Oxide Semiconductor TFT Light Shielding for Threshold Voltage Stability
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Solution Overview
Problem
The active matrix substrate experiences a positive shift in threshold voltage during operation, leading to undesirable TFT characteristics and reduced reliability, particularly in oxide semiconductor TFTs used for pixel and circuit applications.
Innovation Solution
The implementation of a light shielding structure in oxide semiconductor TFTs, where a light shielding layer is strategically placed between the oxide semiconductor layer and the substrate, either overlapping the entire channel region or only a portion of it, to intentionally induce a negative shift in threshold voltage through light degradation, thereby offsetting the positive shift and maintaining TFT reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a light shielding layer is disposed between the oxide semiconductor layer and the substrate to suppress light exposure, then the positive shift of threshold voltage is suppressed, but the manufacturing process complexity increases
Solution Approach 1:
The patent applies light shielding selectively only to circuit TFTs in the non-display region, while pixel TFTs in the display region maintain their conventional structure without additional light shielding layers. This local application approach suppresses threshold voltage shifts in circuit TFTs without unnecessarily complicating the overall device structure.
Solution Approach 2:
The light shielding layer serves multiple functions: it suppresses light-induced threshold voltage shifts in circuit TFTs, and can be integrated with existing pixel electrode structures in the display region. This multi-functional approach reduces the need for separate components.
2Manufacturing precision
If the same oxide semiconductor film is used for both pixel TFTs and circuit TFTs to simplify the manufacturing process, then manufacturing precision is improved, but circuit TFTs are more susceptible to threshold voltage positive shift
Solution Approach 1:
The patent maintains uniform oxide semiconductor film formation across the entire substrate for both pixel and circuit TFTs, ensuring consistent material properties and simplified manufacturing. However, it introduces light shielding specifically in the non-display region where circuit TFTs are located, creating a local differentiation that protects circuit TFTs from light-induced threshold voltage shifts while leaving pixel TFTs unchanged.
3Area of stationary object
If the non-display region is narrowed to reduce device size, then the area of the display region is increased, but the space for forming drive circuits and SSD circuits is reduced
Solution Approach 1:
The patent changes the operational parameters of circuit TFTs by introducing light shielding, which induces a negative threshold voltage shift. This parameter change compensates for the inherent positive threshold voltage shift in circuit TFTs, enabling stable circuit operation even in the narrowed non-display region where circuit integration density is increased.
4Speed
If oxide semiconductor TFTs are used instead of amorphous silicon TFTs to achieve higher mobility, then the operating speed is improved, but the threshold voltage becomes more sensitive to light exposure
Solution Approach 1:
The patent converts the harmful effect of light exposure on oxide semiconductor TFTs into a beneficial effect. By intentionally exposing circuit TFTs to light through strategic light shielding design, a controlled negative threshold voltage shift is induced that offsets the natural positive threshold voltage shift during operation. This transforms light degradation, which is typically harmful, into a mechanism that stabilizes threshold voltage and improves long-term reliability.
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 approach effectively suppresses the positive shift of the threshold voltage, ensuring high off-characteristics for pixel TFTs and improving the reliability of circuit TFTs by adjusting the light shielding ratio to manage the amount of light exposure, thus maintaining stable TFT performance.
Implementation Method 1
a light shielding layer is disposed between the oxide semiconductor layer and the substrate
Data Source
AI summary
An active matrix substrate includes a plurality of source bus lines and a plurality of gate bus lines and a plurality of oxide semiconductor TFTs that have a plurality of pixel TFTs, each of which is associated with one of the plurality of pixel regions, and a plurality of circuit TFTs constituting a peripheral circuit, in which each of oxide semiconductor TFTs has an oxide semiconductor layer and a gate electrode disposed on a channel region of the oxide semiconductor layer via a gate insulating layer, the plurality of oxide semiconductor TFTs have a plurality of first TFTs, a plurality of second TFTs, and/or a plurality of third TFTs, and the plurality of first TFTs have the plurality of pixel TFTs, and the plurality of second TFTs and/or the plurality of third TFTs have at least a portion of the plurality of circuit TFTs.