Series TFT Switching Device for Display Threshold Shift Reduction
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Solution Overview
Problem
In active display devices, the continuous application of negative gate voltage to thin film transistors (TFTs) leads to threshold voltage shifts, increasing leakage current and deteriorating image quality, especially under light exposure.
Innovation Solution
Implementing a switching device with at least two TFTs connected in series, where a positive gate voltage is applied during pixel charging time to turn on all TFTs, and during other times, at least one TFT is turned off while others remain on, with a negative gate voltage applied to maintain the charged state, thereby minimizing threshold voltage shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If negative gate voltage is continuously applied to TFT to maintain OFF state, then pixel charged state is maintained, but threshold voltage shifts toward negative voltage and leakage current increases
Solution Approach 1:
The switching device is divided into multiple TFTs (first TFT and second TFT) connected in series, each controlled by separate gate lines. This segmentation allows independent control of each TFT's gate voltage, enabling the first TFT to be turned off with negative voltage while the second TFT remains on with positive voltage, thus preventing threshold voltage shift and leakage current in the first TFT while maintaining pixel charge.
Solution Approach 2:
The gate voltage applied to each TFT is dynamically changed based on timing phases. During pixel charging time, both TFTs receive positive gate voltage to turn on. During other times, the first TFT receives negative gate voltage to turn off while the second TFT continues receiving positive voltage to remain on. This dynamic voltage switching prevents continuous negative voltage exposure that causes threshold voltage shift.
2Reliability
If negative gate voltage is applied to TFT, then TFT is turned off to maintain pixel charge, but threshold voltage shifts and image quality deteriorates
Solution Approach 1:
By segmenting the switching function into multiple TFTs with separate gate controls, the patent isolates the TFT that would be exposed to negative voltage (first TFT) from the pixel maintenance function. The second TFT continues to maintain the pixel charged state while the first TFT is protected from threshold voltage shift, thereby maintaining image quality.
3Device complexity
If single TFT is used as switching device, then device complexity is low, but threshold voltage shift occurs under continuous negative voltage
Solution Approach 1:
The switching device is segmented into multiple TFTs (first TFT and second TFT) connected in series, each with its own gate line for independent control. This segmentation increases device complexity slightly but resolves the threshold voltage shift issue by allowing the first TFT to be turned off with negative voltage while the second TFT remains on with positive voltage, thereby maintaining electrical reliability.
Data Source
AI summary
Example embodiments are directed to a switching device of an active display device and a method of driving the switching device, such that electrical reliability of the active display device is improved. The switching device of the active display device includes a plurality of thin film transistors (TFTs) that are connected in series. Except for a refresh time duration during which the plurality of TFTs of the switching device are simultaneously turned ON, a positive voltage is applied to at least one of the plurality of TFTs of the switching device so that a reliability of the switching device may be improved.