Oxide TFTs with Top and Bottom Gate Structures for OLED Displays
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Solution Overview
Problem
Conventional organic light emitting displays using amorphous silicon TFTs face low mobility issues, while poly-silicon TFTs require complex compensation circuits and high fabrication costs, and are challenging to apply to large substrates due to high processing costs and difficult control.
Innovation Solution
The use of oxide TFTs with both top and bottom gate structures, fabricated simultaneously in the same process, allowing for different characteristics in a single pixel, with the first TFT having an inverted staggered top gate structure and the second TFT having an inverted staggered bottom gate structure, using silicon oxide and silicon nitride gate insulation layers respectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If amorphous silicon TFTs are used, then fabrication is simple, but mobility is low
Solution Approach 1:
The patent changes the material parameter from amorphous silicon to oxide semiconductor, which fundamentally alters the electrical properties while maintaining compatibility with low-temperature fabrication processes. This parameter change enables high mobility without requiring complex high-temperature processing.
Solution Approach 2:
The patent employs a composite structure combining oxide semiconductor layers with specific insulation layers (silicon oxide, silicon nitride) and metal electrodes. This composite material approach achieves both high mobility and ease of fabrication by selecting materials that work together in the low-temperature process window.
2Speed
If poly-silicon TFTs are used, then mobility is increased, but threshold voltage distribution is not uniform
Solution Approach 1:
The patent changes the semiconductor material from poly-silicon to oxide semiconductor, which inherently provides more uniform electrical properties. The oxide semiconductor's amorphous structure with controlled stoichiometry eliminates the grain boundary effects that cause threshold voltage variation in poly-silicon.
Solution Approach 2:
The patent uses a standardized oxide semiconductor layer formation process that can be uniformly applied across the entire substrate, ensuring consistent threshold voltage characteristics without requiring device-specific compensation circuits.
3Reliability
If poly-silicon TFTs with compensation circuits are used, then mobility and threshold voltage uniformity are improved, but device complexity increases
Solution Approach 1:
The patent extracts and eliminates the need for compensation circuits by using oxide semiconductor TFTs that inherently provide uniform threshold voltage. This removes the additional TFTs and capacitors required in poly-silicon-based compensation circuits, simplifying the overall device structure.
Solution Approach 2:
The oxide semiconductor material itself provides the uniform electrical characteristics needed for reliable operation, making external compensation mechanisms unnecessary. The material's intrinsic properties serve the function that previously required additional circuitry.
4Speed
If low temperature poly-silicon (LTPS) method is used, then mobility is improved, but applicability to large substrates is reduced
Solution Approach 1:
The patent changes the material system from LTPS to oxide semiconductor, which enables high mobility at lower processing temperatures. This parameter change allows uniform material deposition and processing across large substrate areas without the temperature gradients and control issues that limit LTPS scalability.
Solution Approach 2:
The patent employs low-cost, low-temperature processing techniques that do not require expensive laser annealing equipment or complex temperature control systems. This makes the fabrication process economically viable and technically feasible for large-area substrate production.
Data Source
Figure 1A~1B
Figure 2~3
Figure 4A~4B
AI summary
An organic light emitting display includes data lines and scan lines intersecting each other, a scan driving unit for supplying a scan signal to the scan lines, a data driving unit for supplying a data signal to the data lines, and pixels defined at intersection points of the data and scan lines, each pixel having an organic light emitting diode, a first TFT with an inverted staggered top gate structure and connected to the organic light emitting diode, the first TFT including an oxide semiconductor as an active layer, and a second TFT with an inverted staggered bottom gate structure and configured to receive the scan signal from the scan lines, the second TFT including an oxide semiconductor as an active layer.