Oxide TFT Buffer Layer Structure for Low-Resistance Ohmic Contact
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Solution Overview
Problem
Thin film transistors with oxide semiconductor films face challenges in achieving high-speed operation and reliability due to increased contact resistance and capacitance, particularly in large-area display devices, where signal delay and display unevenness occur due to the direct contact between low-resistance metal electrodes and oxide semiconductor films.
Innovation Solution
An inverted staggered thin film transistor structure is implemented with an oxide semiconductor film containing indium, gallium, and zinc, using a buffer layer with higher carrier concentration than the semiconductor layer to reduce contact resistance and improve ohmic contact, and a titanium film is used for the source and drain electrodes to minimize resistance and hillock formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If low-resistance metal material is used for source and drain electrodes to reduce wiring resistance, then signal delay is reduced, but contact resistance between the metal electrodes and oxide semiconductor film increases due to Schottky junction formation
Solution Approach 1:
An n-type buffer layer is introduced as an intermediary between the low-resistance metal electrode and the oxide semiconductor film. This buffer layer has higher carrier concentration than the semiconductor layer, enabling ohmic contact with the metal while maintaining proper electrical connection to the semiconductor channel, thus resolving the Schottky junction problem
Solution Approach 2:
The carrier concentration parameter is strategically varied: the buffer layer is designed with higher carrier concentration than the semiconductor layer to achieve ohmic contact, while the semiconductor layer maintains lower carrier concentration for proper transistor operation. This parameter differentiation resolves the contact resistance issue
2Device complexity
If direct contact between source and drain electrodes and oxide semiconductor film is formed to simplify structure, then device complexity is reduced, but parasitic capacitance increases which hinders high-speed operation
Solution Approach 1:
The n-type buffer layer serves as a mediator that spatialally separates the metal electrode from the semiconductor channel region. This separation reduces the overlap area between charged regions, thereby decreasing parasitic capacitance while maintaining a relatively simple overall structure
Solution Approach 2:
The buffer layer extends the interaction region in the vertical dimension between electrode and channel, allowing the electrode to contact the buffer layer rather than directly overlapping with the channel region. This dimensional arrangement reduces parasitic capacitance
3Productivity
If oxide semiconductor film is used for channel formation to enable thin film transistor operation, then device integration is improved, but variation in electric properties increases leading to display unevenness
Solution Approach 1:
The carrier concentration parameter is precisely controlled and differentiated: the buffer layer has higher carrier concentration for stable ohmic contact, while the semiconductor layer has lower carrier concentration for proper transistor characteristics. This controlled parameter variation reduces device-to-device variation
Solution Approach 2:
Different regions of the oxide semiconductor structure are assigned different local properties: the buffer layer region has high carrier concentration for contact stability, while the channel region has lower carrier concentration for transistor operation. This local differentiation improves overall device uniformity
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
The solution results in a thin film transistor with reduced contact resistance, small parasitic capacitance, and improved dynamic characteristics, leading to high-speed operation and reliable performance with reduced variation in electric properties, thereby enhancing the reliability and uniformity of display devices.
Implementation Method 1
Each carrier concentration of the first n-type buffer layer and the second n-type buffer layer is higher than that of the semiconductor layer
Data Source
AI summary
An embodiment is to include an inverted staggered (bottom gate structure) thin film transistor in which an oxide semiconductor film containing In, Ga, and Zn is used as a semiconductor layer and a buffer layer is provided between the semiconductor layer and a source and drain electrode layers. The buffer layer having higher carrier concentration than the semiconductor layer is provided intentionally between the source and drain electrode layers and the semiconductor layer, whereby an ohmic contact is formed.