Display Device Diode Connection Transistors Gray Scale Control
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Solution Overview
Problem
Active matrix organic EL display devices face challenges in achieving good gray scale expression due to subthreshold characteristics of drive transistors, where small gate voltage differences result in abrupt current changes, leading to gray scale unevenness and sensitivity to transistor characteristic variations, especially at low luminance.
Innovation Solution
Incorporating a plurality of diode connection transistors connected in series to the source side of the drive transistor, with their sources connected to the back gate of the drive transistor, to adjust the relationship between gate voltage and current in the subthreshold characteristics, thereby reducing the effect of transistor characteristic variations and improving gray scale expression.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If subthreshold characteristics of drive transistor are used to reduce current at low luminance, then power consumption is reduced, but gray scale expression deteriorates due to abrupt current changes with small gate voltage differences
Solution Approach 1:
A capacitor is introduced as an intermediary element connected between the gate and source of the drive transistor. This capacitor mediates the voltage relationship, allowing the gate-source voltage to be maintained at a level that provides gradual current control rather than abrupt subthreshold behavior, thereby improving gray scale expression while maintaining low luminance operation
Solution Approach 2:
The invention changes the electrical parameters by introducing a capacitor that alters the voltage-time relationship at the gate-source junction. This parameter change transforms the abrupt subthreshold current-voltage characteristic into a more gradual response, enabling better gray scale control without increasing power consumption
2Quantity of substance
If subthreshold region is used for low luminance display, then current through organic EL element is reduced, but gray scale unevenness is generated due to characteristic variation of drive transistor
Solution Approach 1:
The capacitor serves as a mediator that decouples the direct relationship between gate voltage and source voltage. By storing and releasing charge, it smooths out voltage variations and reduces the impact of transistor characteristic variations, thereby eliminating gray scale unevenness while maintaining reduced current levels
Solution Approach 2:
The capacitor provides beforehand cushioning by pre-storing charge that compensates for upcoming voltage variations. This cushioning effect protects against the harmful impact of transistor characteristic variations before they can cause gray scale unevenness, ensuring uniform display quality
Data Source
AI summary
A display device includes a display element emitting a light by a current flowing, a drive transistor configured to control the current flowing through the display element, and a plurality of diode connection transistors connected in series to a source side of the drive transistor, and a source of any on the drive transistor or the plurality of diode connection transistors is connected to a back gate of the drive transistor.