Pixel Circuit Segmentation for Leakage Suppression
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Solution Overview
Problem
Display quality in organic electroluminescent (EL) display apparatuses is compromised due to variations in drive transistor properties, leading to issues like black level degradation and white level reduction, which affect the accuracy of color gradation and overall image quality.
Innovation Solution
The display apparatus incorporates a pixel circuit configuration with a first and second write transistor and a counter transistor, along with a counter voltage line, to actively control the voltage at an intermediate node, reducing the source drain voltage of the second write transistor and minimizing off-leakage current, thereby maintaining consistent voltage retention in capacitors and improving display quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional pixel circuit with a single write transistor is used, then the device complexity is low, but off-leakage current flows through the write transistor during non-selection periods, causing voltage retention errors and display quality degradation
Solution Approach 1:
The single write transistor is segmented into two separate write transistors (first write transistor and second write transistor) with different gate control signals. This segmentation allows independent control of each transistor, enabling the first transistor to block leakage current while the second transistor maintains voltage retention, thereby solving the off-leakage current problem without requiring a complete redesign of the pixel circuit architecture
Solution Approach 2:
A counter transistor is introduced as an intermediary component connected to an intermediate node between the first and second write transistors. This counter transistor actively compensates for voltage variations by controlling the voltage at the intermediate node, thereby preventing off-leakage current effects and maintaining accurate voltage retention in the capacitor during non-selection periods
2Reliability
If threshold voltage correction and mobility correction are performed, then drive transistor property variations are corrected, but display quality still degrades under certain display conditions due to residual off-leakage effects
Solution Approach 1:
The circuit design converts the potentially harmful off-leakage current into a beneficial effect by using the first write transistor's off-state leakage as a controlled path that can be compensated by the counter transistor. The intermediate node voltage control mechanism transforms what would be a harmful leakage current into a controllable parameter that can be actively managed to maintain display quality
Data Source
AI summary
A display apparatus includes pixels two-dimensionally arranged. Each of the pixels includes a light-emitting element, a capacitor which retains a voltage of a data signal, a drive transistor which feeds a current according to the voltage of the data signal to the light-emitting element, a first write transistor connected between a data signal line and a gate electrode of the drive transistor, a second write transistor connected between the first write transistor and a gate electrode of the drive transistor, and a counter transistor including a source electrode and a drain electrode, one of the source electrode and the drain electrode being connected between the first write transistor and the second write transistor. The other of the source electrode and the drain electrode of the counter transistor is connected to a counter voltage line which feeds a counter voltage.