Oxide Semiconductor Pixel Circuit for Low-Leakage Global Shutter
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Solution Overview
Problem
Conventional CMOS image sensors with global shutter systems face challenges in suppressing charge leakage, leading to image distortion and high off-state current issues, particularly when capturing fast-moving objects, due to the prolonged charge holding period and high off-state current of transistors.
Innovation Solution
The use of a semiconductor device with a pixel circuit structure that includes transistors with oxide semiconductor channels, specifically designed to minimize off-state current, allowing for simultaneous charge accumulation across all pixels and reducing charge leakage, thereby enabling image capture without distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a global shutter system is adopted in a CMOS sensor to capture fast-moving objects without image distortion, then image quality is improved, but charge leakage increases due to the prolonged charge holding period
Solution Approach 1:
The patent changes the material parameter of the transistor from conventional semiconductor to oxide semiconductor, which fundamentally alters the off-state current characteristics. This material substitution enables the transistor to maintain extremely low leakage current over the extended charge holding period required by global shutter systems, thus resolving the contradiction between image quality and charge leakage.
2Productivity
If the charge holding period is extended to read data sequentially from all pixels in a global shutter system, then complete image capture is achieved, but charge flows out by leakage current and original data is lost
Solution Approach 1:
The patent modifies the electrical parameter of the transistor by using oxide semiconductor material, which exhibits extremely low off-state current. This enables the transistor to hold charge data stable throughout the entire sequential reading process from all pixels, preventing charge data loss even during extended holding periods.
3Device complexity
If conventional transistors are used in pixel circuits, then device complexity is reduced, but off-state current is high causing significant charge leakage
Solution Approach 1:
The patent changes the material composition parameter of the transistor channel from conventional silicon-based semiconductor to oxide semiconductor. This material substitution fundamentally improves the off-state current characteristic without adding structural complexity, maintaining the simple transistor design while achieving superior electrical performance.
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
The implementation of oxide semiconductor transistors in the CMOS image sensor significantly reduces off-state current, effectively suppressing charge leakage and allowing for clear, undistorted images of moving objects by ensuring consistent charge holding across all pixels.
Implementation Method 1
a photodiode 101, a signal charge accumulation portion 112, and a plurality of transistors 103, 104
Data Source
AI summary
In a CMOS image sensor in which a plurality of pixels is arranged in a matrix, a transistor in which a channel formation region includes an oxide semiconductor is used for each of a charge accumulation control transistor and a reset transistor which are in a pixel portion. After a reset operation of the signal charge accumulation portion is performed in all the pixels arranged in the matrix, a charge accumulation operation by the photodiode is performed in all the pixels, and a read operation of a signal from the pixel is performed per row. Accordingly, an image can be taken without a distortion.