Oxide TFT Active-Layer Structure for Threshold Stability and Low Leakage
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Solution Overview
Problem
High-mobility oxide thin film transistors face reliability issues when securing a positive threshold voltage, as excess oxygen in the active layer or gate insulating film can degrade the device, leading to leakage current and increased power consumption.
Innovation Solution
A thin film transistor substrate design featuring multiple active layers, including a crystalline oxide semiconductor first active layer and an amorphous oxide semiconductor second active layer, with the second active layer having higher mobility and lower electrical resistance, and a crystalline oxide semiconductor third active layer for improved reliability, allowing for high mobility and reliable operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If excess oxygen is added to the active layer or gate insulating film to shift threshold voltage in the positive direction, then leakage current is reduced, but the active layer becomes easily degraded and reliability decreases
Solution Approach 1:
The active layer is divided into multiple sub-layers with different oxygen concentrations. The first active layer (closer to gate electrode) has higher oxygen concentration to prevent leakage current, while the second active layer (farther from gate electrode) has lower oxygen concentration to maintain high mobility and avoid degradation. This segmentation allows simultaneous optimization of both reliability and mobility characteristics.
Solution Approach 2:
Different regions of the active layer are assigned different oxygen concentrations based on their functional requirements. The region closer to the gate electrode (first active layer) requires higher oxygen to suppress leakage, while the region farther away (second active layer) requires lower oxygen to maintain high carrier mobility. This local differentiation resolves the contradiction between preventing leakage and maintaining reliability.
2Reliability
If oxygen content is increased in the gate insulating film to secure positive threshold voltage, then threshold voltage control is improved, but the thin film transistor reliability deteriorates
Solution Approach 1:
The gate insulating film is segmented into multiple sub-layers with different oxygen concentrations. The first gate insulating film (closer to active layer) has higher oxygen concentration to enable positive threshold voltage control, while the second gate insulating film (farther from active layer) has lower oxygen concentration to prevent degradation and maintain reliability. This resolves the contradiction between threshold voltage control and reliability.
Solution Approach 2:
Different regions of the gate insulating film are assigned different oxygen concentrations based on their functional roles. The region adjacent to the active layer requires higher oxygen for threshold voltage control, while the region farther away requires lower oxygen to avoid degradation. This local quality differentiation enables simultaneous achievement of both goals.
3Speed
If high mobility oxide semiconductor is used in the active layer, then signal delay is reduced, but threshold voltage shifts in negative direction causing leakage current
Solution Approach 1:
The active layer is segmented into multiple sub-layers where the second active layer (with high mobility and low oxygen) handles fast signal transmission, while the first active layer (with lower mobility but high oxygen) stabilizes threshold voltage and prevents leakage. This segmentation allows the system to achieve both high speed and stable threshold voltage.
Solution Approach 2:
The active layer uses a composite structure combining materials with different oxygen concentrations and mobility characteristics. The high-mobility material (low oxygen) provides fast signal transmission, while the lower-mobility material (high oxygen) provides threshold voltage stability. This composite approach resolves the contradiction between speed and reliability.
Data Source
AI summary
A thin film transistor substrate includes a substrate; and a gate electrode and an active layer spaced up and down on the substrate, wherein the active layer includes a first active layer and a second active layer, the first active layer is disposed closer to the gate electrode than the second active layer, a gate insulating film is additionally disposed between the first active layer and the gate electrode, the first active layer and the gate insulating film are in contact with each other, and, the first active layer includes a crystalline oxide semiconductor material, and the second active layer includes an amorphous oxide semiconductor material.