# Visible Light Communication System Technology Review: Devices, Architectures, and Applications
## 3 VLC Receiver Technology
Transit time is the time it takes for carriers generated by an optical signal to be swept out of the detector's active region. The velocity of the carriers is given by the following Equation (6):  
\[v=\mu E \tag{6}\]  
where \(\mu\) is the carrier mobility and \(E\) is the electric field. When biased with a high enough electric field, the carriers will reach a saturation velocity, \(v_{sat}\). The mobility is thus modified to be as Equation (7):  
\[\mu_{sat}=\frac{\mu}{\sqrt{1+\left(\frac{\mu E}{v_{sat}}\right)^{2}}} \tag{7}\]  
The bandwidth is then given by Equation (8) [102]:  
\[f_{transit}=0.38\frac{v_{sat}}{h_{PD}} \tag{8}\]  
where \(h_{PD}\) is the full height of the photodetector. Another parameter in determining the bandwidth of a detector is the electrical impedance characteristic of the detector circuit, also known as the RC parasitic response. The bandwidth due to RC limitations is shown in Equation (9):  
\[f_{RC}=\frac{1}{2\pi RC} \tag{9}\]  
The total bandwidth, which includes both \(RC\) and transit time effects, is given by Equation (10):  
\[BW=\left(\frac{1}{f_{RC}^{2}}+\frac{1}{f_{transit}^{2}}\right)^{-1/2} \tag{10}\]  
DC current may occur in the detector, even with the absence of the photons, which is known as dark current. Sufficient levels of dark current can result in a decrease in the signal-to-noise ratio. Bulk generation and surface generation are considered when calculating the total dark current. Bulk generation is a volume-dependent mechanism in which a significant lattice mismatch between germanium and silicon causes threading dislocations, allowing for mid-bandgap states, and results mainly from the Shockley-Read-Hall process [103]. Although the bulk current density is relatively constant at a low electric field, as the electric field increases, band bending will result in a higher bulk current density that increases exponentially with the applied electric field. Surface generation is another factor that leads to dark current, and results from surface defects such as dangling bonds. Surface passivation is more difficult with germanium than silicon as germanium is not fully passivated by silicon dioxide. Other materials such as germanium oxide have been used for passivation with some success [104]. The total dark current is given by Equation (11):  
Figure 7:0  
where \(A\) is the junction area. Suppose we assume that bulk current dominates; then, the dark current scales with detector area. There is an approximately linear increase of dark current with the area of the detector. The shot noise is usually the dominant noise factor in germanium on silicon photodetectors. The shot noise due to the dark current is given by Equation (12):  
Figure 7:1  
The avalanche photodiode (APD), PIN photodetector, metal-semiconductor-metal photodetector (MSM-PD), superlattice avalanche photodetector (SL-APD), waveguide photodetector (WGPD), and cavity-enhanced photodetector (CE-PD), are the most used photodetectors in optical communications. The free-space VLC receiver with large-area APD has a \(-3\) dB modulation bandwidth of 420 MHz [105]. With a data rate of 56 Mbps, triple cationic perovskite solar cells can also be used as VLC receivers [106]. The bandwidth is limited by the RC time constant in the perovskite solar cell receiver. A high-performance surface-absorbing semipolar InGaN/GaN PIN-based micro-PD demo a \(-3\) dB modulation bandwidth of 347 MHz, suggesting its promising performance for VLC applications [107]. A black silicon photodetector is developed that shows high gain in the visible-NIR regime (400-1200 nm). A high responsivity of 1097.60 A/W at 1080 nm is reported, indicating its advantages for weak light detection. Table 3 summarizes the recent research achievements in receiver technology for VLC.