# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 3 Sources and Detectors for Lidar Imaging Systems in Autonomous Vehicles
### Photodetectors
#### 3.2.2 Photodetectors in Lidar Imaging Systems  
(a) PIN photodiodes  
A PIN photodiode is a diode with a wide, undoped intrinsic semiconductor region between a p- and a n-doped regions. When used as a photodetector, the PIN diode is reverse-biased. Under this condition the diode is not a conductor, but when a photon with enough energy enters the depletion region it creates an electron-hole pair. The field of the reverse bias then sweeps the carriers out of the region creating a current proportional to the number of incoming photons. This gives rise to a photocurrent sent to an external amplification circuit. The depletion region stays completely within the intrinsic region, and it is much larger than in a PN diode and almost constant-sized, regardless of the reverse bias applied to the diode. When using PIN diodes as receivers, most of photons are absorbed in the I-region, and carriers generated therein can efficiently contribute to the photocurrent. They can be manufactured with multiple different materials including Si, InGaAs, CdTe,... yielding wide options regarding their spectral response, although the most usual single-element detectors used are based on Si and InGaAs. PIN photodiodes do not have any gain (G = 1) (Figure 12), but may present very large bandwidths (up to 100 GHz, depending on its size and capacitance), dimensions of millimeters, low cost, low bias and large QE. However, due to their features their sensitivity is not enough for low light applications. In lidar imaging systems, they are typically used as detectors in pulsed lidars to raise the start signal for the time-to-digital converter at the exit of the laser pulse. (b) Avalanche photodiodes  
An avalanche photodiode (APD) is a diode-based photodetector with an internal gain mechanism operated with a relatively high reverse bias voltage (typically tens or even hundredths of volts), sometimes just below the breakdown of the device, which happens when a too-large reverse bias is applied. As with a conventional photodiode detector, the absorption of incident photons generates a limited number of electron-hole pairs. While under high bias voltage, a strong internal electric field is created which accelerates the carriers generated and creates additional secondary electrons, in this case by impact ionization. The resulting electron avalanche process, which takes place over a distance of only a few micrometers, can produce gain factors up to a few hundredths but is still directly proportional to the incoming optical power and to the reverse bias. The amplification factor of the APD dictates the number of photoelectrons that are created with the successful detection of each photon, and, thus, the effective responsivity of the receiver. Gain may vary from device to device and strongly depends on the reverse voltage applied. However, if the reverse voltage is increased further, a voltage drop occurs due to the current flowing through the device load resistance. This means that the value of the maximum gain has a dependence on the photocurrent. Although there is a large linear region of operation where the output photocurrent presents gain proportional to the power of the incoming light, when the APD is operated near its maximum gain (and thus close to the breakdown voltage) the APD response is not linear anymore, and then the APD is said to operate in Geiger mode. The level of gain required to obtain the optimal SNR value will often be dependent on the amount of incident light (Figure 12). Several excellent reviews of APDs in detail are available, e.g., [128]. APDs are very sensitive detectors. However, the avalanche process itself creates fluctuations in the generated current and thus noise, which can offset the advantage of gain in the SNR. The noise associated with the statistical fluctuations in the gain process is called excess noise. Its amount depends on several factors: the magnitude of the reverse voltage, the properties of the material (in particular, the ionization coefficient ratio), and the device design.