ZnO Transistor Multi-Layered Channel for Threshold Control
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Solution Overview
Problem
Conventional oxide transistors with an n-type oxide layer as the channel layer face challenges in controlling the threshold voltage, leading to reduced ON/OFF current ratio and increased subthreshold slope, making it difficult to manufacture enhancement mode transistors with high mobility and appropriate threshold voltage.
Innovation Solution
A transistor with a multi-layered channel structure, where one layer closer to the gate is made of high mobility and high carrier density materials like indium zinc oxide (IZO) or indium tin oxide (ITO), and a second layer farther from the gate is made of gallium indium zinc oxide (GIZO), allowing for independent control of mobility and threshold voltage by adjusting the thickness of each layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the carrier concentration of the n-type oxide layer is reduced to control threshold voltage, then threshold voltage control is improved, but mobility is reduced, ON/OFF current ratio is reduced and subthreshold slope is increased
Solution Approach 1:
The channel layer is divided into two distinct layers: a first oxide semiconductor layer in contact with the gate insulator and a second oxide semiconductor layer above it. This segmentation allows each layer to have different carrier concentrations optimized for their respective functions - the first layer for high mobility and the second layer for threshold voltage control.
Solution Approach 2:
Different regions of the channel structure are assigned different material properties. The first oxide semiconductor layer has higher carrier concentration for high mobility, while the second oxide semiconductor layer has lower carrier concentration for threshold voltage control. This local differentiation resolves the contradiction between mobility and threshold voltage control.
2Productivity
If the carrier concentration of the n-type oxide layer is increased to improve mobility and ON/OFF current ratio, then mobility is improved, but threshold voltage is reduced to negative side making enhancement mode transistors impossible
Solution Approach 1:
The channel is segmented into two layers with different carrier concentrations. The first layer (higher carrier concentration) provides high mobility and ON/OFF current ratio, while the second layer (lower carrier concentration) ensures positive threshold voltage for enhancement mode operation. This resolves the contradiction by distributing different functional requirements to different layers.
Solution Approach 2:
The first oxide semiconductor layer is designed with high carrier concentration for optimal mobility, while the second layer has lower carrier concentration for proper threshold voltage control. This local quality differentiation allows simultaneous achievement of high mobility and positive threshold voltage.
3Device complexity
If a single n-type oxide layer is used as channel layer, then device structure is simple, but it is difficult to simultaneously achieve high mobility, high ON/OFF current ratio, small subthreshold slope, and controlled threshold voltage
Solution Approach 1:
The channel layer is segmented into two oxide semiconductor layers with different carrier concentrations. This segmentation enables independent optimization of mobility (first layer) and threshold voltage control (second layer), achieving superior operational characteristics that cannot be obtained with a single layer.
Solution Approach 2:
The channel structure uses a composite of two oxide semiconductor materials with different electrical properties. The first oxide semiconductor provides high carrier concentration for mobility, while the second oxide semiconductor provides lower carrier concentration for threshold voltage control, creating a composite structure with optimized overall performance.
Data Source
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AI summary
Disclosed are a channel layer and a transistor including the channel layer. The channel layer may include a multi-layered structure. Layers forming the channel layer may have different mobilities and/or carrier densities. The channel layer may have a double layered structure including a lower layer and an upper layer which may be formed of different oxides. Characteristics of the transistor may vary according to materials used to form the lower layer and the upper layer and thickness thereof.