AMOLED Pixel Circuit Parameter Extraction via Charge Pump
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Solution Overview
Problem
In active matrix organic light emitting device (AMOLED) displays, extracting parameters such as threshold voltage and mobility of drive transistors is challenging due to non-uniform behavior caused by aging and process variations, which affects image quality and requires additional components in the driver circuit, reducing pixel aperture.
Innovation Solution
A method for extracting circuit parameters from pixel circuits by turning off the drive device, supplying a predetermined voltage, and reading the voltage while the light emitting device is off, using a charge-pump amplifier to determine parameters like threshold voltage and mobility, and measuring current and voltage while changing the gate and drain voltage to operate in different regimes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If additional components are added to extract parameters (threshold voltage, mobility) from drive transistors, then parameter extraction capability is improved, but pixel aperture area is reduced
Solution Approach 1:
The pixel circuit components (drive transistor, storage capacitor, switch transistor) are designed to serve dual purposes: normal display operation and parameter extraction. By controlling the timing and state of existing components, the circuit extracts threshold voltage and mobility parameters without requiring additional dedicated extraction components, thus maintaining maximum pixel aperture.
Solution Approach 2:
The pixel circuit uses its own internal components and operating states to perform self-diagnosis and parameter extraction. The drive transistor's own characteristics are measured by manipulating its state (on/off transitions) and reading the resulting voltage changes, eliminating the need for external measurement equipment or additional circuitry within the pixel.
2Area of moving object
If parameter extraction is performed with minimal components, then pixel aperture is maximized, but extraction complexity increases
Solution Approach 1:
Parameter extraction is performed during specific time intervals (horizontal blanking period, vertical blanking period) when the display is not actively refreshing. The circuit alternates between normal display mode and parameter extraction mode by controlling switch transistor states at different timing phases, simplifying the overall circuit design by reusing existing components at different times rather than maintaining separate extraction circuitry.
Solution Approach 2:
The pixel circuit is programmed with specific voltage sequences before measurement to prepare the drive transistor in known states (fully on, fully off, transitioning). This preliminary programming establishes controlled initial conditions that simplify the extraction process and enable accurate parameter measurement using minimal components.
3Device complexity
If drive transistor parameters are not compensated for aging and process variations, then device complexity is reduced, but image quality deteriorates
Solution Approach 1:
The extracted parameters (threshold voltage, mobility) are fed back to adjust the programming voltage applied to the drive transistor. This feedback mechanism compensates for aging and process variations by dynamically adjusting operating conditions to maintain consistent display performance, thereby improving image quality and reliability without requiring overly complex compensation circuits.
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 for efficient extraction of parameters with minimal components, maximizing pixel aperture and enabling in-pixel compensation for optimal lifetime performance and image quality maintenance.
Implementation Method 1
a first capacitor having a first capacitance value coupled between the gate and the source of the drive transistor
Implementation Method 2
The OLEDs emit light based on current supplied through drive transistors controlled by programming voltages
Data Source
AI summary
A system reads a desired circuit parameter from a pixel circuit that includes a light emitting device, a drive device to provide a programmable drive current to the light emitting device, a programming input, and a storage device to store a programming signal. One embodiment of the extraction system turns off the drive device and supplies a predetermined voltage from an external source to the light emitting device, discharges the light emitting device until the light emitting device turns off, and then reads the voltage on the light emitting device while that device is turned off. The voltages on the light emitting devices in a plurality of pixel circuits may be read via the same external line, at different times. In-pixel, charge-based compensation schemes are also discussed, which can be used with the external parameter extraction implementations.