Pixel-Grouping Image Sensor With Asymmetrical Charge Storage
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Solution Overview
Problem
MOS technology image sensors face challenges in grouping pixels for improved sensitivity and resolution without compromising charge-to-voltage conversion efficiency, as existing methods either complicate individual pixel reading or result in lower conversion factors and inefficiencies, especially at low light levels.
Innovation Solution
A matrix of pixels with a temporary storage grid between adjacent photodiodes, controlled by a succession of potentials to allow charge grouping without affecting the charge-voltage conversion factor, featuring an asymmetrical grid for directional charge transfer, enabling independent or grouped charge collection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If pixels are grouped together to improve sensitivity, then sensitivity is improved, but the charge-to-voltage conversion factor decreases
Solution Approach 1:
The patent divides the charge transfer path into segments: photodiodes transfer charges to a temporary storage grid, which then transfers to the charge storage node. This segmentation allows the conversion factor to be determined by the small charge storage node capacitance rather than the total grouped pixel capacitance, maintaining high conversion factors even when pixels are grouped for improved sensitivity.
Solution Approach 2:
The temporary storage grid acts as an intermediary between the photodiodes and the charge storage node. It temporarily holds charges from multiple pixels before transferring them to the storage node, enabling charge grouping while preserving the conversion factor because the node capacitance remains small and unchanged.
2Productivity
If a temporary storage grid is introduced for charge grouping, then charge transfer efficiency is improved, but device complexity increases
Solution Approach 1:
The temporary storage grid is merged with the existing pixel structure, sharing the same physical space and control mechanisms. The grid uses the same gate control infrastructure as the transfer transistor, and charges flow through existing conductive paths, thereby improving transfer efficiency without proportionally increasing device complexity.
Solution Approach 2:
The temporary storage grid serves multiple functions: it enables charge grouping from multiple pixels, maintains the charge-to-voltage conversion factor, and can operate in both grouped and ungrouped modes. This multi-functionality justifies the added complexity by providing versatile charge handling capabilities.
3Measurement precision
If pixels are read independently for better resolution, then resolution is improved, but sensitivity decreases
Solution Approach 1:
The patent makes the charge collection mode dynamic and configurable. The temporary storage grid can be controlled to either collect charges from a single pixel (independent reading mode for high resolution) or from multiple pixels (grouped reading mode for high sensitivity). This dynamic adaptability allows the system to optimize between resolution and sensitivity based on imaging conditions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables efficient charge transfer and reading of grouped pixels without altering the charge-voltage conversion factor, improving sensitivity and resolution while maintaining high transfer efficiency, especially at low light levels.
Implementation Method 1
The grid is controlled by means which apply to the grid a succession of potentials characterised by: a first potential which prohibits the passage of charges between the photodiodes of the two pixels during the integration time of charges
Implementation Method 2
The grid is made up in an asymmetrical way between its upstream side (side which receives charges) and its downstream side (side which provides charges) to ensure directivity of the transfer of charges in this last phase
Implementation Method 3
each pixel comprises at least one photodiode and transistors used to perform a charge-to-voltage conversion in the pixel
Data Source
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AI summary
The invention relates to matrix image sensors organized into pixel rows and columns, and more specifically, to image sensors produced using MOS technology active pixels. The matrix is organized into groups of at least two pixels (PD1 to PD4) using means for grouping the charges produced in the two pixels into one pixel, for the purpose of improving sensitivity. At least one gate (G21) for temporarily storing charges is provided, said gate having a dissymmetrical shape and being arranged between the photodiode (PD1) of the first pixel and the photodiode (PD2) of the second pixel, and a means for applying a series of potentials to the temporary-storage gate, said potentials enabling charges to be initially prohibited from passing between the first and second photodiodes while charges are being integrated, then enabling the charges accumulated in the photodiodes to accumulate beneath the gate, and then enabling the discharge of said charges only into the second photodiode due to the dissymmetry of the gate. The charges that have been thus grouped into a single photodiode are read in the pixel.