Drive Transistor Channel Length Variation for OLED Uniformity
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Solution Overview
Problem
In active matrix organic electroluminescent display devices, the deterioration of current-voltage characteristics in light emitting devices leads to image persistence and non-uniform screen brightness due to variations in threshold voltage of drive transistors, which existing pixel circuits struggle to correct effectively.
Innovation Solution
The implementation of a pixel circuit design with a drive transistor having a longer channel length than switching transistors, along with a bootstrap operation and threshold voltage correction mechanism, to maintain consistent gate voltage and suppress variations in threshold voltage, ensuring uniform screen brightness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional pixel circuit with standard channel length transistors is used, then the device complexity is low and manufacturing is easier, but the threshold voltage variations cause image persistence and non-uniform screen brightness
Solution Approach 1:
The patent applies local quality by differentiating the channel length parameter specifically for the drive transistor versus switching transistors. The drive transistor is designed with a longer channel length (L_drive > L_switching) to reduce threshold voltage variations, while switching transistors maintain standard dimensions for compactness. This localized parameter optimization resolves the contradiction by improving screen uniformity only where needed without increasing overall device complexity.
Solution Approach 2:
The patent implements parameter changes by modifying the channel length parameter of the drive transistor. By increasing the channel length from the standard value used in switching transistors to a longer value, the threshold voltage variation is reduced, thereby improving screen uniformity and preventing image persistence without requiring fundamental redesign of the pixel circuit architecture.
2Reliability
If the channel length of the drive transistor is increased to reduce threshold voltage variation, then screen uniformity improves, but the transistor area and pixel circuit size increase
Solution Approach 1:
The patent applies local quality by optimizing the channel length parameter specifically for the drive transistor's function of maintaining stable threshold voltage, while keeping switching transistor dimensions compact for efficient pixel packing. This selective parameter optimization achieves threshold voltage stability without proportionally increasing the entire pixel circuit area.
Solution Approach 2:
The patent applies partial action by increasing only the channel length parameter of the drive transistor rather than scaling all transistor dimensions. This targeted modification achieves the necessary threshold voltage stability with minimal impact on overall pixel area, as only the longitudinal dimension of the drive transistor channel is extended.
3Reliability
If existing pixel circuit designs are used, then the manufacturing process is simpler, but they cannot effectively correct threshold voltage variations and image persistence occurs
Solution Approach 1:
The patent implements parameter changes by modifying the channel length of the drive transistor during the manufacturing process. This parameter adjustment is integrated into the existing thin-film transistor fabrication process, requiring only modifications to the photolithography patterning step to accommodate the extended channel length, thereby maintaining ease of manufacture while achieving effective correction of threshold voltage variations.
Data Source
AI summary
An image display device includes: a pixel array part formed of first to fourth scanning lines arranged in rows, signal lines arranged in columns, pixel circuits in a matrix connected to the scanning lines and signal lines, and a plurality of power source lines which supplies first to third potentials necessary for the operations of pixel circuit; a signal part which supplies a video signal to the signal lines; and a scanner part which supplies a control signal to the first to fourth scanning lines, and in turn scans the pixel circuit for every row, wherein the pixel circuits include a sampling transistor, a drive transistor, first to third switching transistors, a pixel capacitance, and a light emitting device, and a channel length of the drive transistor is made longer than a channel length of the switching transistors to suppress fluctuations in threshold voltage.