Shift Register Stabilizes TFT Threshold Voltage
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Solution Overview
Problem
Conventional amorphous silicon (a-Si) TFT-based gate driving circuits in display devices experience unstable threshold voltage shifts over time, affecting normal operations and reliability due to process capability limitations and increased manufacturing complexity of low temperature polysilicon (LTPS) TFTs.
Innovation Solution
A shift register design incorporating a pre-charge unit, pull-up unit, first pull-down unit, and second pull-down unit, with specific transistor configurations and control signal timing to manage and stabilize the threshold voltage of TFT components, including phase-inverted pull-down control signals and adjustable time intervals to prevent voltage shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a-Si TFT is used in gate driving circuit, then manufacturing cost is reduced and manufacturing process is simpler, but threshold voltage becomes unstable and shifts over time
Solution Approach 1:
The gate driving circuit is divided into multiple stages with each stage containing independent pull-down units. This segmentation allows periodic resetting of threshold voltage in each stage without affecting the entire circuit, enabling stable operation using simple a-Si TFT manufacturing process
Solution Approach 2:
Pull-down control signals are applied periodically to the pull-down units to reset the threshold voltage of a-Si TFTs. The periodic action prevents cumulative threshold voltage shifts that would otherwise occur during continuous operation, maintaining circuit reliability while using cost-effective a-Si TFTs
2Speed
If LTPS TFT is used in gate driving circuit, then carrier mobility is improved, but manufacturing process complexity increases
Solution Approach 1:
The invention uses a-Si TFTs which are cheaper and easier to manufacture than LTPS TFTs. Although a-Si TFTs have lower carrier mobility and shorter operational life, the periodic threshold voltage resetting mechanism extends their effective operational life, making them a viable alternative to expensive LTPS TFTs
3Manufacturing precision
If bias voltage is added to TFT, then process capability is improved, but threshold voltage increases and driving circuit integration becomes difficult
Solution Approach 1:
The pull-down units are activated periodically to preemptively reset the threshold voltage before it can shift to problematic levels. This preliminary action prevents threshold voltage accumulation that would otherwise complicate driving circuit integration, allowing standard a-Si TFT process capabilities to be used
Data Source
AI summary
A shift register and a display device are provided. The shift register includes a pre-charge unit, a pull-up unit, a first pull-down unit and a second pull-down unit. The pre-charge unit receives first and second input signals, and outputs a pre-charge signal via a first node. The pull-up unit receives the pre-charge signal and a clock signal, and outputs a scanning signal via a second node. The first pull-down unit receives the pre-charge signal, first and second pull-down control signals, and controls whether to pull-down the scanning signal to a reference voltage level. The second pull-down unit receives the pre-charge signal and first and second pull-down control signals, and controls whether to keep the scanning signal at the reference voltage level. The period of the first second pull-down control signal and the period of the second pull-down control signal are in a range from 12 frames to 180 frames.