Adjustable Well Capacity Pixel for CMOS Image Sensors
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Solution Overview
Problem
Conventional CMOS image sensor pixels face challenges in achieving high full well capacity while avoiding spill-back lag, as increasing well size to support higher capacity leads to residual charge retention and performance degradation.
Innovation Solution
The introduction of a well-capacity-adjustment (WCA) gate photo-sensor block that dynamically controls the well capacity by biasing WCA structures to increase well potential during integration and decrease it during charge transfer, avoiding lag and residual charge issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the well size is increased to support higher full well capacity, then the full well capacity is improved, but spill-back lag increases due to residual charge retention
Solution Approach 1:
The patent applies the Dynamics principle by making the well capacity adjustable through a control gate that can dynamically change the potential well depth. The well capacity is no longer fixed but can be adapted based on operating conditions, allowing the system to optimize between full well capacity and spill-back prevention by adjusting the well depth as needed.
Solution Approach 2:
The patent implements Parameter changes by modifying the electrical potential of the well through the control gate. By changing the voltage applied to the control gate, the potential well depth and consequently the well capacity can be adjusted. This allows the system to achieve high full well capacity when needed while preventing spill-back lag by reducing the well capacity when appropriate.
2Quantity of substance
If the well potential is increased to increase full well capacity, then the full well capacity is improved, but residual charge remains after transfer causing lag
Solution Approach 1:
The control gate enables dynamic adjustment of the well potential, allowing the system to optimize between storing maximum charge and enabling complete charge transfer. By dynamically controlling the potential well depth, the system can achieve high full well capacity during integration while ensuring complete charge transfer to prevent lag.
Solution Approach 2:
The patent changes the electrical parameter (well potential) through the control gate to achieve desired full well capacity. By adjusting this parameter, the system can optimize charge storage while ensuring complete charge transfer, thereby preventing lag without sacrificing full well capacity.
3Adaptability or versatility
If a larger well is designed to accumulate more photocharge, then the dynamic range is improved, but the conversion gain decreases due to larger well capacitance
Solution Approach 1:
The adjustable well capacity through the control gate allows the system to dynamically adapt between different operating modes. The system can optimize for dynamic range by increasing well capacity when needed, or optimize for conversion gain by reducing well capacity, providing versatility in performance characteristics based on application requirements.
Solution Approach 2:
By changing the well capacity parameter through control gate biasing, the system can adjust the trade-off between dynamic range and conversion gain. This parameter adjustment allows optimization of one characteristic while accepting the corresponding change in the other, or finding an optimal balance between both.
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 allows for increased full well capacity during integration while minimizing spill-back lag and residual charge, enhancing the dynamic range and conversion gain of CMOS image sensor pixels.
Implementation Method 1
WCA structures (e.g., deep trench regions) form walls at least partially surrounding and capacitively coupling with the doped well, such that biasing of the WCA structures changes the well potential and the corresponding well capacity.
Implementation Method 2
a photosensing element that responds to light to generate photocharge
Data Source
AI summary
An imaging pixel design is provide with a photo-sensor block structure that facilitates dynamic control of well capacity in the photodiode region (i.e., a “well capacity adjustment (WCA) gate photo-sensor block”). The photodiode region includes a doped well in which photocharge is accumulated responsive to exposure to incident illumination. The capacity of the well corresponds to a well potential. WCA structures (e.g., deep trench regions) form walls at least partially surrounding and capacitively coupling with the doped well, such that biasing of the WCA structures changes the well potential and the corresponding well capacity. As such, the WCA structures can be biased during integration to increase the well potential to a high level for large well capacity, and the WCA structures can be differently biased during photocharge transfer to decrease the well potential to a sufficiently low level that avoids lag and/or other conventional concerns.