Micro Light-Emitting Diode Pixel Circuit Based on p-Type Low-Temperature Polycrystalline Silicon Thin-Film Transistor for Mobile Displays
Yong-Hoo Hong, Eun Kyo Jung, Ye-Rim Jeong, Hwarim Im, Yong‐Sang Kim
Abstract
We propose a micro light-emitting diode ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> LED) pixel circuit based on p-type low-temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) for mobile displays. Mobile displays require higher pixels per inch (PPI) than other applications, such as television. Therefore, we reduced the number of TFTs in the pixel circuit by considering the relationship between threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V} _{\text {TH}}$ </tex-math></inline-formula> ) compensation and luminance error rate. In the normal operation mode, the proposed pixel circuit modulates the gray levels using hybrid pulsewidth modulation and pulse amplitude modulation (HPP) to suppress the wavelength shift and improve the low gray-level expression. In the always-on display (AOD) operation mode, which is widely used in commercial mobile devices, the proposed circuit uses only pulse amplitude modulation (PAM) to reduce the power consumption. We verified the operation of the proposed circuit using circuit simulation. The proposed pixel circuit exhibited the HPP operation and successfully expressed 8-bit gray levels in the normal operation mode. In the AOD operation mode, the proposed circuit modulated the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> LED current using PAM. We also investigated the effect of the simplified compensation structure on image quality by comparing the just noticeable difference (JND) and luminance changes ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sf \Delta {L}$ </tex-math></inline-formula> ) under the variation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V} _{\text {TH}}$ </tex-math></inline-formula> . The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sf \Delta {L}$ </tex-math></inline-formula> values of all the gray levels were lower than the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert $ </tex-math></inline-formula> JND <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert $ </tex-math></inline-formula> under the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V} _{\text {TH}}$ </tex-math></inline-formula> variation of ±0.1 V. Consequently, the proposed pixel circuit exhibited stable operation with reliable image quality for mobile displays.