Single Crystal Si Thin Film Transistors for LCD Contrast
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Solution Overview
Problem
Conventional single crystal TFTs in LCDs have thick silicon layers, leading to high photosensitivity and current leakage, which result in low display contrast ratios and image retention, especially in ambient light or bright backlighting conditions.
Innovation Solution
The use of thin film layers of essentially single crystal silicon with a thickness between 100 nm and 200 nm in pixel transistors, along with corresponding pixel and common electrodes, reduces photosensitivity and current leakage, enhancing display contrast and performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional single crystal TFTs with thick silicon layers are used, then high electron mobility and high pixel density are achieved, but photosensitivity increases causing current leakage and low display contrast ratio
Solution Approach 1:
The patent applies parameter changes by reducing the silicon layer thickness from conventional thick layers (300 nm or greater) to a thin film layer (100 nm to 200 nm). This parameter change simultaneously maintains high electron mobility inherent to single crystal silicon while reducing photosensitivity and current leakage, thereby resolving the technical contradiction between reliability and photosensitivity.
2Power
If thick silicon layers are used in pixel TFTs, then sufficient current driving capability is achieved, but current leakage increases due to photosensitivity
Solution Approach 1:
The patent changes the thickness parameter of the silicon layer to 100 nm to 200 nm, which reduces the photosensitive volume and thereby reduces current leakage while maintaining adequate current driving capability for the display application.
Solution Approach 2:
The patent applies local quality by creating lightly doped drain (LDD) regions adjacent to the gate electrode. These LDD regions have different doping concentrations than the source and drain regions, providing localized control over current characteristics and reducing hot carrier effects and current leakage in critical areas.
3Ease of manufacture
If amorphous silicon TFTs are used, then large-size LCD panels can be fabricated, but transistor performance is poor due to low electron mobility
Solution Approach 1:
The patent employs composite materials by combining single crystal silicon with insulating layers (such as silicon dioxide or silicon nitride) to form a thin film structure. This composite approach enables the fabrication of large-size panels while maintaining the high electron mobility and superior transistor performance of single crystal silicon.
Solution Approach 2:
The patent changes the material state from amorphous silicon to single crystal silicon in thin film form, fundamentally altering the electronic properties while maintaining compatibility with LCD fabrication processes. This parameter change in crystal structure enables high electron mobility (>100 cm²/Vs) necessary for high-performance displays.
4Reliability
If polycrystalline silicon TFTs are used, then electron mobility is improved compared to amorphous silicon, but crystalline defects reduce performance below single crystal levels
Solution Approach 1:
The patent changes the crystalline structure parameter from polycrystalline to single crystal, eliminating grain boundaries and crystalline defects. This is achieved through specific deposition or recrystallization processes that produce a continuous single crystal structure in thin film form, thereby maximizing electron mobility and eliminating the performance limitations of polycrystalline silicon.
Data Source
AI summary
A liquid crystal display comprises a display panel that includes at least one pixel transistor, at least one pixel electrode in electrical communication with the pixel transistor, at least one common electrode, and a liquid crystal material between the pixel electrode and the common electrode. The pixel transistor includes a thin film layer of essentially single crystal silicon that has a thickness in a range of between about 100 nm and about 200 nm. The pixel electrode has a thickness in a range of between about 5 nm and about 20 nm. The common electrode has a thickness of between about 50 nm and about 200 nm.