Multi-Transistor Pixel Circuit for Threshold-Independent Display Current
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Solution Overview
Problem
Display devices, such as liquid crystal and organic light emitting displays, suffer from variations in luminance due to process deviations that cause different threshold voltages in driving transistors, leading to inconsistent light emission from pixels.
Innovation Solution
A pixel design incorporating multiple transistors and capacitors to control current flow independently of the driving transistor's threshold voltage, utilizing a complex scan and emission control system to stabilize voltage levels and improve luminance consistency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a simple pixel structure with a single driving transistor is used, then the device complexity is low, but the luminance consistency deteriorates due to threshold voltage variations
Solution Approach 1:
The pixel circuit is divided into multiple functional blocks: a driving transistor for current control, a compensation transistor for threshold voltage compensation, storage capacitors for voltage holding, and emission control transistors for timing control. This segmentation allows each component to address specific aspects of the luminance consistency problem independently.
Solution Approach 2:
A compensation transistor is introduced as an intermediary element that measures and compensates for the driving transistor's threshold voltage. Additionally, storage capacitors act as intermediaries to hold compensation voltages and isolate the light emitting element from direct threshold voltage variations, thereby improving luminance consistency.
2Manufacturing precision
If multiple transistors and capacitors are added to compensate for threshold voltage variations, then the luminance consistency improves, but the device complexity increases
Solution Approach 1:
The compensation transistor serves multiple functions: it measures the driving transistor's threshold voltage, generates compensation signals, and controls current flow during different operation phases. The storage capacitors simultaneously hold compensation voltages and maintain driving currents, reducing the need for separate dedicated components.
Solution Approach 2:
The pixel circuit merges the driving function, compensation function, and emission control function into a single integrated structure. The same transistors and capacitors are reused across different operation phases (data input, compensation, emission, initialization) rather than having separate dedicated components for each function.
3Manufacturing precision
If threshold voltage compensation is implemented, then the luminance consistency improves, but the operation time increases due to additional control phases
Solution Approach 1:
The pixel operates in periodic cycles that include distinct phases: data input phase, compensation phase, emission phase, and initialization phase. By structuring operations periodically with clear phase boundaries, the circuit efficiently manages compensation tasks without continuous intervention, optimizing the balance between compensation accuracy and operation speed.
Solution Approach 2:
The compensation for threshold voltage is performed in advance during the compensation phase, before the emission phase begins. This preliminary compensation action ensures that the driving transistor is pre-calibrated for accurate current control during light emission, eliminating the need for real-time adjustments and reducing overall operation time.
4Ease of manufacture
If oxide transistors are used in the pixel, then the manufacturing simplicity is maintained, but the stain and afterimage characteristics deteriorate
Solution Approach 1:
The initialization circuit is extracted as a separate functional unit that actively removes accumulated charges from the oxide transistor during dedicated initialization phases. By isolating and addressing the charge accumulation problem through a dedicated initialization mechanism, the patent maintains the ease of manufacturing oxide transistors while mitigating their inherent afterimage issues.
Solution Approach 2:
The pixel circuit periodically discards accumulated charges in the oxide transistor during initialization phases and recovers the transistor's electrical characteristics. This periodic resetting process eliminates stain and afterimage effects by removing unwanted charge accumulations, allowing the oxide transistor to maintain its manufacturing simplicity while improving reliability.
Data Source
AI summary
A display device that includes multiple pixels, each pixel includes a light emitting element, a multiple of transistors, multiple voltage sources and capacitors. The pixel circuit is designed and the pixels are driven so that a driving current going through the light emitting element is independent of a threshold voltage of a driving transistor of the multiple transistors. As a result, luminance variation due to threshold voltage can be minimized, and residual image can be reduced.