Polarity Controller for NMOS Threshold Voltage Drift in OLED Displays
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Solution Overview
Problem
The continuous rise in threshold voltage of NMOS transistors in active matrix organic electro-luminescent display devices leads to reduced current supply to the electro-luminescent elements, resulting in degradation of image quality due to constant positive polarity of the gate-source voltage.
Innovation Solution
Incorporating a polarity controller to apply a voltage with a value between the minimum and maximum data voltage to the source terminal of the second switching element, varying the polarity of the gate-source voltage based on the data voltage applied to the gate terminal, and using pulse voltages or dummy data to periodically change the polarity, thereby preventing continuous threshold voltage rise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional active matrix driving system with NMOS transistors is used, then the display device can achieve high resolution and large screen capability, but the threshold voltage of the NMOS transistors continuously rises due to constant positive polarity of gate-source voltage, leading to reduced current supply and degradation of image quality
Solution Approach 1:
The patent applies periodic polarity inversion to the gate-source voltage of the NMOS transistors. By alternating between positive and negative polarity at regular intervals, the system prevents the continuous threshold voltage rise that occurs with constant positive polarity, thereby maintaining stable current supply to electro-luminescent elements and preserving image quality over time while retaining high resolution capabilities
Solution Approach 2:
The patent dynamically changes the polarity parameter of the gate-source voltage from constantly positive to alternately positive and negative. This parameter change allows the NMOS transistors to operate in different modes that prevent threshold voltage drift, ensuring stable current control and consistent image quality throughout the display device's operation
2Ease of operation
If the gate-source voltage maintains constant positive polarity to ensure proper transistor operation, then the transistor can function correctly, but the threshold voltage continuously rises, reducing current supply to electro-luminescent elements and degrading image quality
Solution Approach 1:
The patent implements periodic polarity inversion where the gate-source voltage alternates between positive and negative polarity. During positive polarity periods, the transistor operates normally for current control; during negative polarity periods, the threshold voltage is reset or stabilized. This periodic action maintains both proper transistor operation and stable image quality by preventing continuous threshold voltage rise
Solution Approach 2:
The patent transforms the static constant positive polarity into a dynamic alternating polarity system. The gate-source voltage becomes time-dependent, switching between positive and negative states, which allows the transistor to adapt its operation mode periodically, preventing threshold voltage drift while maintaining functional operation throughout the display device
3Device complexity
If the threshold voltage is allowed to rise continuously, then the transistor operation simplifies with constant positive polarity, but the current supply to electro-luminescent elements decreases, affecting gray scale representation and power efficiency
Solution Approach 1:
The patent employs periodic polarity inversion to maintain stable threshold voltage levels. By alternating between positive and negative polarity, the system prevents continuous threshold voltage rise, ensuring consistent current supply to electro-luminescent elements. This maintains proper gray scale representation and power efficiency without significantly increasing control complexity, as the polarity switching follows a regular periodic pattern
Solution Approach 2:
The patent implements a feedback mechanism where the polarity of gate-source voltage is adjusted based on the operational state of the NMOS transistors. When threshold voltage rise is detected or anticipated, the system applies negative polarity to counteract the drift, and switches to positive polarity when stable operation is restored. This feedback control maintains optimal current supply and power efficiency while managing transistor operation
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution effectively maintains image quality by preventing the continuous rise in threshold voltage, allowing for optimal current supply to the electro-luminescent elements and improving the display's gray scale representation and power efficiency.
Implementation Method 1
The electro-luminescent display displays a video image by electrically exciting fluorescent material using carriers such as electrons and holes
Data Source
AI summary
An organic electro-luminescent display device has a gate-source voltage that selectively has a positive polarity and a negative polarity so that deterioration of the switching element is prevented. The display device includes an electro-luminescent element that emits light. The device includes a first switching element for switching a data voltage in response to a scan signal, a second switching element for adjusting the amount of the current supplied to the electro-luminescent element, and a polarity controller for applying a voltage having a value between a minimum value and a maximum value of the data voltage to a source terminal of the second switching element to vary a polarity of a gate-source voltage of the second switching element according to the data voltage applied to a gate terminal of the second switching element.