OLED Pixel Circuit Threshold Compensation via Transistor Hysteresis
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Solution Overview
Problem
In organic light emitting display devices, manufacturing process errors result in driving transistors with varying threshold voltages, leading to inconsistent current output and light emission, even during full black displays, necessitating additional transistors and control lines, which increases pixel size and reduces pixel density.
Innovation Solution
A pixel circuit comprising a light emitting element, a driving transistor, two switching transistors, and capacitors connected to power supply lines and data lines, with a controller managing these elements to control current flow and voltage levels across frame periods, ensuring consistent luminance and reducing the need for additional control lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional transistors and control lines are added to compensate for threshold voltage variations and control light emission, then display uniformity and reliability are improved, but pixel area increases and pixel density decreases
Solution Approach 1:
The patent combines multiple functions into the driving transistor itself. The driving transistor performs both the primary function of controlling current to the OLED and the secondary function of threshold voltage compensation through its specific configuration with the capacitors. This merging eliminates the need for separate compensation transistors, reducing pixel area while maintaining display uniformity.
Solution Approach 2:
The driving transistor is designed to serve multiple purposes: current control, threshold voltage compensation, and hysteresis effect utilization. The capacitor connected to the gate electrode serves dual purposes of maintaining gate voltage and compensating for threshold variations. This multi-functionality reduces the total number of components needed in the pixel circuit.
2Reliability
If additional transistors and control lines are added to prevent light emission during full black display, then display quality and reliability are improved, but pixel area increases
Solution Approach 1:
The patent utilizes the hysteresis effect of the driving transistor as a feedback mechanism. The specific configuration of the driving transistor with capacitors creates a feedback loop that maintains the transistor in a fully off state during full black display, preventing any light emission. This feedback mechanism eliminates the need for additional active control elements while ensuring display quality.
Solution Approach 2:
The driving transistor circuit is designed to self-regulate and prevent light emission during full black display through its inherent hysteresis characteristics and capacitor configuration. The circuit automatically maintains the appropriate state without requiring additional control transistors or complex control logic, thereby reducing pixel area.
3Manufacturing precision
If more transistors are added to control current flow and voltage levels, then manufacturing precision and display uniformity are improved, but device complexity increases
Solution Approach 1:
The patent achieves threshold voltage compensation by changing the electrical parameters (voltage levels and timing) rather than adding more components. The controller adjusts the voltage applied to the gate electrode and the timing of operations to compensate for manufacturing variations in transistor threshold voltages. This parameter-based approach simplifies the circuit while maintaining manufacturing precision.
Solution Approach 2:
The patent replaces physical hardware additions (more transistors and capacitors) with control-based solutions. The controller uses timing control and voltage level adjustments to achieve the same effect that additional compensation components would provide. This substitution reduces device complexity while maintaining the ability to compensate for manufacturing variations.
4Productivity
If pixel area is reduced to increase pixel density, then resolution is improved, but space for additional control components is reduced
Solution Approach 1:
The patent extracts the threshold voltage compensation function from separate dedicated components and integrates it into the existing driving transistor configuration. By taking out the compensation function and embedding it in the driving transistor with capacitors, the design eliminates the need for separate compensation transistors and control lines, freeing up space for higher pixel density while maintaining the compensation capability.
Data Source
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AI summary
A pixel circuit to be connected to a data line and first and second power supply lines includes a light emitting element connected between the first power supply line and the second power supply line; a driving transistor to control a current flowing from the first power supply line to the second power supply line through the light emitting element according to a voltage of a first node; a first switching element connected between the first node and a second node; a second switching element connected between the second node and a third node; a first capacitor connected between the first power supply line and the first node; and a second capacitor connected between the second node and the data line.