Bidirectional GOA Shift Registers With Segmented Paths for Threshold Stability
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Solution Overview
Problem
In GOA circuits with bidirectional scanning, input transistors experience negative shift of threshold voltage due to negative bias thermal stress, leading to difficulty in maintaining charged levels and potential circuit output failure when scanning direction is switched, especially with oxide transistors like IGZO.
Innovation Solution
A shift register unit with an input circuit, first reset circuit, and output circuit that includes control mechanisms to charge and discharge nodes symmetrically for both forward and reverse scanning, using transistors to maintain zero bias states and prevent threshold voltage deviations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If bidirectional scanning is implemented in GOA circuits, then the versatility of the display device is improved, but the reliability of the circuit output deteriorates due to threshold voltage shift in input transistors
Solution Approach 1:
The input circuit is segmented into separate charging and discharging paths with dedicated transistors (first input transistor for charging, second input transistor for discharging). This segmentation allows independent control of charge and discharge operations, preventing the threshold voltage shift that occurs when a single transistor handles both operations in bidirectional scanning modes.
Solution Approach 2:
A third transistor is introduced as an intermediary discharge transistor connected between the first node and the second power terminal. This intermediary component provides a dedicated discharge path that is independent of the charging path, allowing the circuit to properly discharge nodes during direction switching without being affected by threshold voltage shifts in the input transistors.
2Manufacturing precision
If oxide transistors (IGZO) are used in the GOA circuit, then the manufacturing precision is improved, but the threshold voltage stability deteriorates under negative bias thermal stress
Solution Approach 1:
The circuit performs preliminary discharging actions through dedicated discharge transistors before direction switching occurs. By actively discharging nodes through the second and third transistors in advance, the circuit prevents charge accumulation that would otherwise cause threshold voltage shifts in oxide transistors during negative bias thermal stress conditions.
Solution Approach 2:
The circuit implements feedback control through the discharge paths that monitor and actively manage the charge state of internal nodes. The discharge transistors respond to circuit state changes by actively removing excess charge, providing negative feedback that stabilizes the threshold voltage of oxide transistors during bidirectional operation and thermal stress.
Data Source
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Figure 4B~5A
AI summary
A shift register unit and a driving method, a grid driving circuit and a display device are disclosed. A shift register unit includes an input circuit (110), a first reset circuit (120), and an output circuit (130). The input circuit (110) includes an input terminal (INT) configured to perform a first control on the first control node (PU) and the first node (N1) in response to an input signal of the input terminal (INT), and then perform a second control, which is different from the first control, on the first node (N1) under the control of the level of the first node (N1), the first node (N1) is located in a path where the input signal incurs the first control on the first control node (PU); the first reset circuit (120) is configured to reset the first control node (PU) in response to the first reset signal; the output circuit (130) is configured to output an output signal to an output terminal (OUT) under the control of the level of the first control node (PU). The shift register unit can avoid the phenomenon of no output after switching the scanning direction due to negative bias of the threshold voltage of the transistor at the input end of the shift register unit, enhance the stability of the circuit, and have a larger threshold voltage bias margin.