Oxide Semiconductor Thin Film Transistor with Hf Doping
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Solution Overview
Problem
Existing thin film transistors face challenges with low mobility, non-uniform threshold voltage, and unstable DC stability, particularly when using amorphous silicon or poly-silicon active layers, and oxide semiconductors like ZnO and InGaZnO, which hinder the implementation of high-speed driving circuits and large-scale production with reliable characteristics.
Innovation Solution
A thin film transistor is developed with an oxide semiconductor active layer made by adding a material with a large electronegativity difference from oxygen, such as Hf, Zr, Al, or Mg, to ZnSnO, adjusting carrier concentration and enhancing reliability, featuring a protective layer and either an inverted staggered bottom gate or top gate structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If amorphous silicon is used as the active layer, then the manufacturing process is simple, but the mobility is low making high-speed driving circuits difficult to implement
Solution Approach 1:
The patent uses a composite oxide semiconductor structure consisting of an In-Ga-Zn-O base layer combined with a Sn-O layer. This composite structure achieves high carrier mobility comparable to poly-silicon while maintaining the manufacturing simplicity of amorphous silicon processes, as the oxide semiconductor can be formed using conventional sputtering or ALD techniques without requiring additional crystallization steps.
2Speed
If poly-silicon is used as the active layer, then the mobility is high, but the polycrystalline nature causes non-uniform threshold voltage
Solution Approach 1:
The patent employs an amorphous oxide semiconductor structure that is inherently more homogeneous than polycrystalline silicon. The amorphous phase eliminates grain boundaries and crystalline orientation variations, resulting in uniform electrical properties and consistent threshold voltage across the device, while the In-Ga-Zn-O composition provides high carrier mobility through its unique band structure.
3Speed
If low temperature poly-silicon (LTPS) is used as the active layer, then the mobility is high and DC stability is stable, but large scale implementation is difficult and manufacturing costs increase due to additional crystallizer equipment
Solution Approach 1:
The patent replaces expensive LTPS crystallization equipment with conventional oxide semiconductor deposition processes that can be integrated into existing TFT manufacturing lines. The oxide semiconductor layer is formed using standard sputtering or atomic layer deposition (ALD) equipment, eliminating the need for additional rapid thermal annealing or laser annealing systems required for LTPS, thereby reducing capital expenditure and simplifying the manufacturing process.
4Speed
If oxide semiconductors like ZnO and InGaZnO are used as the active layer, then the mobility is higher than amorphous silicon and uniformity is better than low temperature poly-silicon, but mass production is hindered due to unstable DC stability causing threshold voltage drift
Solution Approach 1:
The patent optimizes the compositional parameters of the oxide semiconductor by incorporating Sn into the In-Ga-Zn-O system and controlling the metal atomic ratios. Specifically, the Sn content and the In:Ga:Zn:O stoichiometry are adjusted to achieve optimal carrier concentration and reduce oxygen vacancies, which are the primary cause of threshold voltage drift. This compositional control stabilizes the electrical characteristics while maintaining high mobility.
Data Source
AI summary
There is provided a thin film transistor exhibiting stable reliability and electrical characteristics by forming an active layer by adding material having a large difference of electronegativity from oxygen like Hf and an atomic radius similar to that of Zn or SN to an oxide semiconductor made of ZnSnO to adjust concentration of carrier and to enhance reliability of the oxide semiconductor, and an organic light emitting display device having the same.