Micro-LED Pixel Circuit Using Low-Temperature Poly-Si Oxide TFTs With Fast Rise and Fall Times of Pulse Width Modulation for Low Gray Expression

We report a micro light-emitting diode ( $\mu $ LED) pixel circuit using low-temperature poly-Si oxide (LTPO) thin film transistors (TFTs). The pixel circuit operates by pulse width modulation (PWM) to prevent the wavelength shift of $\mu $ LED. The proposed PWM can support 10-bit gray scale at a refresh rate of 120 Hz by PWM, thanks to the fast rise and fall times. Note that the circuit also compensates for the variation of the threshold voltage ( $\text{V}_{\text {TH}}{)}$ of the TFTs in the pixel. It is found that the current error rate is 3.7%, and the maximum pulse width error rate is 3.4% for low gray levels when the $\text{V}_{\text {TH}}$ shift is 0.5 V. The fabricated pixel circuit successfully demonstrates PWM of variable gray levels, with measured rise and fall times of $3~\mu \text{s}$ and $0.3~\mu \text{s}$ , respectively. Therefore, the proposed pixel circuit is suitable for $\mu $ LED display capable of expressing 10-bit gray scale at a refresh rate of 120 Hz.