Buried Channel Oxide Semiconductor Transistor Design
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Solution Overview
Problem
Transistors using oxide semiconductor films face issues such as gate-bias stress photodegradation due to silicon diffusion, increased oxygen vacancies leading to variation in transistor characteristics, and challenges in forming minute island-shaped patterns due to etching processes, which affect their reliability and performance.
Innovation Solution
A semiconductor device structure with three oxide semiconductor layers is used, where a first layer with a large bandgap and low electron affinity is in contact with the gate insulating film, a third layer with low oxygen vacancies acts as a protective film, and a second layer with a small bandgap and high electron affinity forms the channel, allowing for a buried channel structure that reduces interface scattering and enhances field-effect mobility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a silicon oxide film is used as a gate insulating film, then good insulation performance is achieved, but silicon diffuses into the oxide semiconductor layer causing trap states and gate-bias stress photodegradation
Solution Approach 1:
A first oxide semiconductor layer with a large bandgap and small electron affinity is introduced as an intermediary layer between the silicon oxide gate insulating film and the channel formation region. This intermediary layer prevents silicon diffusion from the gate insulating film into the channel region, thereby eliminating trap states and preventing gate-bias stress photodegradation while maintaining good insulation performance.
Solution Approach 2:
The oxide semiconductor film is segmented into three distinct layers with different material compositions and properties: a first layer (large bandgap, small electron affinity) for interface protection, a second layer (small bandgap, large electron affinity) for channel formation, and a third layer for protective functions. This segmentation allows each layer to perform its specific function optimally without interfering with others.
2Productivity
If an oxide semiconductor film with small bandgap and large electron affinity is used, then high field-effect mobility is achieved, but oxygen vacancies increase causing threshold voltage shift and normally-on state
Solution Approach 1:
The oxide semiconductor film is divided into three layers where the second layer has small bandgap and large electron affinity for high field-effect mobility, while the first and third layers have different properties to prevent oxygen vacancies and stabilize the threshold voltage. This segmentation allows the channel formation region to achieve high mobility without the reliability issues of oxygen vacancies.
Solution Approach 2:
Different regions of the oxide semiconductor film are assigned different material qualities: the first layer uses material with large bandgap and small electron affinity for interface stability, the second layer uses material with small bandgap and large electron affinity for high mobility channel formation, and the third layer provides protective qualities. Each local region is optimized for its specific function.
3Ease of manufacture
If a single-layer oxide semiconductor film is used, then simple manufacturing is achieved, but it cannot simultaneously satisfy interface protection, high mobility, and oxygen vacancy reduction requirements
Solution Approach 1:
The oxide semiconductor film is segmented into three layers, each with specific material properties tailored for different functions: interface protection, channel formation, and protective stabilization. While this increases structural complexity, it enables simultaneous achievement of interface protection, high mobility, and oxygen vacancy reduction that cannot be achieved with a single layer.
Solution Approach 2:
The oxide semiconductor film uses composite material structure with three different oxide semiconductor layers, each composed of materials with specific bandgap and electron affinity characteristics. This composite structure allows optimization of each layer's properties for its specific function, achieving overall superior transistor performance.
Data Source
AI summary
To provide a semiconductor device which has transistor characteristics with little variation and includes an oxide semiconductor. The semiconductor device includes an insulating film over a conductive film and an oxide semiconductor film over the insulating film. The oxide semiconductor film includes a first oxide semiconductor layer, a second oxide semiconductor layer over the first oxide semiconductor layer, and a third oxide semiconductor layer over the second oxide semiconductor layer. The energy level of a bottom of a conduction band of the second oxide semiconductor layer is lower than those of the first and third oxide semiconductor layers. An end portion of the second oxide semiconductor layer is positioned on an inner side than an end portion of the first oxide semiconductor layer.