Display Drive Circuit Threshold Shift Reduction
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Solution Overview
Problem
The existing drive circuits for image display devices using amorphous silicon thin-film transistors (a-Si TFTs) suffer from threshold voltage shifts due to prolonged operation in high positive bias voltages, leading to instability and increased power consumption, particularly when used in environments with high temperatures.
Innovation Solution
A drive circuit design that includes unit drive circuits with selective pulse output and power source circuits that alternate between ON and OFF states during non-output periods, using transistors to manage the connection between output terminals and power sources, thereby reducing threshold voltage shifts and power consumption by applying a reverse bias voltage during OFF states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the transistor is kept in an ON state during non-output periods to maintain output terminal at L level, then the output terminal can be held at a stable potential, but the threshold voltage shifts in the positive direction and power consumption increases
Solution Approach 1:
The patent applies periodic action by alternating the transistor between ON and OFF states during different time periods. During the output operation period, the transistor is turned ON to output the selective pulse. During the non-output period, the transistor is turned OFF to reduce power consumption and prevent threshold voltage shift, while the output terminal is pulled to L level through a pull-down resistor. This periodic switching resolves the contradiction between maintaining output stability and reducing power consumption.
Solution Approach 2:
The patent introduces a pull-down resistor as an intermediary element to replace the continuous ON state of the transistor during non-output periods. The pull-down resistor acts as a mediator that maintains the output terminal at L level without requiring the transistor to remain ON, thereby eliminating the harmful effects of continuous conduction while preserving the desired output level.
2Reliability
If the transistor is kept in an ON state during non-output periods to maintain output terminal at L level, then the output terminal can be held at a stable potential, but the threshold voltage shifts in the positive direction
Solution Approach 1:
The patent uses periodic action to switch the transistor between ON and OFF states. During the non-output period, turning OFF the transistor stops the continuous positive bias that causes threshold voltage shift, while the pull-down resistor maintains the output terminal at the required L level. This periodic operation preserves both output stability and threshold voltage stability.
Solution Approach 2:
The pull-down resistor serves as an intermediary that maintains the output terminal potential without requiring the transistor to remain ON. This eliminates the continuous positive bias stress on the transistor, thereby preventing threshold voltage shift while still achieving the desired output level stability.
3Device complexity
If a single power source is used for both output operation period and non-output period, then the circuit structure is simple, but the transistor experiences prolonged high bias voltage leading to threshold voltage shift
Solution Approach 1:
The patent segments the power supply function into two distinct power sources: VGH for the output operation period and VGL for the non-output period. This segmentation allows the transistor to experience appropriate bias conditions during each period, preventing prolonged high bias voltage and its harmful effects on threshold voltage stability.
Solution Approach 2:
The patent makes the power supply dynamic by switching between different power sources (VGH and VGL) based on the operational state. The power source is not fixed but dynamically adapted to the current operation phase, allowing the circuit to maintain simplicity while preventing threshold voltage shift through appropriate biasing.
Data Source
AI summary
In a unit drive circuit in each stage in a shift register, a transistor which is maintained in ON state during a period where the unit drive circuit in the stage does not perform an outputting operation is configured not to generate Vth shift. As switches, transistors T6A, T6B are connected between the output terminal OUT and AC power sources VA, VB. At least one of T6A, T6B is brought into ON state and T6A, T6B are alternately brought into OFF state during the period other than the outputting operation period. VA, VB supply L level during a period where T6A, T6B are in ON state, while VA, VB supply a ground potential GND which is an intermediate potential between an H level and an L level during a period where T6A, T6B are in OFF state.