NMOS Pixel Circuit for Electroluminescent Displays
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Solution Overview
Problem
Amorphous silicon TFTs have low electron mobility and unstable threshold voltages, limiting their use in active matrix pixels due to low switching speed and instability, while microcrystalline silicon TFTs also face challenges in implementing PMOS transistors, restricting their application in electroluminescent display devices.
Innovation Solution
An active matrix electroluminescent display device is designed with amorphous or microcrystalline silicon NMOS transistors connected between the anode of the display element and a power supply, along with a storage capacitor to hold the gate-source voltage constant, enabling accurate current source operation using NMOS transistors, and a second drive transistor supplies a holding voltage to the anode, allowing for improved carrier mobility and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If amorphous silicon TFTs are used in active matrix pixels, then the device can be manufactured with low cost and simple process, but the switching speed is slow and the threshold voltage is unstable
Solution Approach 1:
The patent changes the material parameter from amorphous silicon to microcrystalline silicon, which fundamentally alters the carrier mobility and threshold voltage stability characteristics while maintaining the TFT manufacturing process compatibility
Solution Approach 2:
The patent uses a composite transistor design combining microcrystalline silicon channel with carefully engineered source/drain regions and gate structures to achieve both high stability and fast switching
2Ease of manufacture
If amorphous silicon TFTs are used, then the manufacturing process is simple, but the switching speed is limited due to low electron mobility
Solution Approach 1:
The patent changes the crystalline structure parameter of the silicon material from amorphous to microcrystalline, which increases carrier mobility by several orders of magnitude while maintaining compatibility with existing TFT fabrication processes
Solution Approach 2:
The patent optimizes the local properties of different regions within the transistor, using microcrystalline silicon specifically in the channel region where high mobility is critical, while maintaining other regions suitable for standard manufacturing
3Speed
If microcrystalline silicon TFTs are used, then the carrier mobility is improved, but PMOS transistors cannot be formed due to material limitations
Solution Approach 1:
The patent inverts the conventional approach by using NMOS transistors instead of PMOS for the electroluminescent display pixel circuit, which is enabled by the microcrystalline silicon material properties and results in improved performance
Solution Approach 2:
The patent changes the circuit design parameters to accommodate NMOS-only implementation, including modified threshold voltage requirements and adjusted biasing schemes that exploit the high mobility of microcrystalline silicon NMOS transistors
4Ease of operation
If current source transistor is used with gate voltage control, then the display function is achieved, but different transistor characteristics cause image artefacts
Solution Approach 1:
The patent implements a feedback mechanism through the microcrystalline silicon transistor's stable threshold voltage characteristic, which naturally compensates for variations in device parameters and maintains uniform current distribution across the display
Solution Approach 2:
The patent changes the transistor material parameter to microcrystalline silicon, which has much lower threshold voltage variation with time and process parameters, thereby eliminating the image artefacts caused by transistor characteristic differences
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
This configuration enables accurate current source operation and improved switching speed by maintaining a constant source voltage, enhancing the performance and stability of the display device, allowing for the integration of driver circuitry on the same substrate as the active plate.
Implementation Method 1
A storage capacitor holds the gate voltage after the addressing phase
Implementation Method 2
Matrix display devices employing electroluminescent, light-emitting, display elements
Data Source
AI summary
An active matrix electroluminescent display device has pixels using an amorphous silicon or microcrystalline silicon drive NMOS transistor (22) connected between the anode of the display element (2) and a power supply line (26). A storage capacitor (24) is connected between the anode of the display element and the gate of the drive transistor (22). An amorphous silicon or microcrystalline silicon second drive NMOS transistor (30) supplies a holding voltage to the anode of the display element (2). This arrangement enables the voltage across the display element to be held while the transistor gate drive voltage is stored on the storage capacitor. This enables an accurate current source pixel circuit to be implemented using NMOS transistors.