.
1. A pixel sensor comprising:
a photodiode characterized by a first light conversion efficiency and a main photodiode saturation exposure;
a floating diffusion node;
a gate that selectively connects said photodiode to said floating diffusion node;
a readout amplifier that generates a signal indicative of a potential on said floating diffusion node, said signal being coupled to a first conductor in response to a select signal being received by said pixel sensor; and
a reset gate that selectively connects said floating diffusion node to a reset bus at a reset potential, wherein
said floating diffusion node comprises a parasitic photodiode characterized by a second light conversion efficiency, said first light conversion efficiency being greater than or equal to 30 times said second light conversion efficiency and said second light conversion efficiency being sufficient to measure light exposures greater than said main photodiode saturation exposure.
2. The pixel sensor of claim 1 further comprising an overflow gate connected to said photodiode, said overflow gate removing charge from said photodiode if a potential on said photodiode exceeds an overflow potential.
3. The pixel sensor of claim 1 wherein said floating diffusion node comprises an n-type implanted region in a p-type semiconductor substrate.
4. The pixel sensor of claim 1 wherein said photodiode comprises an n-type implanted region in a p-type substrate.
5. The pixel sensor of claim 1 wherein said photodiode is a pinned photodiode.
6. An imaging array comprising a plurality of pixel sensors connected to a readout line, said pixel sensors comprising:
a photodiode characterized by a first light conversion efficiency and a main photodiode saturation exposure;
a floating diffusion node;
a gate that selectively connects said photodiode to said floating diffusion node;
a readout amplifier that generates a signal indicative of a potential on said floating diffusion node, said signal being coupled to a first conductor in response to a select signal being received by said pixel sensor;
a reset gate that selectively connects said floating diffusion node to a reset bus at a reset potential; and
a readout gate that connects that pixel sensor to said readout line, wherein
said floating diffusion node comprises a parasitic photodiode characterized by a second light conversion efficiency, said first light conversion efficiency being greater than or equal to 30 times said second light conversion efficiency and said second light conversion efficiency being sufficient to measure light exposures greater than said main photodiode saturation exposure.
7. The imaging array of claim 6 further comprising an overflow gate connected to said photodiode, said overflow gate removing charge from said photodiode if a potential on said photodiode exceeds an overflow potential.
8. The imaging array of claim 6 wherein said floating diffusion node comprises an n-type implanted region in a p-type semiconductor substrate.
9. The imaging array of claim 6 wherein said photodiode comprises an n-type implanted region in a p-type substrate.
10. The imaging array of claim 6 wherein said photodiode is a pinned photodiode.