Image Sensor Pixel Charge Overflow Circuit for Global Shutter
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Solution Overview
Problem
High Dynamic Range (HDR) CMOS image sensors operating in Global Shutter (GS) mode face challenges in efficiently storing charge without increasing pixel size, leading to thermal noise and compromised low light level resolution due to the need for additional charge storage nodes and sequential scanning.
Innovation Solution
Incorporating a charge overflow circuit with a buried channel MOS or JFET transistor and capacitor/high value resistor to divert excess charge during high light levels, allowing 90% of charge to be stored externally while maintaining 10% in the pixel, enabling efficient global shutter operation without enlarging the pixel size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If additional charge storage nodes are added to each pixel for global shutter operation, then global shutter capability is achieved, but pixel area increases and cost increases
Solution Approach 1:
The patent extracts the charge overflow problem from the pixel by introducing a separate charge overflow path through a dedicated transistor and capacitor external to the pixel. This removes excess charge before it can blooming into adjacent pixels, enabling global shutter operation without requiring larger in-pixel storage nodes.
Solution Approach 2:
The patent introduces an intermediary charge overflow path consisting of a transistor and capacitor that mediates between the photodiode and the readout circuitry. This intermediary structure handles overflow charge externally, allowing the pixel to maintain its compact size while achieving global shutter capability.
2Adaptability or versatility
If charge overflow is allowed during high light levels, then dynamic range is improved, but thermal noise increases and low light sensitivity is compromised
Solution Approach 1:
The patent applies partial action by selectively diverting only the overflow portion of charge during high illumination conditions, while preserving the full charge signal during low light conditions. The charge overflow transistor is activated only when the floating diffusion voltage exceeds a threshold, allowing the pixel to operate in two distinct modes: full-signal mode for low light and overflow-mode for high light.
Solution Approach 2:
The patent introduces dynamic control of the charge overflow path through a voltage-controlled transistor switch. The transistor's conductance state dynamically changes based on the floating diffusion voltage level, automatically routing charge through the overflow path only when necessary during high illumination, thereby adapting the pixel's charge handling behavior to lighting conditions.
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 reduces thermal noise and preserves low light sensitivity by compressing the pixel dynamic range, allowing for high dynamic range imaging without increasing pixel size or sacrificing resolution.
Implementation Method 1
Typical image sensors sense light by converting impinging photons into electrons (or holes) that are integrated (collected) in sensor pixels
Implementation Method 2
However, removing charge from the floating diffusion node using the reset transistor generates thermal kTC-reset noise
Data Source
AI summary
An image sensor may include an array of image sensor pixels. Each pixel may have a photodiode, a charge storage region, and a charge overflow circuit. The charge storage region may be used to operate the image sensor array in global shutter mode. During high light level illumination, the charge overflow circuit may divert charge away from the photodiode such that only a predetermined portion of the accumulated charge remains in the photodiode. During low light level illumination all of the accumulated charge may be stored in the pixel photodiode. The charge overflow circuit may include a transistor and a resistor or capacitor. By implementing a charge overflow circuit, the size of the charge storage region may be reduced while still preserving the high dynamic range and low noise of the image sensor during all light illumination conditions.