Oxide TFT Logic Circuit for Low Off-State Current
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Solution Overview
Problem
Thin film transistors using oxide semiconductors face issues with low on-off ratios and high off-state currents due to excess oxygen and hydrogen impurities, leading to unstable operation and increased power consumption.
Innovation Solution
A logic circuit with a thin film transistor having a channel formation region made from an oxide semiconductor with reduced hydrogen concentration (5×10^19 atoms/cm^3 or less) and an energy gap of 2 eV or more, minimizing impurities and off-state current, thereby stabilizing the circuit and reducing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If oxide semiconductor is used with conventional impurity levels, then device fabrication is easier, but on-off ratio is low and off-state current is high
Solution Approach 1:
The patent applies parameter changes by strictly controlling the hydrogen concentration in the oxide semiconductor to 5×10^19/cm³ or less and carrier density to 5×10^14/cm³ or less. These quantitative parameter specifications transform the oxide semiconductor from a conventional high-impurity state to a highly purified intrinsic state, achieving an on-off ratio of 10^8 or more while maintaining fabrication feasibility through defined processing conditions.
Solution Approach 2:
The patent employs inert atmosphere techniques by performing film formation and heat treatment in highly purified inert gas environments (nitrogen or rare gases) with controlled purity levels. This creates an inert environment that prevents hydrogen and moisture contamination during manufacturing, enabling the achievement of ultra-low hydrogen concentrations without requiring excessively complex fabrication processes.
2Ease of manufacture
If oxide semiconductor contains hydrogen impurities, then film formation is simpler, but power consumption increases due to high off-state current
Solution Approach 1:
The patent reduces power consumption by changing the hydrogen concentration parameter to 5×10^19/cm³ or less through controlled film formation processes. This parameter change decreases off-state current to 10^-21 A or less, thereby reducing leakage energy loss while maintaining reasonable fabrication simplicity through optimized deposition conditions.
Solution Approach 2:
The patent applies preliminary action by performing heat treatment in an inert atmosphere before final device assembly to pre-remove hydrogen impurities from the oxide semiconductor. This preliminary purification step reduces hydrogen concentration to target levels, preventing subsequent contamination and ensuring low off-state current without requiring complex post-processing.
3Reliability
If oxide semiconductor has high carrier density, then conductivity is better, but circuit operation becomes unstable
Solution Approach 1:
The patent resolves this contradiction by precisely controlling the carrier density parameter to 5×10^14/cm³ or less through strict hydrogen concentration control (5×10^19/cm³ or less). This parameter optimization achieves both high stability (on-off ratio ≥10^8) and adequate conductivity for logic circuit operation, demonstrating that extreme purity rather than high doping is the key to reliable oxide semiconductor device performance.
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
The reduced hydrogen concentration in the oxide semiconductor channel formation region enhances the on-off ratio, stabilizes the logic circuit, and decreases power consumption by minimizing off-state current and leakage of electric charge.
Implementation Method 1
a first heating treatment in a nitrogen atmosphere or a rare gas atmosphere
Implementation Method 2
when there is no electric field, the oxide semiconductor serves as an insulator or a semiconductor which is close to an insulator
Data Source
AI summary
A logic circuit includes a thin film transistor having a channel formation region formed using an oxide semiconductor, and a capacitor having terminals one of which is brought into a floating state by turning off the thin film transistor. The oxide semiconductor has a hydrogen concentration of 5×1019 (atoms/cm3) or less and thus substantially serves as an insulator in a state where an electric field is not generated. Therefore, off-state current of a thin film transistor can be reduced, leading to suppressing the leakage of electric charge stored in a capacitor, through the thin film transistor. Accordingly, a malfunction of the logic circuit can be prevented. Further, the excessive amount of current which flows in the logic circuit can be reduced through the reduction of off-state current of the thin film transistor, resulting in low power consumption of the logic circuit.