Oxide Semiconductor Pixel Circuit for Image Sensor Data Retention
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Solution Overview
Problem
Image sensors face challenges in maintaining data for one frame period at high frame rates due to high leakage currents in silicon semiconductor transistors, which require large capacitors, compromising resolution and aperture ratio, and current mirror circuits increase pixel complexity without enhancing resolution.
Innovation Solution
A semiconductor device with a photosensor in each pixel using a transistor with a small leakage current, such as one formed in an oxide semiconductor layer, to store electric charge for one frame period, and a configuration that includes photodiodes, capacitors, and transistors to perform difference operations without a current mirror circuit, allowing for a reduced number of elements in the pixel circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If a transistor with silicon semiconductor layer is used as switch element, then data can be held for about one frame period at most, but data rewriting is required every frame
Solution Approach 1:
The patent changes the material parameter of the transistor channel from silicon semiconductor to oxide semiconductor. This material substitution fundamentally alters the leakage current characteristics, enabling data to be held for one frame period or longer without rewriting, thereby resolving the contradiction between data holding duration and retention reliability.
2Duration of action of moving object
If capacitor area is enlarged to increase capacitance, then data can be held for one frame period, but resolution increases and aperture ratio decreases
Solution Approach 1:
By changing the transistor material parameter from silicon to oxide semiconductor, the leakage current is reduced to extremely low levels. This enables the use of smaller capacitance values in the capacitor, allowing data to be held for one frame period without requiring large capacitor area, thus maintaining both high resolution and aperture ratio.
3Adaptability or versatility
If current mirror circuit is provided in each pixel to conduct difference operation, then difference operation is achieved, but number of elements increases and resolution decreases
Solution Approach 1:
The patent extracts and removes the current mirror circuit from the pixel circuit configuration. Instead of using a current mirror circuit for difference operation, the invention employs a simplified approach using the oxide semiconductor transistor's low leakage current特性 combined with capacitor-based data storage, achieving difference operation without the complexity of current mirror circuits.
Solution Approach 2:
The patent uses capacitor-based data storage to copy and hold previous frame data, replacing the need for current mirror circuits. The capacitor stores the electrical signal representing previous frame information, enabling difference operation through simpler circuitry and reducing pixel element count while maintaining functionality.
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 image sensors to maintain data for extended frame periods with reduced leakage current, achieving a wide dynamic range and fewer elements in the pixel circuit, thus improving resolution and aperture ratio.
Implementation Method 1
Each pixel accumulates electric charge in a data holding portion for one frame period or longer, and an output of the photosensor changes in accordance with the electric charge accumulated in the data holding portion
Data Source
AI summary
An image sensor is provided which is capable of holding data for one frame period or longer and conducting a difference operation with a small number of elements. A photosensor is provided in each of a plurality of pixels arranged in a matrix, each pixel accumulates electric charge in a data holding portion for one frame period or longer, and an output of the photosensor changes in accordance with the electric charge accumulated in the data holding portion. As a writing switch element for the data holding portion, a transistor with small leakage current (sufficiently smaller than 1×10−14 A) is used. As an example of the transistor with small leakage current, there is a transistor having a channel formed in an oxide semiconductor layer.