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.