OLED Pixel Circuit Reducing Transistor Count via Gate Capacitance
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Solution Overview
Problem
Conventional OLED display pixel circuits require a large number of transistors and capacitors, limiting pixel size reduction and the number of pixels that can be accommodated in a predetermined screen size, due to the need for multiple TFTs and capacitors with low relative permittivity dielectric layers.
Innovation Solution
A display device and driving method that reduces element counts per pixel by using a configuration with fewer transistors and capacitors, where the gate voltage of the driver transistor is restored to a default state, and the voltage of the capacitor terminals is adjusted to compensate for threshold voltage variations, allowing desired currents to be fed to the electro-optical element regardless of the threshold voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional OLED pixel circuits use multiple TFTs and capacitors with low relative permittivity dielectric layers, then the circuit can provide stable current control, but the pixel size becomes large and the number of pixels that can be accommodated in a predetermined screen size is limited
Solution Approach 1:
The patent combines multiple functions into fewer transistors. Specifically, the driver transistor serves both as a current control element and as a storage element through its gate capacitance, eliminating the need for separate storage capacitors. The switching transistor performs both selection and current control functions, reducing the total transistor count from 4-6 down to 2 transistors per pixel circuit.
Solution Approach 2:
The driver transistor is designed to perform multiple functions: it acts as a current regulator for the OLED, stores gate voltage through its inherent gate capacitance, and controls current flow to the electro-optical element. This multi-functionality eliminates the need for dedicated storage capacitors and reduces the overall circuit complexity.
2Area of moving object
If the number of TFTs and capacitors per pixel is reduced, then pixel size can be decreased and more pixels can be fitted in a given area, but the ability to compensate for threshold voltage variations may be compromised
Solution Approach 1:
The patent implements a feedback mechanism where the capacitor connected to the gate of the driver transistor stores the difference between the data voltage and the threshold voltage. During the write period, the capacitor charges to reflect the threshold voltage variation, and this stored voltage compensates for the threshold variation during the display period, ensuring stable current control despite transistor parameter variations.
Solution Approach 2:
The patent changes the operating parameters of the driver transistor by adjusting the gate voltage through the capacitor. The capacitor voltage is dynamically adjusted during the write period to compensate for threshold voltage variations, effectively changing the gate-source voltage parameter to maintain constant current output despite transistor parameter drift.
3Ease of manufacture
If conventional circuits use capacitors with low relative permittivity dielectric layers, then manufacturing is simplified, but larger capacitor area is required which increases pixel size
Solution Approach 1:
The patent merges the function of separate storage capacitors with the gate capacitance of the driver transistor. By utilizing the inherent gate capacitance of the driver transistor as the storage element, the need for large-area separate capacitors is eliminated, significantly reducing the pixel area required while maintaining the same manufacturing processes.
Solution Approach 2:
The patent transitions from using large-area planar capacitors to utilizing the vertical gate structure of the transistor. The gate capacitance provides the necessary storage function in a three-dimensional configuration rather than a two-dimensional planar capacitor, effectively using the vertical dimension to reduce area requirements.
Data Source
AI summary
A display device in accordance with the present invention changes to Vc the voltage of a terminal of a capacitor C2 the other terminal of which is connected to the gate of a driver TFT Q1. Thus, a desired voltage Vda is fed from a source line Sj to the drain of the driver TFT Q1 so as to adjust the threshold voltage Vth of the driver TFT Q1. The device then changes the voltage of the terminal of the capacitor C2 to Va to render the gate voltage of the driver TFT Q1 Vda-Vth-Vc+Va. A power supply voltage Vp is fed from the source of the driver TFT Q1.