Thin Film Transistor Bias Electrode Threshold Stability
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Solution Overview
Problem
Existing thin film transistors (TFTs) used in active matrix systems for displays and radiation image pickup devices face challenges such as instability in threshold value drive, high carrier concentration leading to normally-on operation, and increased power consumption due to complex manufacturing processes and insulator effects on back-channel electrification.
Innovation Solution
A thin film transistor design with a single gate electrode and a bias electrode independently fixed at earth electric potential, using an In-Ga-Zn-O-based amorphous oxide semiconductor and an amorphous gallium oxide interlayer insulating film, which reduces threshold value variation and power consumption while enabling stable normally-off operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the carrier concentration in the active layer is increased to suppress threshold value variation, then the threshold value stability is improved, but the TFT exhibits normally-on operation which increases power consumption
Solution Approach 1:
The invention changes the electrical parameters of the active layer by precisely controlling the carrier concentration to be 1×10^17 to 1×10^18 cm^-3, and adjusts the insulating film thickness ratio (tg/ts) to 0.01 to 10. These parameter changes enable the TFT to achieve normally-off operation with stable threshold voltage and low power consumption, resolving the contradiction between threshold stability and power consumption.
2Speed
If a double-gate structure is used to increase electron field-effect mobility, then the mobility is improved, but the manufacturing process becomes complex and cost increases
Solution Approach 1:
The invention extracts the essential function of the double-gate structure (controlling carrier concentration and mobility) and implements it through a simplified single-gate configuration combined with specific active layer material composition (In-Ga-Zn-O) and insulating film thickness ratio control. This eliminates the need for complex dual gate electrode fabrication while achieving comparable or superior performance.
3Strength
If an insulator is placed on the back-channel side to protect the TFT, then the structural integrity is improved, but static charge electrification varies the threshold value
Solution Approach 1:
The invention changes the electrical properties of the insulating film by precisely controlling its thickness ratio relative to the active layer thickness (tg/ts = 0.01 to 10). This parameter control prevents static charge accumulation and electrification effects while maintaining the protective function, thus resolving the contradiction between structural integrity and threshold stability.
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 solution achieves stable normally-off operation with reduced power consumption and suppressed threshold value shifts, enhancing the reliability and efficiency of TFTs in displays and radiation image pickup devices.
Implementation Method 1
a second interlayer insulating film that separates the bias electrode from the source and drain electrodes and the active layer, wherein the second interlayer insulating film is an amorphous gallium oxide film
Implementation Method 2
a gate electrode that controls current flowing between the source and drain electrodes via the active layer
Implementation Method 3
an active layer that contacts the source and drain electrodes and contains an oxide semiconductor
Data Source
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AI summary
A thin film transistor including: source and drain electrodes, an active layer that contacts the source and drain electrodes and contains an oxide semiconductor, a gate electrode that controls current flowing between the source and drain electrodes via the active layer, a first insulating film that separates the gate electrode from the source and drain electrodes and the active layer, a bias electrode that is arranged at the opposite side of the active layer from the gate electrode, and has an electric potential fixed independently from the gate electrode, and a second insulating film that separates the bias electrode from the source and drain electrodes and the active layer.