Oxide Semiconductor TFT Lower Layer for Threshold Stability
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Solution Overview
Problem
In liquid crystal display devices using oxide semiconductor thin film transistors, defects on the surface can lead to holes being excited by light, which are trapped in the gate insulating film, causing a shift in the threshold voltage and resulting in reliability issues and display inhomogeneity.
Innovation Solution
A thin film transistor design that includes a gate electrode, a gate insulating film, an oxide semiconductor layer, a lower semiconductor layer between the oxide semiconductor and the gate insulating film, and source and drain electrodes, where the lower semiconductor layer is present in overlapping regions to prevent photoexcited holes from reaching the gate insulating film.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If oxide semiconductor thin film transistors are used in liquid crystal display devices, then higher mobility and smaller size can be achieved, but defects on the surface can lead to holes being excited by light and trapped in the gate insulating film, causing threshold voltage shift and reliability issues
Solution Approach 1:
A lower semiconductor layer is introduced as an intermediary between the gate insulating film and the oxide semiconductor layer. This lower semiconductor layer acts as a mediator that captures photoexcited holes before they can reach and be trapped in the gate insulating film, thereby preventing threshold voltage shift while allowing the oxide semiconductor to maintain its high mobility characteristics
Solution Approach 2:
The lower semiconductor layer is positioned in advance between the gate insulating film and the oxide semiconductor layer to preemptively capture holes before they can migrate to the gate insulating film. This preliminary protective structure prevents the harmful effect of hole trapping before it occurs, ensuring stable threshold voltage under light irradiation
2Reliability
If the lower semiconductor layer is added to prevent hole trapping, then threshold voltage stability is improved, but device structure becomes more complex
Solution Approach 1:
The lower semiconductor layer is made of the same oxide semiconductor material as the main channel layer, ensuring homogeneous material properties throughout the device. This approach maintains consistent electrical characteristics and simplifies the manufacturing process while still providing the necessary protective function against hole trapping
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 design stabilizes the threshold voltage even under light irradiation, enhancing the reliability of the thin film transistor and reducing display inhomogeneity by preventing hole trapping in the gate insulating film.
Implementation Method 1
defects on the surface can lead to holes being excited by light
Implementation Method 2
holes being excited by light, which are trapped in the gate insulating film
Data Source
AI summary
A TFT in which a channel region is formed of an oxide semiconductor is provided. Threshold voltage shift due to holes photoexcited in the vicinity of a source electrode and a drain electrode is prevented so that reliability is enhanced. A lower semiconductor layer is partially provided between an oxide semiconductor layer and a gate insulating film. The lower semiconductor layer is present in at least one of a source overlapping region where the oxide semiconductor layer overlaps a source electrode and a drain overlapping region where the oxide semiconductor layer overlaps a drain electrode. In contrast, a region where the lower semiconductor layer is absent is provided between the source overlapping region and the drain overlapping region.