Dual Gate Charge Transfer Transistors for Adjustable Blooming Control
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Solution Overview
Problem
CMOS image sensor pixels face challenges in simultaneously reducing dark current, providing efficient blooming control, and ensuring complete charge transfer, often requiring trade-offs that compromise pixel performance, and the blooming performance is fixed once the pixel is designed, limiting charge well capacity for anti-blooming operations.
Innovation Solution
The introduction of a dual gate charge transferring transistor structure with a first gate adjacent to the photodiode and a second gate adjacent to the floating diffusion node, allowing for modulation of the punch-through potential and dynamic range compression by adjusting the bias of the second gate during charge integration and readout, respectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single gate charge transferring transistor is used, then the pixel circuit is simpler and smaller, but the blooming control is fixed and cannot be adjusted, sacrificing charge well capacity
Solution Approach 1:
The charge transferring transistor is divided into two separate transistors: a first charge transferring transistor for transferring charge from the photodiode to the floating diffusion, and a second charge transferring transistor for controlling blooming by transferring charge from the photodiode to the substrate. This segmentation allows independent control of charge transfer and blooming suppression functions.
Solution Approach 2:
The first charge transferring transistor serves multiple functions: it transfers signal charge during normal operation and also suppresses blooming when activated. The dual-transistor architecture provides universal functionality for both charge transfer and blooming control in a compact pixel design.
2Ease of manufacture
If the transfer gate length and doping levels are fixed during manufacturing, then the pixel design is simpler, but the blooming performance becomes fixed and charge well capacity is reduced
Solution Approach 1:
The pixel design transitions from static, fixed blooming control to dynamic, adjustable blooming control through the second charge transferring transistor. This transistor can be activated or deactivated based on lighting conditions, allowing the pixel to adapt its blooming suppression level dynamically rather than being fixed at manufacturing.
Solution Approach 2:
The blooming control mechanism changes from fixed physical parameters (gate length and doping levels) to controllable electrical parameters (transistor activation state). This allows the blooming performance to be adjusted by changing the operational state of the second charge transferring transistor rather than requiring physical modifications to the pixel structure.
3Quantity of substance
If charge is accumulated in the pixel to increase charge well capacity, then the pixel can handle overexposure better, but dark current increases and blooming control becomes less efficient
Solution Approach 1:
The harmful effect of dark current accumulation is extracted and removed by providing a direct path from the photodiode to the substrate through the second charge transferring transistor. This allows dark current charges to be drained away before they can accumulate and increase the dark current level, while signal charges can still be accumulated in the floating diffusion.
Solution Approach 2:
The second charge transferring transistor provides preliminary anti-action against blooming and dark current accumulation by being ready to drain excess charges before they can cause harmful effects. The transistor can be activated in advance when overexposure is detected to prevent charge overflow into neighboring pixels.
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 enables flexible blooming control and dynamic range compression, improving pixel performance by reducing dark current and increasing charge well capacity, while allowing for adjustable blooming performance without sacrificing charge transfer efficiency.
Implementation Method 1
Typical image sensors sense light by converting impinging photons into electrons (or holes) that are integrated (collected) in sensor pixels
Data Source
AI summary
An image sensor may include an array of image sensor pixels. Each pixel may have a photodiode, a floating diffusion node, and a charge transferring transistor. The charge transferring transistor may be a dual gate transistor having first and second gate terminals. A suitable bias may be applied to the second gate terminal to alter the capacitance of the floating diffusion node. The amount of electrons that may be accommodated by the floating diffusion node may be altered with application of a varying voltage level bias at the second gate terminal. By implementing a dual gate transistor, dynamic range compression and anti-blooming charge overflow may be implemented directly in the pixel to reduce image sensor pixel size and cost.