OLED Display Pixels With Silicon Buffer Transistor for Leakage Control
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Solution Overview
Problem
Organic light-emitting diode (OLED) displays experience luminance non-uniformity, luminance drop, and color shifts over time due to the reliability issues of semiconducting-oxide switching transistors, particularly the drifting threshold voltage affecting the current flowing through the light-emitting diodes.
Innovation Solution
Incorporating a silicon thin-film transistor between the semiconducting-oxide transistor and the gate terminal of the drive transistor, along with a matching capacitor to reduce the sensitivity of the emission current to the threshold voltage, and using a pulse width modulation scheme to adjust the luminance compensation, such as increasing the duty cycle every 100-1000 hours to maintain luminance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a semiconducting-oxide switching transistor is used to control the gate terminal of the drive transistor, then leakage at the gate terminal is reduced, but the threshold voltage drifts over time causing luminance non-uniformity and color shifts
Solution Approach 1:
An intermediate transistor (second transistor) is introduced between the semiconducting-oxide switching transistor and the gate terminal of the drive transistor. This intermediate transistor acts as a buffer that isolates the gate terminal from the threshold voltage drift of the semiconducting-oxide transistor, while the semiconducting-oxide transistor continues to provide low leakage control.
2Device complexity
If the threshold voltage of the semiconducting-oxide transistor is allowed to drift, then device complexity is reduced, but luminance uniformity and color stability deteriorate
Solution Approach 1:
The intermediate transistor serves as a mediator that allows the semiconducting-oxide transistor to maintain its simple low-leakage switching function while protecting the drive transistor gate from threshold voltage variations, thereby maintaining luminance uniformity without significantly increasing device complexity.
Solution Approach 2:
The switching control function is segmented into two separate transistors: the semiconducting-oxide transistor handles the switching control with low leakage, while the intermediate transistor handles the isolation and protection of the gate terminal from threshold voltage drift. This segmentation allows each transistor to specialize in one function.
3Reliability
If a silicon thin-film transistor is added between the semiconducting-oxide transistor and the gate terminal, then sensitivity of emission current to threshold voltage is reduced, but device complexity increases
Solution Approach 1:
The silicon thin-film transistor acts as an intermediary element that reduces the sensitivity of the emission current to the threshold voltage of the semiconducting-oxide transistor. This intermediate stage buffers the variations, ensuring more stable emission current while maintaining a manageable device structure.
Data Source
AI summary
A display may have an array of organic light-emitting diode display pixels operating at a low refresh rate. Each display pixel may include a drive transistor coupled in series with one or more emission transistors and a respective organic light-emitting diode (OLED). A semiconducting-oxide transistor may be coupled between a drain terminal and a gate terminal of the drive transistor to help reduce leakage during low-refresh-rate display operations. A silicon transistor may be further interposed between the semiconducting-oxide transistor and the gate terminal of the drive transistor. One or more capacitor structures may be coupled to the source terminal and/or the drain terminal of the semiconducting-oxide transistor to reduce rebalancing current that might flow through the semiconducting-oxide transistor as it is turned off. Configured in this way, any emission current flowing through the OLED will be insensitive to any potential drift in the threshold voltage of the semiconducting-oxide transistor.