Micro Light-Emitting Diode Pixel Circuit Based on p-Type Low-Temperature Polycrystalline Silicon Thin-Film Transistor for Mobile Displays

• Published in: IEEE transactions on electron devices Vol. 70; no. 9; pp. 4662 - 4668
• Main Authors: Hong, Yong-Hoo, Jung, Eun Kyo, Jeong, Ye-Rim, Im, Hwarim, Kim, Yong-Sang
• Format: Journal Article
• Language: English
• Published: New York IEEE 01-09-2023; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Always-on display (AOD) operation mode; Circuits; Compensation; Displays; Error analysis; Gray-scale; hybrid pulsewidth modulation and pulse amplitude modulation (HPP); Image quality; just noticeable difference (JND); Light emitting diodes; Low temperature; Luminance; micro light-emitting diode (μLED); Photovoltaic cells; pixel circuit; Pixels; Polycrystals; Polysilicon; Power consumption; Pulse amplitude modulation; Pulse duration; Pulse width modulation; Semiconductor device measurement; Semiconductor devices; Silicon; Silicon films; Thin film transistors; Threshold voltage; Voltage
• ISSN: 0018-9383; 1557-9646
• DOI: 10.1109/TED.2023.3296068

We propose a micro light-emitting diode (<inline-formula> <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> <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> <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> <tex-math notation="LaTeX">\sf \Delta {L} </tex-math></inline-formula>) under the variation of <inline-formula> <tex-math notation="LaTeX">{V} _{\text {TH}} </tex-math></inline-formula>. The <inline-formula> <tex-math notation="LaTeX">\sf \Delta {L} </tex-math></inline-formula> values of all the gray levels were lower than the <inline-formula> <tex-math notation="LaTeX">\vert </tex-math></inline-formula>JND<inline-formula> <tex-math notation="LaTeX">\vert </tex-math></inline-formula> under the <inline-formula> <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.