MOx Switching TFT Sub-Pixel Circuit for Smaller Storage Capacitors
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Solution Overview
Problem
High-resolution OLED displays face challenges in reducing pixel footprint due to the difficulty in miniaturizing storage capacitors, as their size is determined by frame rate and leakage current through the switching TFT, limiting further pixel size reduction.
Innovation Solution
The use of metal oxide (MOx) switching TFTs, which offer significantly lower leakage current compared to low-temperature polysilicon (LTPS) TFTs, allows for reduced storage capacitor size, enabling higher pixel density by maintaining capacitance for longer frame holding times without loss, and enabling resolutions up to 1200 pixels per inch or higher.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional LTPS TFTs are used for switching, then the TFT can provide sufficient switching performance, but the leakage current is high which requires large storage capacitor size
Solution Approach 1:
The patent changes the material parameter of the switching TFT from LTPS to metal oxide (MOx), which fundamentally alters the electrical characteristics. This material substitution reduces the leakage current by several orders of magnitude while maintaining adequate switching performance, directly resolving the contradiction between switching performance and energy loss.
2Quantity of substance
If storage capacitor size is reduced to increase pixel density, then higher resolution is achieved, but the capacitance may be insufficient to maintain charge for the required frame holding time
Solution Approach 1:
By changing the switching TFT material to MOx with extremely low leakage current, the patent enables storage capacitors to retain charge effectively over the required frame holding period even at reduced sizes. This parameter change in material properties allows the capacitor to maintain adequate charge retention despite the reduced capacitance value associated with smaller size.
3Area of moving object
If pixel size is reduced for high resolution displays, then the display density increases, but the circuitry footprint including storage capacitor becomes too small to maintain required capacitance
Solution Approach 1:
The patent applies material parameter changes by using MOx switching TFTs that exhibit negligible leakage current. This enables the storage capacitor to maintain its charge retention reliability even when the capacitor physical size and capacitance value are reduced to fit within the smaller high-resolution pixel footprint.
4Reliability
If storage capacitor size is maintained at conventional dimensions, then sufficient capacitance is available, but the pixel footprint becomes too large for high resolution displays
Solution Approach 1:
By changing the switching TFT material to metal oxide with ultra-low leakage characteristics, the patent decouples the relationship between capacitor size and charge retention reliability. This allows the storage capacitor to be miniaturized while maintaining adequate capacitance and charge holding capability through the superior electrical properties of the MOx switching device.
Data Source
AI summary
Disclosed herein is a sub-pixel circuit for a display device. The sub-pixel circuit has a driving TFT and at least one switching TFT. The at least one switching TFT is an oxide TFT. The sub-pixel circuit additionally has at least one storage capacitor wherein the storage capacitor has a capacitance between about 1 fF and about 55 fF.