# Pixel Circuit Designs for Active Matrix Displays
## 4 Micro-LED Circuit Design  
Micro-LED micro-displays are still in the early stages of development and may gradually replace existing display technologies such as silicon-based LCDs or silicon-based OLEDs in certain applications in the future [96]. We compared some pixel circuit design features of glass-based and silicon-based micro-LEDs, as shown in Table 3.  
TFT Micro-LED Pixel CircuitsMicro-LEDs and OLEDs are current-driven devices, so the pixel circuits are relatively similar, with the basic 2T1C circuitry. There are two methods to drive micro-LEDs: pulse amplitude modulation (PAM) and pulse width modulation (PWM). PAM directly controls the brightness of micro-LEDs by adjusting the amplitude of the drive current, and its pixel circuit structure is simple, which is conducive to achieving high pixel density. However, due to the high leakage current of the driving transistors, it needs to rely on complex external compensation circuits to maintain voltage stability [21], leading to an increase  
Figure 8: Source-following pixel circuits (**a,b**) [92; 93] and capacitive voltage divider pixel circuits (**c,d**) [94; 95]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Feature** & **Glass Substrate (TFT)** & **Silicon Substrate (CMOS)** \\ \hline \multirow{3}{*}{Substrate characteristics} & Low cost & High cost \\  & Easy to achieve large sizes & High integration \\  & Low carrier mobility & High carrier mobility \\ \multirow{2}{*}{Circuit complexity} & Limited circuit design & Flexible circuit design \\  & High demand for compensation circuits & Easy integration of compensation circuits \\ \multirow{2}{*}{Compatible driving techniques} & Suitable for PWM (easy to implement) & Suitable for PAM (high-precision control) \\  & Difficult to implement PAM & Easy to implement PWM (high refresh rate) \\ \multirow{2}{*}{Power consumption} & Limited implementation of PHM & Easy to implement PHM (high dynamic range) \\  & High & Low \\ \multirow{2}{*}{Manufacturing process} & Compatible with existing & \multirow{2}{*}{Requires high-precision CMOS processes} \\  & TFT-LCD processes & \\ \multirow{2}{*}{Cost} & Lower difficulty in mass transfer & High difficulty in mass transfer \\  & Low & High \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison table of glass-based (TFT) and silicon-based (CMOS) micro-LEDs. in system power consumption. PWM indirectly controls the brightness by adjusting the luminous time (duty cycle) at a fixed current, and its pixel circuits usually require more transistors to achieve precise timing control [96]. Despite the higher complexity of PWM's pixel circuit, its core advantage is to avoid the wavelength shift of the centre of the micro-LEDs caused by current variations, thus solving the problem of colour casting of the display due to PAM [97]. In addition, the external circuit design of PWM varies depending on the implementation: analogue PWM requires the generation of a linearly varying \(V\)sweep signal, while digital PWM requires the generation of EM (enable) signals with different duty cycles via a timing controller, and the overall complexity may be higher than that of the PAM scheme. However, in low-refresh-rate scenarios, PWM can achieve lower power consumption by reducing the switching frequency. Samsung has solved the problem of wavelength shift in micro-LED displays due to variations in current density by using PAM and PWM driver units with external and internal compensation of \(V_{\text{TH}}\), respectively [98]. This circuit is not a PHM (pulse hybrid modulation) drive mode, as mentioned subsequently. The conventional PWM method combined with internal compensation circuitry makes it difficult to achieve high bits. [PERSON], [PERSON] et al. used a 7T1C pixel circuit [99] to improve the bit of a 240 Hz 10-line panel from 3 to 8 bits by converting equal subfields to unequal subfields and combining PWM method and internal compensation. [PERSON], [PERSON] et al.