Seo Jin Kang1, Eun Seong Yu1, Hyuk Su Lee1 and Byung Seong Bae1
© 2022 ECS – The Electrochemical Society
ECS Meeting Abstracts, Volume MA2022-02, H02: Thin Film Transistors 16 (TFT 16)Citation Seo Jin Kang et al 2022 Meet. Abstr. MA2022-02 1291DOI 10.1149/MA2022-02351291mtgabs
Author affiliations
1 Hoseo University
ORCID iDs
Seo Jin Kang https://orcid.org/0000-0001-8478-4161
Eun Seong Yu https://orcid.org/0000-0002-2140-8721
Hyuk Su Lee https://orcid.org/0000-0002-3400-9607
Byung Seong Bae https://orcid.org/0000-0002-3328-0286
Abstract
Micro LED (μ-LED) refers to a micro-light emitting diode with a chip size of 5 to 100 μm. Since micro-LED displays use individual LED chips as a light source of the pixel, they do not need backlight and color filters, which are suitable for implementing flexible displays. In addition, micro-led displays have many advantages over OLED displays, such as excellent color gamut, power consumption, lifetime, and response time. With these advantages, micro-LED displays are attracting attention as next-generation displays, and accordingly, interest in micro-LED driving technology is increasing. While existing OLED displays express grayscale by controlling the amount of current flowing through the light-emitting device, micro-LED can have a problem with color change depending on the grayscale because the light-emitting wavelength shifts when the amount of current changes. Therefore, to express a certain color, it is necessary to express the grayscale by adjusting the emission time at the same current density. That is, it is more suitable to use the PWM(pulse width modulation) for each pixel than to use the PAM(pulse amplitude modulation) for the grayscale representation of the OLED display. Therefore, unlike OLED, the micro-LED display requires a new pixel circuit because it must use PWM driving. In addition, unlike LCDs, micro-led displays are sensitive to changes in the electrical characteristics of TFTs depending on the location of each pixel or external stress. Thus, threshold voltage compensation is essential because changes and degradation of the characteristics of the thin film transistor backplane, such as threshold voltage shift, cause non-uniform brightness. Therefore, we propose a PWM compensation pixel circuit based on a-IGZO TFT that can be fabricated on a flexible substrate and has a lower process cost than LTPS and greater mobility than a-Si TFT.
The operation of proposed pixel is divided into (1) reset 1, (2) PWM data compensation, (3) reset 2, (4) CCG (constant current generation)compensation, and (5) emission. The circuit is divided into a CCG unit that generates a constant current and a PWM unit that adjusts emission time by a ramping signal. Figure 1 (a) shows the output current simulation result according to PWM data voltage variation. When the PWM data voltages vary from -1 to ~ – 5V, the currents were constant at 70.9 μA, and the smaller the PWM data voltage, the longer the emission time was. In addition, Figure 1 (b) shows the effect of the threshold voltage compensation. The current and emission time are constant for various threshold voltages such as 1, 2, and 3 V.
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