# Mini-LED and Micro-LED: Promising Candidates for the Next Generation Display Technology
## 3 Micro-LED
### RGB Micro-LED Full-Color Display  
The principle of RGB full-color display is based on the law of three primary colors that can be combined to create all colors in nature via a certain ratio setting (Figure 8). Therefore, for RGB LEDs, different currents are applied to control the brightness of each LED to realize the combination of three primary colors and achieve the full-color display. It is the method that is usually employed in outdoor LED large screens. In the RGB full-color display method, each pixel contains a set of RGB micro-LEDs. Generally, the P and N electrodes of the three-color micro-LEDs are connected to the circuit substrate by means of bonding or flip-chip. After that, a dedicated full-color driver chip is used to drive each color of micro-LEDs via the pulse width modulation (PWM) current. The PWM current drive method can achieve digital dimming by setting the duty cycle of the current. For example, an 8-bit PWM full-color driver chip can achieve  
Figure 8: Mechanism of RGB micro-LED full-color display. \begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & **Company** & **Principle** & **Description** \\ \hline Electrostatic array & LuxVue & \begin{tabular}{c} Electrical \\ pick-Up array \\ \end{tabular} & The transfer heads are divided by the dielectric layer to form a pair of silicon electrodes, which are positively and negatively charged, respectively, before picking up the target LED. \\ \hline Magnetic array & ITRI & \begin{tabular}{c} Electromagrenfe \\ pick-Up array \\ \end{tabular} & Micro-LEDs are adsorbed and placed by the electromagnetic force generated by the coil. \\ \hline Elastomer stamp & X-Celeprint & \
\begin{tabular}{c} Visco-elastic \\ pick-up array \\ \end{tabular} & The pick-up and transfer processes are aided by the Van der Waals forces between the viscoelastic elastomer stamp and the solid micro-LEDs. \\ \hline Roll to plate & KIMM &
\begin{tabular}{c} A roll-based transfer technology for transferring \\ nanoscale objects from a donor substrate to a target substrate with high yields and productivity. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Massively selective transfer printing methods. \(2^{8}=256\) kinds of dimming effects of a single-color micro-LED, then for a pixel containing three-color LED theoretically can achieve 256\({}^{3}=\) 16,777,216 kinds of dimming effect which means the same number of colors can be displayed. Figure 9 demonstrates the specific full-color display driving principle [38]. However, the RGB micro-LED based technology suffers from a severe disadvantage in mass production. For example, in order to manufacture a 4K resolution display, nearly 25 million micro-LEDs are needed to be assembled and connected without a single error in an economical and efficient way, with placement accuracy of 1 \(\upmu\)m or less. Obviously, it is very difficult to transfer or grow such a huge number of three different micro-LEDs on the same substrate. As an ideal solution, all RGB micro-LEDs would be composed of the same material with the same behaviors and driving conditions. The challenge is to find one material capable of spanning the blue to red spectrum. Theoretically, the InGaN alloy can cover the entire visible range by adjusting the indium content to fine tune the peak emission wavelength. Unfortunately, high indium content in GaN-based LEDs results in poor quality because of lattice mismatches between the GaN buffer layers and the InGaN quantum wells. To address these issues, [PERSON] et al. developed an innovative substrate called InGaNOS (InGaN pseudo-substrates) that overcomes lattice mismatch [39]. The substrate has a top relaxed InGaN layer that can be used as a seed layer for full InGaN LED growth, as shown in Figure 10a. The experimental results illustrate that InGaN structures grown on InGaNOS substrates can span the spectrum from blue (482 nm) to red (617 nm) [40], as shown in Figure 10b.