Skimmed Charge Capture for High Dynamic Range CMOS Pixels
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Solution Overview
Problem
Conventional CMOS imagers face challenges in achieving high dynamic range without suffering from pixel-to-pixel response variation and the need for additional transistors, which complicates spatial resolution and pixel size constraints.
Innovation Solution
The solution involves modifying the 6T global shutter pixel by adding an extra transfer gate between the photosensor and storage gate, allowing non-destructive charge sensing and storage of excess charge information in the floating diffusion region, enabling multiple scans during integration to estimate and manage charge exceeding predetermined limits, and using a comparator for global shutter operation to reset pixels uniformly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional CMOS imager pixel structures are used, then pixel-to-pixel response variation occurs and dynamic range is limited, but adding more transistors to increase full well capacity increases device complexity and pixel size
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: a photosensor region for charge generation, a storage region for charge accumulation, and a readout region with transistors. This segmentation allows the photosensor and storage region to be optimized independently, improving pixel response consistency while keeping the transistor count limited to the readout circuitry.
Solution Approach 2:
The invention extracts the charge storage function from the photosensor region and places it in a separate storage region. This extraction allows the photosensor to maintain optimal dimensions for consistent response while the storage region provides the additional capacity needed for extended dynamic range without adding transistors to the photosensor area.
2Reliability
If additional transistors are added to increase full well capacity, then dynamic range improves, but pixel size increases and spatial resolution deteriorates
Solution Approach 1:
The invention utilizes the vertical dimension by forming the storage region as a pinned photodiode extending beneath the photosensor. This three-dimensional charge storage structure increases full well capacity without expanding the lateral pixel footprint, thereby maintaining spatial resolution while improving dynamic range.
Solution Approach 2:
The storage region is nested beneath the photosensor region in a stacked configuration. The pinned photodiode storage region is positioned directly under the photosensor, allowing charge to be transferred vertically. This nesting arrangement maximizes charge storage capacity within the available vertical space without increasing the lateral pixel area.
3Quantity of substance
If photosensor size is increased to improve full well capacity, then charge storage capacity increases, but pixel density and spatial resolution decrease
Solution Approach 1:
The invention applies local quality by creating a pinned photodiode structure with specific doping characteristics in the storage region beneath the photosensor. This localized structural modification provides enhanced charge storage capacity in a confined volume, increasing pixel density while maintaining or improving full well capacity through the specialized photodiode properties rather than simply enlarging the pixel area.
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
This approach enhances the effective full well capacity of pixels, allowing for high dynamic range imaging beyond typical linear response limits, while maintaining small pixel sizes and avoiding spatial resolution trade-offs.
Implementation Method 1
The illustrated pixel 110 contains a pinned photodiode photosensor 112
Implementation Method 2
transfer gate 114, a floating diffusion region FD to collect charge transferred from the photosensor 112
Implementation Method 3
The source follower transistor 118 converts the charge stored at the floating diffusion region FD into an electrical output voltage signal Vout
Data Source
AI summary
An imaging device having pixels that store charge from a photosensor under at least one storage gate during a sampling period. A driver used to operate the at least one storage gate, estimates how much charge in the pixel exceeds a predetermined limit during a non-destructive pixel sensing operation. A specific voltage is stored on the pixel's floating diffusion region to flag how many times the pixel exceeded the limit. The final pixel signal and the stored information is readout at the end of integration period to create a sum that represents the pixel's final signal value.