Organic EL Pixel Drive Circuit Saturation Region Control
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Solution Overview
Problem
Conventional organic EL display technologies face challenges in achieving high luminance and reliability due to the inferior long-term reliability of nMOS transistors, which are prone to deterioration from electron implantation, affecting the luminescence of the organic EL device and leading to reduced power supply voltage application.
Innovation Solution
The use of a transistor switch with a gate voltage controlled on a binary ON/OFF basis, where the operating point falls in a saturation region, and a second transistor switch with a smaller gate voltage amplitude, connected between the light emitting device and the luminance control circuit, helps to avoid deterioration of the pixel drive circuit by controlling the voltage applied to the nMOS transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If nMOS transistors are used in the pixel drive circuit, then the display can achieve high luminance, but the long-term reliability deteriorates due to electron implantation into the gate insulator
Solution Approach 1:
The patent changes the operating parameters of the nMOS transistor by introducing a series connection with a capacitor that maintains the transistor in the saturation region. This parameter change (ensuring Vds > Vgs - Vth) prevents the transistor from entering the triode region where electron implantation occurs, thereby maintaining both high luminance and long-term reliability.
Solution Approach 2:
The patent applies beforehand cushioning by pre-charging the capacitor in series with the nMOS transistor before the transistor operates. This capacitor maintains a voltage that ensures the transistor remains in the saturation region, preventing electron implantation damage before it can occur during normal operation.
2Power
If the power supply voltage is applied to the nMOS transistor, then the light emitting device can operate, but the voltage-induced stress causes deterioration of the pixel drive circuit
Solution Approach 1:
The patent introduces a capacitor as an intermediary element in series with the nMOS transistor. This intermediary maintains the transistor in the saturation region by ensuring the drain-to-source voltage remains sufficiently high, thereby allowing full power supply voltage to be applied while preventing voltage-induced stress and deterioration.
3Ease of operation
If the nMOS transistor operates in the triode region, then the circuit is simpler to control, but electron implantation into the gate insulator occurs causing deterioration
Solution Approach 1:
The patent changes the operating region parameter of the nMOS transistor from triode to saturation by introducing a series capacitor that maintains Vds > Vgs - Vth. This parameter change ensures the transistor operates in the saturation region where electron implantation does not occur, while the control remains simple through the capacitor's voltage maintenance function.
Data Source
AI summary
An image display apparatus capable of effecting displays high in both reliability and luminance. With the image display apparatus, a transistor switch is provided between a light emitting device and a drive transistor, and a voltage value expressed by formula (a gate voltage at an on-time of the transistor switch)—(the threshold voltage Vth of the drive transistor) is rendered smaller than a value of a voltage applied to a common electrode of the light emitting device.