Double Gate Thin-Film Transistor for Threshold Voltage Stability
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Solution Overview
Problem
Thin-film transistors (TFTs) in flat panel displays face challenges with variation in threshold voltage and electrical reliability, particularly in oxide semiconductor TFTs which require bias voltage and suffer from instability over time.
Innovation Solution
A double gate structure is implemented in TFTs, with two gate electrodes formed above and below the semiconductor pattern, and a second gate electrode is included in the same conductive layer as the source and drain electrodes, reducing threshold voltage variation and enhancing electrical reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single gate electrode structure is used in oxide semiconductor TFTs, then the manufacturing process is simpler, but the threshold voltage varies with time and electrical reliability deteriorates
Solution Approach 1:
The single gate electrode is segmented into two separate gate electrodes (first gate electrode and second gate electrode) positioned at opposite sides of the semiconductor pattern. This segmentation allows independent control of threshold voltage and improves electrical reliability by preventing charge accumulation that causes threshold voltage drift over time.
Solution Approach 2:
The gate control is extended from one dimension (single gate above or below) to two dimensions (gates above and below the semiconductor pattern). This dual-gate configuration enables better electrostatic control and stabilizes threshold voltage by balancing electric fields from both sides.
2Reliability
If oxide semiconductor TFTs are used, then high charge mobility is achieved, but bias voltage must be applied to one electrode causing threshold voltage variation
Solution Approach 1:
The gate control function is segmented between two gate electrodes, distributing the voltage control burden and eliminating the need for complex bias voltage application to a single electrode. This segmentation stabilizes threshold voltage by balancing electric fields.
Solution Approach 2:
The dual gate structure creates a more symmetric electric field distribution across the semiconductor channel, achieving better equipotential conditions that stabilize threshold voltage and reduce the need for compensating bias voltages.
3Reliability
If poly-Si TFTs are used instead of a-Si TFTs, then charge mobility increases and deterioration is reduced, but manufacturing process complexity and cost increase
Solution Approach 1:
The invention changes the material parameter from polycrystalline silicon to oxide semiconductor, achieving high charge mobility and stability through material composition rather than through complex crystallization processes. This parameter change maintains reliability benefits while simplifying manufacturing.
Solution Approach 2:
The use of oxide semiconductor materials (such as IGZO - indium gallium zinc oxide) provides a composite material solution that combines the high mobility of polycrystalline silicon with the manufacturing simplicity of amorphous materials, achieving both reliability and ease of manufacture.
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 double gate structure decreases threshold voltage variation, increases charge mobility, and improves the operational current and stability of TFTs, maintaining reliability even with thick organic insulation layers or color filter layers.
Implementation Method 1
a first gate electrode electrically insulated from the semiconductor pattern... a second gate electrode electrically connected to the first gate electrode
Data Source
AI summary
A thin-film transistor includes a semiconductor pattern, a first gate electrode, a source electrode, a drain electrode and a second gate electrode. The semiconductor pattern is formed on a substrate. A first conductive layer has a pattern that includes the first gate electrode which is electrically insulated from the semiconductor pattern. A second conductive layer has a pattern that includes a source electrode electrically connected to the semiconductor pattern, a drain electrode spaced apart from the source electrode, and a second gate electrode electrically connected to the first gate electrode. The second gate electrode is electrically insulated from the semiconductor pattern, the source electrode and the drain electrode.