Oxide TFT Threshold Voltage Stabilization via Reverse Bias
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Solution Overview
Problem
Thin-film transistors (TFTs) using oxide semiconductors for the semiconductor layer in display devices experience significant variations in threshold voltage due to stress, affecting luminance control and requiring frequent threshold-voltage compensation, which is challenging to maintain over time.
Innovation Solution
A display device with a matrix of pixels, each including a light-emitting element, a capacitor for voltage storage, and a drive transistor that receives current from the capacitor, with a voltage supplier applying a reference voltage during initialization and a reverse bias voltage before initialization, maintaining the threshold voltage within an operable range by using specific switch configurations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If oxide semiconductor TFT is used for the drive transistor, then power consumption is reduced and manufacturing cost is lowered, but threshold voltage varies easily by stress leading to poor display quality
Solution Approach 1:
The patent applies a reverse bias voltage to the drive transistor in a predetermined period before the initialization period, when the light emitting element is not emitting light. This preliminary action prevents threshold voltage variation caused by stress before it occurs during the light emission period, thereby maintaining display quality stability without requiring longer compensation periods
Solution Approach 2:
The patent implements periodic application of reverse bias voltage during non-light-emission periods and forward bias voltage during initialization periods. This periodic action continuously resets the threshold voltage to an appropriate value, preventing cumulative stress effects and maintaining stable display quality over extended operation
2Duration of action of stationary object
If threshold voltage compensation operation is performed, then threshold voltage variation is reduced, but the operable voltage range cannot be maintained for long time
Solution Approach 1:
The patent continuously maintains the threshold voltage within the operable range by applying reverse bias voltage periodically during non-light-emission periods. This continuous useful action extends the duration for which the operable voltage range can be maintained, allowing stable luminance control over longer periods without requiring frequent compensation operations
Solution Approach 2:
The reverse bias voltage applied before the initialization period acts as a preliminary anti-action that prevents threshold voltage from drifting out of the operable range. This preemptive measure counteracts stress-induced voltage variation before it compromises luminance control stability, extending the maintenance time of the operable voltage range
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows the display device to maintain the threshold voltage of the drive transistor within an operable range for a longer time, even when adequate compensation periods are difficult to ensure, thereby improving display quality and reducing variations in luminance control.
Implementation Method 1
applies a reverse bias voltage to the drive transistor in a predetermined period before the initialization period, the reverse bias voltage providing a reverse bias between a gate electrode and a source electrode of the drive transistor
Implementation Method 2
applies a forward bias voltage to the drive transistor, the forward bias voltage providing a forward bias between the gate electrode and the source electrode of the drive transistor
Data Source
AI summary
A display device comprising pixel circuits arranged in a matrix, in which each of the pixel circuits includes: an EL element; a capacitor for storing voltage; a drive transistor that provides, to the EL element, a current corresponding to the voltage stored in the capacitor to cause the EL element to emit light; a voltage supplier that applies a reference voltage to the drive transistor in an initialization period, and applies a reverse bias voltage to the drive transistor in a predetermined period before the initialization period, the reference voltage being higher than a threshold voltage of the drive transistor and providing a forward bias between the gate and source electrodes of the drive transistor, the initialization period being a period for initializing the pixel circuit, the reverse bias voltage providing a reverse bias between the gate and source electrodes of the drive transistor.