Oxide Semiconductor Memory Transistor Low Off-State Current
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Oxide semiconductor transistors exhibit low on-off ratios due to high off-state current, primarily caused by excess oxygen and hydrogen impurities, which affect the electric conductivity and stability of the device.
Innovation Solution
A highly purified oxide semiconductor film with reduced hydrogen concentration (less than 5×10^19 cm^-3) is used for the channel formation region, with an energy gap of 2 eV or more to minimize impurities, resulting in a transistor with low off-state current and improved field-effect mobility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If oxide semiconductor is used for transistor channel formation, then field-effect mobility is improved, but off-state current increases leading to low on-off ratio
Solution Approach 1:
The patent applies parameter changes by controlling the carrier concentration of the oxide semiconductor to be less than or equal to 1×10^14 cm^-3, and hydrogen concentration to be less than or equal to 5×10^19 cm^-3. This precise control of material parameters reduces off-state current while maintaining adequate field-effect mobility for transistor operation.
Solution Approach 2:
The patent implements local quality by creating different regions within the oxide semiconductor layer with distinct carrier concentration profiles. The channel formation region maintains lower carrier concentration to reduce off-state current, while source and drain regions have higher carrier concentration to ensure good electrical contact and current flow when the transistor is on.
2Reliability
If oxide semiconductor with electron carrier concentration less than 10^18 cm^-3 is used, then electric conductivity is improved, but on-off ratio remains low at about 10^3
Solution Approach 1:
The patent significantly advances the parameter control by reducing carrier concentration from the conventional <10^18 cm^-3 to <10^14 cm^-3, and hydrogen concentration to <5×10^19 cm^-3. This four-order-of-magnitude reduction in carrier concentration is the key to achieving low off-state current while maintaining reliable electric conductivity through optimized material composition and structure.
3Ease of manufacture
If hydrogen is present in oxide semiconductor during thin film formation, then film formation is facilitated, but O-H bonds form serving as electron donors and increasing off-state current
Solution Approach 1:
The patent applies preliminary action by performing dehydration and dehydrogenation heat treatment on the oxide semiconductor layer after film formation but before transistor operation. This preliminary removal of hydrogen and water prevents the formation of O-H bonds that would act as electron donors, thereby reducing off-state current while maintaining the benefits of oxide semiconductor film formation.
Solution Approach 2:
The patent converts the harmful effect of hydrogen presence during manufacturing into a benefit by deliberately allowing hydrogen incorporation during easy film formation, then subsequently removing it through controlled heat treatment. This two-step approach leverages the ease of hydrogen incorporation during deposition, then eliminates its harmful effects to achieve low off-state current.
Data Source
AI summary
An object is to provide a memory device including a memory element that can be operated without problems by a thin film transistor with a low off-state current. Provided is a memory device in which a memory element including at least one thin film transistor that includes an oxide semiconductor layer is arranged as a matrix. The thin film transistor including an oxide semiconductor layer has a high field effect mobility and low off-state current, and thus can be operated favorably without problems. In addition, the power consumption can be reduced. Such a memory device is particularly effective in the case where the thin film transistor including an oxide semiconductor layer is provided in a pixel of a display device because the memory device and the pixel can be formed over one substrate.