TFT Channel Orientation on Flexible Substrates
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Solution Overview
Problem
Thin film transistors (TFTs) formed on flexible substrates experience changes in electrical properties due to bending, which affects charge mobility and strain-induced drain current, especially when the substrate bending direction is parallel to the channel direction.
Innovation Solution
The longitudinal direction of the channel region of TFTs is oriented at a predetermined angle, ranging from 45° to 90°, relative to the substrate bending direction, using a flexible substrate made of metal or plastic materials like stainless steel or titanium, with a semiconductor layer of low temperature polysilicon and germanium or its compounds, to minimize electrical property changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the channel region of TFT is oriented parallel to the substrate bending direction, then the device structure is simplified and manufacturing is easier, but the electrical property changes significantly due to strain-induced drain current
Solution Approach 1:
The patent applies asymmetry by deliberately orienting the channel region at a specific angle (45° or more, preferably 90°) relative to the substrate bending direction, rather than using the symmetric parallel alignment. This asymmetric orientation reduces the strain-induced drain current effect while maintaining manufacturability through defined angular relationships in the TFT array layout.
2Reliability
If glass substrate is used for TFT formation, then high manufacturing temperature is achievable and TFT performance is stable, but the substrate is fragile, heavy and difficult to mass-produce in large sizes
Solution Approach 1:
The patent changes the substrate material parameter from traditional glass to flexible substrates (plastic films or thin metal films), and相应地 adjusts the manufacturing process temperature parameter to low-temperature ranges. This enables the use of flexible substrates while maintaining TFT functionality, achieving both flexibility and operational performance through coordinated parameter changes.
3Strength
If film-shaped plastic substrate is used for TFT formation, then the substrate is flexible and lightweight, but the substrate is vulnerable to heat and sensitive to temperature changes requiring low-temperature manufacturing
Solution Approach 1:
The patent changes the substrate material parameter from glass to flexible substrates (plastic films or thin metal films), and相应地 adjusts the manufacturing process temperature parameter to low-temperature ranges. This enables the use of flexible substrates while maintaining TFT functionality, achieving both flexibility and operational performance through coordinated parameter changes.
4Temperature
If thin film-shaped metal substrate is used, then high temperature processes are enabled and moisture/oxygen resistance is improved, but the substrate is less flexible compared to plastic substrates
Solution Approach 1:
The patent applies local quality by using thin metal films with controlled thickness (e.g., aluminum alloy 0.5-5 μm) that provide sufficient thermal and barrier properties in the regions requiring high-temperature processing, while maintaining overall substrate flexibility. The metal film thickness and composition are locally optimized to balance rigidity for manufacturing with flexibility for final application.
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 configuration reduces the strain-induced changes in current flow and maintains high performance and resolution in flexible display devices, even when the substrate is bent, by minimizing the impact of stress on the TFTs.
Implementation Method 1
when stress is applied to the semiconductor layer that forms the TFT, charge mobility and effective charge mass are affected by the stress so that the electrical property of the TFT changes. To be specific, when the direction in which the substrate is bent is parallel to the direction in which the channel of the TFT is formed (that is, the direction in which current flows), a change in a strain-induced drain current increases so that an electrical property of the TFT changes.
Data Source
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AI summary
An organic light emitting display device including a flexible substrate (100) and a plurality of thin film transistors (TFTs) (130,140,150,160) formed on the substrate (100). The plurality of TFTs formed on the substrate include a pixel transistor (150,160) for driving a pixel and a driver circuit transistor (130,140) for driving a driver circuit, and a longitudinal direction of a channel region (L) of the pixel transistor (150,160) makes a first predetermined angle with a direction in which the substrate is bent. As such, it is possible to minimize a change in the electrical property of the TFTs formed on the flexible substrate and to thus reduce a change in the amount of current that flows in the channels of the TFTs.