Oxide TFT Stack With Back-Gate Threshold Voltage Control
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Solution Overview
Problem
Transistors with oxide semiconductors often have negative threshold voltages, leading to normally-on characteristics, which are unsuitable for integrated circuits as they are difficult to control and not suited for semiconductor devices.
Innovation Solution
A transistor structure with oxide semiconductor stacked layers and a back gate electrode to control the threshold voltage, where the channel formation region is thinner than other regions, and the gate electrode layers have a higher work function, allowing for a normally-off switching element by adjusting the applied potential.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If oxide semiconductor stacked layers are used to form a transistor channel, then the transistor can achieve high electron mobility and low off-state current, but the threshold voltage becomes negative resulting in normally-on characteristics that are difficult to control
Solution Approach 1:
The oxide semiconductor layer is divided into multiple stacked layers with different thicknesses. The first oxide semiconductor layer has a first thickness and the second oxide semiconductor layer has a second thickness different from the first. This segmentation allows different regions to contribute differently to the channel formation, enabling threshold voltage control while maintaining the benefits of oxide semiconductor materials.
Solution Approach 2:
Different regions of the oxide semiconductor stacked layers are given different local properties through varying thickness. The thinner region provides better threshold voltage control characteristics while the thicker region maintains high electron mobility. This local quality variation resolves the contradiction between controllability and performance.
2Power
If the oxide semiconductor layer thickness is increased to improve electron mobility, then the on-state current increases, but the threshold voltage shifts further into negative values making the transistor normally-on
Solution Approach 1:
The oxide semiconductor layer is segmented into stacked layers of different thicknesses. By controlling the thickness distribution across layers, the patent achieves high on-state current through the thicker layers while the overall structure maintains positive threshold voltage through the combined effect of varying thicknesses.
Solution Approach 2:
The patent uses a composite structure of multiple oxide semiconductor layers with different thicknesses rather than a single uniform layer. This composite approach allows optimization of both electron mobility (through thicker regions) and threshold voltage control (through thinner regions), resolving the contradiction between power and manufacturing precision.
Data Source
AI summary
A transistor includes oxide semiconductor stacked layers between a first gate electrode layer and a second gate electrode layer through an insulating layer interposed between the first gate electrode layer and the oxide semiconductor stacked layers and an insulating layer interposed between the second gate electrode layer and the oxide semiconductor stacked layers. The thickness of a channel formation region is smaller than the other regions in the oxide semiconductor stacked layers. Further in this transistor, one of the gate electrode layers is provided as what is called a back gate for controlling the threshold voltage. Controlling the potential applied to the back gate enables control of the threshold voltage of the transistor, which makes it easy to maintain the normally-off characteristics of the transistor.