Active pixel sensor cell that utilizes a parasitic transistor to reset the photodiode of the cell
AssigneeNational Semiconductor Corporation
InventorsMin-hwa Chi, Richard Billings Merrill, Albert Bergemont
Life of this patent
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Abstract
The amount of silicon real estate consumed by a photodiode-based active pixel sensor cell is reduced by utilizing a parasitic transistor to reset the voltage on the photodiode in lieu of the conventional use of a reset transistor. The parasitic transistor is formed by forming a doped region a distance apart from the well region of the photodiode, which defines a parasitic channel region therebetween, and a reset gate over the parasitic channel region.
Claims9
A row of active pixel sensor cells formed in a semiconductor substrate of a first conductivity type, the row of cells comprising:
- a reset region of a second conductivity type formed in the substrate;
- a plurality of spaced-apart photodiode regions of the second conductivity type formed in the substrate, the first photodiode region of the plurality of photodiode regions being formed a distance apart from the reset region, the junctions between each photodiode region and the substrate forming a photodiode;
- a first parasitic channel region defined between the reset region and the first photodiode region;
- a plurality of second parasitic channel regions defined between the photodiode regions such that a second parasitic channel region is defined between each pair of adjacent photodiode regions;
- a layer of insulation material formed over the first parasitic channel region and the second parasitic channel regions;
- a plurality of reset gates formed over the layer of insulation material such that a reset gate is formed over the first parasitic channel region and a reset gate is formed over each second parasitic channel region;
- a reset line that contacts each of the reset gates;
- and
- a plurality of sense transistors corresponding to the plurality of photodiode regions, each sense transistor having a gate connected to a corresponding photodiode region.
A method for converting light energy into a plurality of pixel signals with a corresponding plurality of active pixel sensor cells formed in a semiconductor substrate of a first conductivity type, wherein the plurality of active pixel sensor cells include:
- a reset region of a second conductivity type formed in the substrate;
- a plurality of spaced-apart photodiode regions of the second conductivity type formed in the substrate, each photodiode region accumulating photogenerated electrons for a single pixel during an integration period, the first photodiode region of the plurality of photodiode regions being formed a distance apart from the reset region;
- a first parasitic channel region defined between the reset region and the first photodiode region;
- a plurality of second parasitic channel regions defined between the photodiode regions such that a second parasitic channel region is defined between each pair of adjacent photodiode regions;
- a layer of insulation material formed over the first parasitic channel region and the second parasitic channel regions;
- a plurality of reset gates formed over the layer of insulation material such that a reset gate is formed over the first parasitic channel region and a reset gate is formed over each second parasitic channel region;
- a reset line that contacts each of the reset gates;
- a plurality of sense transistors corresponding to the plurality of photodiode regions, each sense transistor having a gate connected to a corresponding photodiode region;
- and
- a plurality of row select transistors corresponding to the plurality of sense transistors, each row select transistor being connected to a corresponding sense transistor;
- the method comprising the steps of:
- applying a first voltage to the substrate;
- resetting each of the plurality of photodiode regions to a preintegration voltage which reverse biases each of the substrate to photodiode region junctions;
- collecting photogenerated electrons in each of the photodiode regions, the photogenerated electrons in each photodiode region changing the preintegration voltage to a post integration voltage;
- and
- applying a read voltage to the gates of each of the row select transistors an integration time after the photodiode regions are reset.
Dependencies are inferred from the claim text.