Back-Gated Oxide Semiconductor Transistor for Normally-Off Operation
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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 layer, where the channel formation region is thinner than other regions, and a gate electrode layer with a higher work function is used to control the threshold voltage, allowing for normally-off characteristics by applying a bias voltage to the back gate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If oxide semiconductor is used for transistor channel, then material availability and manufacturing ease are improved, but threshold voltage becomes negative causing normally-on characteristics that are difficult to control
Solution Approach 1:
The patent applies parameter changes by modifying the thickness of the oxide semiconductor layer in the channel formation region to be smaller than in other regions. This thickness parameter change enables the transistor to achieve positive threshold voltage and normally-off characteristics while maintaining the ease of manufacturing oxide semiconductor transistors.
Solution Approach 2:
The patent implements local quality by creating a non-uniform thickness distribution in the oxide semiconductor layer, where the channel formation region has a different (smaller) thickness compared to regions near the source and drain electrodes. This local variation in thickness allows different regions to serve different functions: the thin channel region enables normally-off operation while other regions maintain good electrical contact.
2Reliability
If threshold voltage is made positive to achieve normally-off characteristics, then controlability is improved, but donor density and oxygen vacancies must be precisely controlled which increases manufacturing difficulty
Solution Approach 1:
The patent uses parameter changes by adjusting the thickness of the oxide semiconductor layer as a key control parameter. By making the channel formation region thinner than other regions, the transistor achieves positive threshold voltage and normally-off characteristics. This physical parameter change provides a straightforward manufacturing approach that does not require extremely precise control of donor density or oxygen vacancies.
3Reliability
If channel formation region thickness is reduced to achieve normally-off characteristics, then threshold voltage control is improved, but other regions may have insufficient thickness for good electrical contact
Solution Approach 1:
The patent applies local quality by creating a non-uniform thickness distribution in the oxide semiconductor layer. The channel formation region has a smaller thickness to enable normally-off operation and positive threshold voltage, while other regions (near source and drain electrodes) maintain sufficient thickness to ensure good electrical contact. This spatial variation in thickness optimizes both electrical performance and contact quality.
Solution Approach 2:
The patent segments the oxide semiconductor layer into regions with different thicknesses: a thin channel formation region for normally-off operation and thicker regions near the electrodes for good electrical contact. This segmentation allows each region to be optimized for its specific function without compromising the overall device performance.
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.