N-Type Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
Organic electro-luminescent display apparatuses face challenges in achieving uniform image quality due to varying threshold voltages of driving transistors, which become more pronounced in larger displays, leading to unstable image brightness and sensitivity to emission voltage and temperature changes when using N-type transistors.
Innovation Solution
A pixel circuit design utilizing N-type transistors with specific configurations, including capacitors and transistors, that allows for sufficient initialization time and threshold voltage compensation, ensuring stable operation and uniform image quality by managing scan signals and power supply voltages to isolate the driving transistor's gate-source voltage from power supply variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If N-type transistors are used in pixel circuits, then device complexity is reduced and manufacturing is simplified, but threshold voltage variations cause unstable image brightness and poor image quality
Solution Approach 1:
The patent applies preliminary action by implementing threshold voltage compensation before the pixel circuit operates normally. A compensation transistor is configured in parallel with the driving transistor, and during an initialization period, the compensation transistor is activated to measure and compensate for threshold voltage variations. This preliminary compensation ensures that subsequent image display maintains stable brightness despite N-type transistor threshold voltage variations.
Solution Approach 2:
The patent implements feedback mechanisms through capacitor coupling between the compensation transistor and driving transistor. The compensation transistor's gate voltage is adjusted based on measured threshold variations and fed back to compensate the driving transistor's gate voltage. This feedback loop continuously corrects for threshold voltage drift, maintaining stable image brightness throughout operation.
2Use of energy by moving object
If N-type transistors are used in pixel circuits, then power consumption is reduced, but insufficient initialization time and threshold voltage compensation time degrade image quality
Solution Approach 1:
The patent applies periodic action by dividing the pixel circuit operation into distinct time periods: an initialization period where compensation transistors are activated to perform threshold voltage compensation, and a normal operation period where image data is displayed. This periodic switching between compensation and display modes ensures sufficient compensation time is allocated without continuously consuming high power, achieving low power consumption while maintaining image quality.
3Reliability
If threshold voltage compensation is implemented, then image brightness stability improves, but device complexity and circuit configuration increase
Solution Approach 1:
The patent applies universality by designing compensation transistors that serve multiple functions: they act as switching elements during initialization, compensation elements during threshold voltage compensation, and can be integrated with the driving transistor structure. The same capacitor structures serve both as storage elements and as coupling elements for feedback signals. This multi-functionality reduces the need for separate dedicated compensation circuits, thereby limiting the increase in device complexity.
Solution Approach 2:
The patent merges the compensation circuit with the driving transistor structure. The compensation transistor is positioned adjacent to and shares structural elements with the driving transistor, such as common source/drain regions and capacitor structures. This merging approach allows threshold voltage compensation to be implemented without adding completely separate compensation circuits, thereby reducing the overall increase in device complexity while maintaining brightness stability.
Data Source
AI summary
A pixel circuit includes a light emitting device, an N-type driving transistor for outputting a driving current according to a voltage applied to the gate electrode, a first capacitor coupled to a gate electrode of the driving transistor, a second capacitor including a first terminal coupled to the gate electrode of the driving transistor and a second terminal coupled to the first electrode of the light emitting device, and second through sixth N-type transistors. An initialization period is reduced for a driving operation and the threshold voltage compensation time of a driving transistor is controlled.