LTPS Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
Micro light emitting diode displays face issues such as wavelength shifts and brightness deviations due to uneven threshold voltages between thin film transistors, which can lead to significant image distortion and non-uniform brightness across pixels.
Innovation Solution
A pixel circuit is designed to include specific transistors and capacitors that compensate for threshold voltage variations, using low-temperature polycrystalline silicon (LTPS) and oxide TFTs to maintain consistent brightness and prevent wavelength shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If pulse amplitude modulation method is applied to control gray scale, then brightness control is achieved, but wavelength shifts occur causing image distortion
Solution Approach 1:
The patent replaces the pulse amplitude modulation method (electrical control) with pulse width modulation method (temporal control). By substituting the control mechanism from amplitude-based to time-based modulation, the system achieves brightness control without inducing wavelength shifts, thus resolving the contradiction between brightness control and wavelength consistency.
Solution Approach 2:
The patent changes the control parameter from amplitude (voltage intensity) to time (pulse duration). By modifying the gray scale control from amplitude-based to time-based parameters, the system maintains wavelength consistency while achieving brightness control through different physical parameters.
2Device complexity
If conventional transistor design is used, then device complexity is low, but threshold voltage unevenness causes brightness deviations
Solution Approach 1:
The patent segments the transistor gate into two independent gates (first gate and second gate) that can be controlled separately. This segmentation allows independent adjustment of threshold voltage and drive current, enabling compensation for threshold voltage unevenness while maintaining a relatively simple transistor structure based on conventional TFT architecture.
Solution Approach 2:
The dual-gate transistor structure provides multi-functionality: the first gate controls threshold voltage while the second gate controls drive current. This universal structure can simultaneously perform threshold compensation and current driving functions, resolving the contradiction between device simplicity and threshold voltage uniformity.
3Ease of manufacture
If simple pixel circuit is used, then ease of manufacture is high, but brightness uniformity across pixels deteriorates
Solution Approach 1:
The pixel circuit is segmented into distinct functional blocks: threshold voltage compensation circuit (first transistor and first capacitor), drive circuit (second transistor and second capacitor), and light emission control circuit (third and fourth transistors). This segmentation enables independent optimization of each function while maintaining overall circuit simplicity and manufacturability, achieving both ease of manufacture and brightness uniformity.
Solution Approach 2:
The circuit performs preliminary threshold voltage compensation before data signal writing and preliminary drive current adjustment before light emission. By performing these compensation actions in advance through the first transistor and capacitor, the system ensures brightness uniformity across pixels without complicating the overall manufacturing process.
Data Source
AI summary
A pixel circuit may include a first transistor that provides a data signal to a first node according to a scan signal, a second transistor that initializes the first node, a first capacitor that is coupled between one terminal to which a light emission signal is provided and the first node, a second capacitor that is coupled between the first node and a second node, a third transistor that includes a gate coupled to the second node and one terminal coupled to a third node, a fourth transistor that includes a gate coupled to the second node and one terminal coupled to the third node, a fifth transistor that is coupled between the second node and the third node, a drive transistor that includes a gate to which a voltage corresponding to the voltage at the third node is supplied, and a micro light emitting diode that is coupled to the drive transistor.