Clear and Color Pixel Array Blooming Mitigation
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Solution Overview
Problem
CMOS image sensors suffer from 'blooming' due to excess charge overflow from clear pixels, which affects neighboring pixels and degrades image quality, especially in high-light conditions or prolonged exposure.
Innovation Solution
Implementing clear pixel access transistors that transfer more current per unit time than color pixel access transistors, and providing an excess-charge transfer path for clear pixels to quickly dissipate excess charge, while conserving area by omitting this path for color pixels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If clear pixels use the same access transistor configuration as color pixels, then device complexity is reduced, but blooming occurs due to excess charge overflow from clear pixels
Solution Approach 1:
The patent applies local quality by providing different access transistor configurations for clear pixels versus color pixels. Specifically, clear pixels receive a first gate voltage that is more negative than the second gate voltage for color pixels, creating locally optimized performance for each pixel type to prevent blooming while maintaining overall system functionality.
Solution Approach 2:
The patent changes the gate voltage parameter dynamically based on pixel type. By adjusting the gate voltage to be more negative for clear pixels during integration periods, the access transistor conductivity is reduced, preventing excess charge overflow. This parameter change resolves the contradiction by improving blooming performance without requiring complete redesign of the access transistor architecture.
2Reliability
If an excess-charge transfer path is added for clear pixels, then blooming is reduced, but array density decreases due to additional circuit area
Solution Approach 1:
The patent applies partial action by providing an excess-charge transfer path only for clear pixels, not for color pixels. This selective approach addresses the blooming issue where it occurs (in clear pixels that receive full spectrum light and generate excess charge) while avoiding unnecessary circuit elements in color pixels, thereby maintaining overall array density.
Solution Approach 2:
The patent segments the pixel array into clear pixels and color pixels with different circuit configurations. By isolating the excess-charge transfer path to only clear pixels, the design segments the functionality based on operational needs, preventing blooming in clear pixels while conserving area in the overall array.
3Speed
If clear pixel access transistors transfer more charge per unit time, then excess charge dissipates faster, but image quality degrades due to reduced charge transfer precision
Solution Approach 1:
The patent applies periodic action by using different gate voltages during different operational phases. During integration periods, clear pixels receive a more negative gate voltage to slow charge transfer and prevent blooming. During readout periods, the gate voltage is adjusted to allow accurate charge transfer to the charge-to-voltage converter, ensuring both blooming prevention and measurement precision through time-dependent voltage control.
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
Effectively reduces blooming issues in clear pixels, maintaining image quality by efficiently managing excess charge and preventing misleading light intensity readings in neighboring pixels.
Implementation Method 1
the clear pixel access transistor is configured to transfer more charge per unit time between the clear pixel photodetector and the charge storage node than the color pixel access transistor transfers between the color pixel photodetector and the charge storage node
Implementation Method 2
providing an excess-charge transfer path for clear pixels to quickly dissipate excess charge
Implementation Method 3
Each pixel includes a photodetector, such as a photodiode
Data Source
AI summary
This disclosure provides pixel arrays made up of a clear pixel and a color pixel. The color pixel includes a first photo-detecting element and a color pixel access transistor to selectively couple the first photo-detecting element to a first charge-storage node. The clear pixel includes a second photo-detecting element and a clear pixel access transistor to selectively couple the second photo-detecting element to a second charge-storage node. The color pixel access transistor transfers a first charge per unit time between the first photo-detecting element and the first charge-storage node. The clear pixel access transistor transfers a second charge per unit time between the clear pixel access transistor and the second charge-storage node. The first charge per unit time is less than the second charge per unit time to mitigate blooming. In other embodiments, the clear pixel includes an excess-charge transfer path that couples the clear pixel to a DC supply node to mitigate blooming.