The light signals are converted into electrical signals that have a modulated radiant power. In a LiDAR system, the photo sensors are used for high demands in regard to the sensitivity and response speed of these components.
The sensitivity of the light is based on the principle of a semiconductor diode where the absorption of photons is a resulting generation of charge carrier pairs.
A simple version of silicon photo sensors was formed by a p (+)/n (-) diode. The formation is done by a lightly doped n-substrate with a highly doped p+ layer at an implanted surface. The layer is a light-sensitive but thin as possible with many photos that are transferred in the p/n junction for penetration. The capacity is provided by p/n junction, operating in blocking direction to determine the dynamic behaviour of the photo sensor.
Another case of a diode is a PIN diode, where an intrinsic layer is used to divide the p/n junction. A simplified version is created of a plate capacitor structure where the capacitance depends upon the intrinsic layer’s depth. The parasitic capacitance is reduced due to a simple p/n photodiode where the response speed can be reduced or an increase in the bandwidth is possible significantly.
The third case is about APD, known as avalanche photodiodes. The p-doping method is used for the transition from the intrinsic to the n+-layer. A high field strength zone is created in such an area where the charge carriers are further impacted by ionisation. The ionisation process is known as the avalanche effect or charge carrier multiplication. It is said that like its contemporary PIN diode, APD function, also in the same way but with an additional internal amplification with a factor of several 100.
The disadvantage of APD is the increased sensitivity where a reverse voltage of over 100V is applied which results in increased costs and the costs of the application circuit.
For close-up ranges up to 100 m approx., PIN diodes application is used for optical distance measurement.
APDs are used for a longer distance for representative measurements.