Oxide Semiconductor Pixel Transistors for Low-Capacitance LCDs
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Solution Overview
Problem
Existing liquid crystal display devices face issues with parasitic capacitance between signal lines and other wirings, limiting high-speed operation and leading to residual images, especially at higher resolutions.
Innovation Solution
Utilizing a transistor with an oxide semiconductor layer that is highly purified to reduce impurities, thereby minimizing leakage current and eliminating the need for a capacitor in each pixel, reducing parasitic capacitance and allowing for higher driving frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional transistor is used in each pixel, then the pixel can control the data signal, but parasitic capacitance between signal lines and other wirings increases, limiting high-speed operation and causing residual images
Solution Approach 1:
The patent changes the material parameter of the transistor by using an oxide semiconductor layer instead of conventional semiconductor materials. This parameter change reduces the off-state current to extremely low levels (10^-21 to 10^-24 A/μm), which eliminates the need for capacitor wiring and reduces parasitic capacitance, thereby enabling high-speed operation and eliminating residual images while maintaining display quality
Solution Approach 2:
The patent extracts and eliminates the capacitor component from the pixel structure by using the oxide semiconductor transistor's inherent low leakage current characteristics. The capacitor wiring that normally intersects with signal lines is removed, taking out the source of parasitic capacitance while still maintaining the ability to hold voltage in the pixel
2Reliability
If a capacitor is included in each pixel to hold voltage, then the voltage applied to the liquid crystal element can be maintained, but the aperture ratio is reduced due to the area occupied by the capacitor and its wiring
Solution Approach 1:
The patent extracts and removes the capacitor component from the pixel structure by relying on the oxide semiconductor transistor's extremely low off-state current to hold voltage. This eliminates the capacitor wiring and capacitor area, increasing the aperture ratio while maintaining voltage holding capability through the transistor's inherent charge retention
Solution Approach 2:
The oxide semiconductor transistor performs multiple functions: it controls the data signal during the write period and simultaneously holds the voltage during the display period due to its extremely low leakage current. This multi-functionality eliminates the need for a separate capacitor while maintaining voltage holding capability
3Productivity
If the signal line operates at high speed to enable double-frame rate driving, then residual images can be reduced, but parasitic capacitance between the signal line and capacitor wiring increases, preventing high-speed operation
Solution Approach 1:
The patent extracts and removes the capacitor wiring that creates parasitic capacitance with the signal line. By eliminating this wiring through the use of oxide semiconductor transistors, the signal line can operate at high speeds for double-frame rate or higher refresh rates without the limiting parasitic capacitance effect
Solution Approach 2:
The patent changes the transistor material parameter to oxide semiconductor, which reduces off-state current to 10^-21 to 10^-24 A/μm. This parameter change enables the system to operate at higher refresh rates by eliminating the parasitic capacitance bottleneck while maintaining the ability to hold voltage for display
Data Source
AI summary
An object is to reduce parasitic capacitance of a signal line included in a liquid crystal display device. A transistor including an oxide semiconductor layer is used as a transistor provided in each pixel. Note that the oxide semiconductor layer is an oxide semiconductor layer which is highly purified by thoroughly removing impurities (hydrogen, water, or the like) which become electron suppliers (donors). Thus, the amount of leakage current (off-state current) can be reduced when the transistor is off. Therefore, a voltage applied to a liquid crystal element can be held without providing a capacitor in each pixel. In addition, a capacitor wiring extending to a pixel portion of the liquid crystal display device can be eliminated. Therefore, parasitic capacitance in a region where the signal line and the capacitor wiring intersect with each other can be eliminated.