# Pixel Circuit Designs for Active Matrix Displays
## 4 Micro-LED Circuit Design
proposed to combine the constant current generation unit in PAM with the PWM drive as shown in Figure 9a for the 13T3C circuit [100]. Applying PWM drive to the proposed pixel circuit to fix the current density of micro-LEDs similarly improves the problem of wavelength drift at the centre of micro-LEDs. Combining the PWM pixel circuit with the pixel compensation circuit, we provide constant current through analogue \(V_{\text{DATA}}\) modulation and improve display uniformity by compensating for PWM TFT and driving TFT \(V_{\text{TH}}\) shift [101]. [PERSON], [PERSON] et al. proposed a new uncompensated analogue PWM pixel circuit [102], which simplified the circuit structure by achieving micro-LED operation at constant current through a pull-up control scheme. The brightness uniformity of the fabricated LED arrays is only 74% due to the fact that the brightness uniformity is affected by factors such as the mobility variation in the driving TFT and IR drop. [PERSON], [PERSON] et al. proposed a new pixel circuit based on LTPO TFTs, which combines the internal \(V_{\text{TH}}\) compensation and the PWM method. This circuit has a micro-LED current error rate of 3.7% and an emission pulse width error rate of 3.4% at a \(V_{\text{TH}}\) variation of 0.5 V [38]. Figure 9: 13T3C pixel circuit **(a)**[100], 4T2C pixel circuit for CCCS **(b)**[103], and 6T1C pixel circuit **(c)**[96]. Although the PWM solution can better solve the central wavelength shift of micro-LEDs with the current value, its complex pixel circuitry occupies a large pixel area. To improve the PPI of the display, the PAM solution is introduced. Good display uniformity and emission stability can also be achieved using a current control current source (CCCS). [PERSON], [PERSON] et al. proposed a 4T2C pixel circuit using CCCS [103] (Figure 9b). The circuit uses the principle of current mirroring to make \(I_{\text{LED}}\) proportional to \(I_{\text{DATA}}\) (affected by the \(W/L\) of T3 and T4). When T1 and T2 are switched off, the voltage stored in the capacitor continuously keeps the T4 switched on, thus providing stable current to the micro-led throughout the entire frame cycle. The PWM scheme can better solve the centre wavelength shift problem of micro-LEDs, but it has lower brightness control accuracy at high grey levels and may face flickering problem. On the contrary, PAM is able to achieve high-precision brightness control and stability at high grey levels but has poor brightness uniformity and low efficiency at low grey levels. Therefore, the PHM drive mode combines the advantages of PWM and PAM. In low greyscale, PWM mode is used to adjust the light emitting time to keep the micro-LED current at a high and constant level. At a high grey level, the PAM mode is used to improve the brightness control accuracy. [PERSON], [PERSON] et al. proposed the 11T4C pixel circuit based on the PHM driving mode. The circuit exhibits good brightness uniformity and stability at different grey levels and has good aging resistance [104]. [PERSON], [PERSON] et al. connected multiple LED driver circuits in series by daisy-chaining connections based on the PHM driving mode to facilitate the expansion of the LED driver-integrated circuits and to achieve synchronous transmission of clock and data signals [105]. 1. Silicon-based micro-LED pixel circuits MOSFETs, being more mature than TFTs, enable smaller-sized displays and integrated peripheral circuits, making them a suitable solution for micro-displays smaller than 1 inch [106]. MOSFET circuits have unique properties such as low power consumption, high noise immunity, good integration, and stability [107]. Complementary metal oxide semiconductor (CMOS) inverters (i.e., n-type and p-type transistors) can easily reduce the size of the device, place analogue and digital circuits on the same chip, and have low fabrication costs.