CMOS Image Sensor Buried Channel Transfer Gate Dark Current Drain
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Solution Overview
Problem
CMOS image sensors face challenges in achieving global shutter capability with small pixel size, low dark current, and low power supply voltage, as existing solutions consume valuable pixel area and increase cost, and generate significant dark current noise.
Innovation Solution
A novel transfer gate design with two stacked charge channels, where the shallow channel collects and drains dark current, and the underlying bulk channel is used for charge transfer and storage, allowing efficient charge handling without signal degradation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a second pinned photodiode is used for charge storage to implement global shutter, then global shutter capability is achieved, but pixel area increases significantly
Solution Approach 1:
The patent combines the charge transfer gate and charge storage function into a single integrated structure. The transfer gate region simultaneously serves as the charge storage site during the global shutter phase, eliminating the need for a separate second photodiode and reducing pixel area while maintaining global shutter capability
Solution Approach 2:
The transfer gate structure is designed to perform multiple functions: charge transfer from the first photodiode, charge storage during the transfer and readout phase, and dark current drainage. This multi-functional design replaces the need for dedicated separate structures, reducing overall pixel area
2Productivity
If a standard transfer gate is used for charge transfer, then charge transfer function is achieved, but dark current increases significantly
Solution Approach 1:
The patent segments the charge transfer gate into two distinct charge channels: a surface channel for collecting and draining dark current, and a bulk channel for transferring signal charge. This segmentation allows dark current and signal charge to be handled separately, maintaining charge transfer efficiency while eliminating dark current contamination
Solution Approach 2:
The patent introduces an intermediate bulk channel between the surface channel and the floating diffusion node. This bulk channel acts as a mediator that allows signal charge to pass through to the floating diffusion while blocking dark current generated at the silicon-silicon dioxide interface, preventing dark current from reaching the output
3Adaptability or versatility
If additional charge storage sites are added to implement global shutter, then global shutter capability is achieved, but device complexity increases
Solution Approach 1:
The patent merges the charge storage function with the existing transfer gate structure rather than adding a separate storage site. The transfer gate region is configured to hold charge during the global shutter phase, eliminating the need for additional storage structures and reducing circuit complexity
Solution Approach 2:
The transfer gate structure is designed to perform multiple functions: charge transfer, charge storage during global shutter, and dark current drainage. This multi-functional design reduces the number of separate components needed, simplifying the overall pixel circuit
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 CMOS image sensors to operate in both rolling and global shutter modes with low dark current and power consumption, maintaining high pixel performance without increasing pixel size or power requirements.
Implementation Method 1
Typical image sensors sense light by converting impinging photons into electrons that are integrated (collected) in the sensor pixels
Implementation Method 2
A novel transfer gate design with two stacked charge channels, where the shallow channel collects and drains dark current, and the underlying bulk channel is used for charge transfer and storage
Data Source
AI summary
The invention describes a solid-state CMOS image sensor array and in particular describes in detail image sensor array pixels having global and rolling shutter capabilities that are using a dual channel transfer-storage gate for charge transfer from a PD to a TX gate well and from the TX gate well onto a FD. The dual channels are stacked above each other where a shallow charge channel is used to drain surface generated dark current away from the pixel structure, while a buried bulk channel provides for standard charge transfer and storage functions. This feature thus improves the sensor noise performance and prevents signal contamination and various shading effects caused by the dark current buildup during a prolonged charge storage sequence in pixels of image sensor arrays using the global shutter mode of operation. Several embodiment of this concept are described including pixels which utilize shared circuitry, a complete PD reset capability, and an efficient anti-blooming control.