Global Shutter Image Sensor With Low-Leakage Charge Storage
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Solution Overview
Problem
Global shutter image sensors face issues with leakage current and image distortion due to longer storage times, leading to varying image quality across rows and reduced fill factor caused by large storage transistors, which affects low-light performance.
Innovation Solution
Applying a negative voltage to the storage transistor gate to accumulate holes and reduce surface state leakage, combined with transistor sizing optimizations to increase fill factor and reduce the need for a shutter gate, allowing for simultaneous charge transfer and readout across all pixels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a global shutter is used to capture fast-moving objects, then image capture speed is improved, but leakage current increases due to longer storage times
Solution Approach 1:
The patent applies a negative voltage to the storage transistor gate to accumulate holes and reduce surface state leakage. This parameter change in gate voltage directly addresses the leakage current issue while maintaining the global shutter functionality for fast-moving object capture
2Quantity of substance
If large storage transistors are used to store image charge, then storage capacity is improved, but fill factor decreases due to larger transistor area
Solution Approach 1:
The patent optimizes transistor sizing by eliminating the need for a large shutter transistor through design modifications. This allows adequate charge storage capacity while increasing the fill factor by reducing the area occupied by transistors
3Manufacturing precision
If a conventional shutter transistor is used to fully deplete photodiode charges, then charge transfer completeness is improved, but device area increases reducing fill factor
Solution Approach 1:
The patent eliminates the need for a large shutter transistor through design modifications, optimizing transistor sizing to achieve adequate charge transfer without requiring large device area, thus improving fill factor while maintaining charge transfer completeness
4Device complexity
If rows are read out sequentially, then readout complexity is reduced, but leakage current varies across rows causing image non-uniformity
Solution Approach 1:
The patent applies negative gate voltage to accumulate holes and reduce surface state leakage in storage transistors. This parameter change compensates for the varying leakage across rows during sequential readout, improving image uniformity while maintaining simple readout architecture
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 minimizes leakage current, enhances image uniformity, and increases the fill factor of the image sensor, improving low-light performance and reducing image distortion by enabling efficient global shutter operation with reduced transistor size.
Implementation Method 1
Applying a negative voltage to the storage transistor gate to accumulate holes and reduce surface state leakage
Data Source
AI summary
An image sensor includes a photodiode to accumulate an image charge and a storage transistor to store the image charge. A transfer transistor is coupled between the photodiode and an input of the storage transistor to selectively transfer the image charge from the photodiode to the storage transistor. An output transistor is coupled to an output of the storage transistor to selectively transfer the image charge to a readout node and a reset transistor is coupled to the readout node. A controller is configured to apply a negative voltage to a gate of the storage transistor before activating the gate of the storage transistor to store the image charge.