Diode-Connected Transistor for OLED Gray Scale Uniformity
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Active matrix organic EL display devices face challenges in achieving good gray scale expression due to subthreshold characteristics of drive transistors, leading to gray scale unevenness and increased power consumption, especially at low luminance.
Innovation Solution
Incorporating a diode connection transistor between the source of the drive transistor and the display element, with specific channel length and width ratios, and channel capacity relationships, to adjust the gate voltage and current relationship, reducing the effect of drive transistor characteristic variation and optimizing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If subthreshold characteristics of the drive transistor are used to reduce current at low luminance, then power consumption is reduced, but gray scale expression deteriorates due to abrupt current changes and characteristic variation
Solution Approach 1:
A diode connection transistor is introduced as an intermediary component between the drive transistor and the display element. This diode connection transistor mediates the current flow, providing a buffering effect that smooths out the abrupt current changes inherent in subthreshold operation. The diode connection transistor acts as a current regulator that maintains stable gray scale expression even when the drive transistor operates in the subthreshold region for low power consumption.
Solution Approach 2:
The invention utilizes parameter changes in the diode connection transistor's operating characteristics to compensate for the abrupt current variations in the drive transistor. By carefully selecting the dimensions (channel width W2 and length L2) and electrical parameters of the diode connection transistor, the system achieves a combined transfer characteristic that provides gentle current changes with respect to gate voltage, thereby maintaining good gray scale expression across the full range of luminance levels.
2Area of stationary object
If the channel length L2 of the diode connection transistor is made shorter, then device area is reduced, but the ability to smooth current changes deteriorates
Solution Approach 1:
The invention optimizes the channel length L2 of the diode connection transistor by establishing specific relationship conditions with other transistor parameters. Rather than simply minimizing L2 for area reduction, the patent defines L2 such that it satisfies particular ratios relative to the drive transistor's channel length L1 and other dimensional parameters. This parametric optimization ensures that the diode connection transistor maintains sufficient current smoothing capability while minimizing the device area occupied.
3Manufacturing precision
If the channel width W2 of the diode connection transistor is increased, then current smoothing capability is improved, but device area increases
Solution Approach 1:
The invention determines the optimal channel width W2 of the diode connection transistor by establishing specific relationship conditions with the drive transistor's parameters. The patent defines W2 such that it satisfies particular ratio relationships with the drive transistor's channel width W1 and other dimensional parameters. This parametric relationship ensures that the diode connection transistor provides adequate current smoothing capability for uniform gray scale expression while keeping the device area as small as possible.
Data Source
AI summary
A pixel circuit includes an organic EL element configured to emit light, a capacitor configured to hold a data voltage, a drive transistor with a data gate connected to one electrode of the capacitor, and a diode connection transistor connected between a source of the drive transistor and the organic EL element. A source of the diode connection transistor is connected to a back gate of the drive transistor. In a case where a channel length of the drive transistor is taken as L1, a channel length of the diode connection transistor is taken as L2, a ratio of a channel width to a channel length of the drive transistor is taken as (W/L)1, and a ratio of a channel width to a channel length of the diode connection transistor is taken as (W/L)2, a relation of L1<L2 and a relation of (W/L)1<(W/L)2 are satisfied.