Oxide Semiconductor Amplifier Circuit for Image Sensor Noise Reduction
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Solution Overview
Problem
Existing X-ray image sensors face challenges in achieving lower exposure and higher definition due to signal charge degradation, leading to a decrease in signal-to-noise (S/N) ratio, particularly when using a-Si TFTs for signal amplification, which results in reliability issues and fixed pattern noise (FPN) with polycrystalline Si TFTs.
Innovation Solution
An amplifier circuit using an oxide semiconductor transistor with a controller that maintains the source potential equal to the drain potential during non-output periods, preventing threshold voltage variations and reducing noise, thereby enhancing the S/N ratio without reliability issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a-Si TFT is used for signal amplification, then device complexity is reduced and ease of manufacture is improved, but threshold voltage varies continuously over time leading to reliability degradation
Solution Approach 1:
The patent changes the material parameter from conventional a-Si or polycrystalline Si to oxide semiconductor, which fundamentally alters the electrical characteristics to achieve stable threshold voltage. This material parameter change enables high reliability while maintaining ease of manufacture through thin-film fabrication processes.
Solution Approach 2:
The patent applies equipotentiality by controlling the source potential to be equal to the drain potential during non-output periods. This potential control strategy prevents threshold voltage drift by maintaining electrical equilibrium, thereby ensuring reliable operation over time.
2Measurement precision
If polycrystalline Si TFT is used for signal amplification, then initial threshold voltage is stable, but large threshold voltage variations occur due to crystal grain boundaries causing Fixed Pattern Noise
Solution Approach 1:
The patent changes the semiconductor material from polycrystalline Si to oxide semiconductor, eliminating crystal grain boundaries that cause threshold voltage variations. This material parameter change reduces Fixed Pattern Noise while maintaining stable threshold voltage characteristics.
3Measurement precision
If pixel size is reduced for higher definition, then spatial resolution is improved, but signal charge decreases leading to S/N ratio degradation
Solution Approach 1:
The patent substitutes the conventional charge-output mechanism with a voltage-output mechanism using oxide semiconductor TFTs. This substitution enables signal amplification at the pixel level, allowing small pixel sizes to maintain high S/N ratios by amplifying the voltage signal rather than relying on large charge collections.
4Object-affected harmful factors
If X-ray irradiation amount is reduced for lower exposure, then patient dose is reduced, but signal charge detected decreases resulting in S/N ratio degradation
Solution Approach 1:
The patent replaces the charge-detection mechanism with a voltage-amplification mechanism using oxide semiconductor TFTs. This enables effective signal detection even with reduced X-ray irradiation amounts, allowing lower patient dose while maintaining high S/N ratio through the unique voltage-output characteristics of oxide semiconductor devices.
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 solution effectively increases the S/N ratio of the image sensor by stabilizing the threshold voltage and reducing noise, ensuring reliable operation and improved image quality.
Implementation Method 1
a source terminal of the transistor is controlled to have a same potential as a drain terminal during a period other than a period of outputting the output of the amplifier circuit to the reading wiring
Implementation Method 2
an amplifier circuit outputting current in accordance with voltage applied to an input terminal of the circuit
Data Source
AI summary
There is a problem that in an image sensor including an amplifier in each pixel, when a thin-film semiconductor is used as a transistor constituting the amplifier, voltage continues to be applied between source and gate of the transistor and thereby a threshold voltage value of the transistor varies, resulting in a variation of signal voltage. To solve the problem, a thin-film transistor formed with an oxide semiconductor is used as the transistor constituting the amplifier, and during a period other than a period of outputting an output of the amplifier, source potential of the transistor is controlled to be equal to drain potential thereof.