Oxide Semiconductor Channel Crystallinity for Normally-Off Transistors
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Solution Overview
Problem
Transistors using oxide semiconductors face challenges in controlling threshold voltage due to hydrogen impurities, leading to unstable electrical characteristics and difficulty in forming a channel only when a positive voltage is applied, which is essential for proper switching functions in circuits.
Innovation Solution
The transistor is designed with an oxide semiconductor layer having regions with varying crystallinity, where the channel formation region has a higher crystal proportion and lower defect density, reducing hydrogen diffusion and concentration, thereby stabilizing electrical characteristics and improving reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a transistor is formed using an oxide semiconductor layer, then field-effect mobility is improved and the transistor can be applied to large-sized liquid crystal display devices, but the threshold voltage becomes difficult to control due to hydrogen impurities acting as donors
Solution Approach 1:
The patent applies local quality by creating regions with different crystallinity within the oxide semiconductor layer. The channel formation region is made highly crystalline to reduce defect density and hydrogen concentration, while other regions may have different crystalline structures. This localized structural differentiation allows the channel region to exhibit low off-state current and controlled threshold voltage while maintaining high field-effect mobility.
Solution Approach 2:
The patent utilizes parameter changes by controlling the crystalline structure and orientation of the oxide semiconductor layer. Specifically, the c-axis alignment perpendicular to the substrate surface is maintained in the channel formation region, which fundamentally changes the electrical properties by reducing carrier concentration from hydrogen donors while preserving high electron mobility parallel to the c-axis.
2Quantity of substance
If hydrogen impurities are present in the oxide semiconductor, then carrier density increases and a channel forms even without gate voltage, but the threshold voltage shifts negatively and the transistor cannot perform proper switching function
Solution Approach 1:
The patent applies local quality by creating regions with different crystallinity within the oxide semiconductor layer. The channel formation region is made highly crystalline to reduce defect density and hydrogen concentration, while other regions may have different crystalline structures. This localized structural differentiation allows the channel region to exhibit low off-state current and controlled threshold voltage while maintaining high field-effect mobility.
Solution Approach 2:
The patent converts the typically harmful effect of hydrogen impurities into a beneficial outcome. By creating a highly crystalline structure with c-axis alignment in the channel formation region, the patent reduces the negative impact of hydrogen donors, transforming the material properties so that hydrogen concentration is minimized in the critical channel region, enabling proper switching functionality.
3Reliability
If the oxide semiconductor layer has high crystallinity in the channel formation region, then hydrogen diffusion is reduced and threshold voltage control is improved, but the manufacturing process complexity increases
Solution Approach 1:
The patent utilizes parameter changes by controlling the crystalline structure and orientation of the oxide semiconductor layer. Specifically, the c-axis alignment perpendicular to the substrate surface is maintained in the channel formation region, which fundamentally changes the electrical properties by reducing carrier concentration from hydrogen donors while preserving high electron mobility parallel to the c-axis.
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
This approach results in a transistor with reduced off-state current and improved switching characteristics, allowing for a normally-off switching element with low power consumption by minimizing hydrogen concentration in the channel formation region.
Implementation Method 1
reducing hydrogen diffusion and concentration
Data Source
AI summary
A region containing a high proportion of crystal components and a region containing a high proportion of amorphous components are formed separately in one oxide semiconductor film. The region containing a high proportion of crystal components is formed so as to serve as a channel formation region and the other region is formed so as to contain a high proportion of amorphous components. It is preferable that an oxide semiconductor film in which a region containing a high proportion of crystal components and a region containing a high proportion of amorphous components are mixed in a self-aligned manner be formed. To separately form the regions which differ in crystallinity in the oxide semiconductor film, first, an oxide semiconductor film containing a high proportion of crystal components is formed and then process for performing amorphization on part of the oxide semiconductor film is conducted.