High performance A-PWM μLED pixel circuit design using double gate oxide TFTs

Abstract

This paper explores the transition speed of A-PWM (Analog Pulse Width Modulation) pixel circuits and feasibility of integration oxide thin-film transistors (TFTs) for MicroLED (µLED) display. A new fast A-PWM type µLED display pixel circuit design is proposed using double gate oxide TFT in the depletion mode as the pull-up transistor of the inverter, while both the main gate and the auxiliary gate electrodes are connected to the source electrode to obtain a constant Zero-V_GS biasing. Consequently, the pull-up TFT acts as a constant current source for increasing the output resistance. Meanwhile the pull-down transistor is a single-gate device, and the transconductance is modulated by the input sweep voltage via a coupling capacitor. The advantage of this structure is that even using IGZO (indium-gallium-zinc-oxide) TFTs with a mobility of only 6 cm²/V.s, the PWM transition time of the A-PWM pixel can be reduced from 500 µs to 50 µs. Furthermore, this pixel circuit integrates a switched-capacitor structure to extract and compensate for V_T shift. Even with a V_T shift of 2 V, the error rate of the PWM fall time remains as low as 0.74 %. Feasibility of the double gate inverter was well verified through measurement of fabrication results.