# Visible Light Communication System Technology Review: Devices, Architectures, and Applications
## 2 VLC Transmitter Light Source
### Theoretical Background of LED for VLC Application
The polarization in the c-plane-oriented InGaN quantum well produces a robust quantum confinement Stark effect (QCSE), which separates electrons and holes, thereby increasing the time for radiation recombination [31; 32]. As a result, the device's performance will dramatically reduce, and the Auger effect QCSE will cause significant peak changes but also cause a significant drop in luminous efficiency under high injection currents [31; 33]. To mitigate this polarization field, growing quantum wells in semipolar or non-polar directions allows better overlap of electrons and holes, which leads to faster radiation recombination, especially with thicker quantum wells [34; 35; 36; 37; 38; 39; 40]. It was expected that a higher electro-optical bandwidth and potentially reduced Auger effect could be achieved by crystal orientation. With the advantages of semipolar LEDs in luminous efficiency and wavelength stability, researchers have expanded their applications; for instance, in high-resolution displays and high-speed visible light communications. Nevertheless, better implementation of VLC includes improving the modulation characteristics of LED light sources and the emission efficiency under high-speed modulation, which depend on their carrier recombination lifetimes. Semipolar LEDs with less QCSE have a faster radiation recombination rate [30]. However, \(\mu\)-LEDs have a better frequency response, low power consumption, and higher brightness [36]. The high brightness of a \(\mu\)-LED leads to a faster transmission data rate and lower bit error rate (BER). By using a high-speed photodetector to detect the modulated LED photocurrent signal, the \(-3\) dB frequency is related to specific diode parameters. The frequency response has been evaluated using a fixed current. The LED cut-off frequency was affected by the carrier lifetime and Resistor-Capacitor (RC) time constant composed of depletion layer capacitance and the junction differential resistance, which determines the transmission data rate of the LED. The modulation bandwidth can determine the data rate of the VLC application, so the \(-3\) dB frequency bandwidth of LED can be related to the resistance-capacitance time constant (\(\tau_{RC}\)) and the minority carrier lifetime, as shown in the Equation (1) [37]:  
\[f_{-3\text{dB}}=\frac{\sqrt{3}}{2\pi\tau}=\frac{\sqrt{3}}{2\pi}\bigg{(}\frac{1 }{\tau_{r}}+\frac{1}{\tau_{nr}}+\frac{1}{\tau_{RC}}\bigg{)} \tag{1}\]  
where \(\tau_{r}\) and \(\tau_{nr}\) represent the radiative and non-radiative carrier lifetime, respectively. The modulation bandwidth of GaN LEDs is mainly determined by the RC time constant and carrier lifetime [41; 42; 43; 44]. The modulation bandwidth can be increased due to the decrement of the carrier lifetime by increasing the injected current density [38; 45]. However, increasing the current density may cause the quantum efficiency and LED performance to decrease [46]. Several reports have indicated the carrier recombination process in GaN-based LEDs would be influenced by carrier localization [47; 48; 49] and the QCSE [50; 51], \(\mu\)-LEDs can achieve a smaller carrier lifetime and higher modulation bandwidth because they can withstand higher injection current density. As the current density increases, QCSE will become less effective, increasing the optical power and modulation bandwidth [52]. Because the \(\mu\)-LED has a smaller active area, the relative geometric capacitance and RC time constant, the main parameters limiting the modulation bandwidth, will be reduced. Growing LEDs on semipolar or non-polar substrates increased the modulation bandwidth, corresponding to a higher radiation recombination rate. GaN-based \(\mu\)-LEDs have excellent performance in high brightness, high contrast, fast response time, extended service life, and low power consumption. Because of its ultra-small light-emitting area, a single \(\mu\)-LED has a low power emission. By assembling multiple \(\mu\)-LEDs into array, the output light power can be increased.