Avalanche photodiodes (APDs) are a type of photodetector that utilizes avalanche multiplication to amplify optical signals. With high sensitivity, low noise, and fast response, APDs have found wide applications in the field of optoelectronic detection.
APDs belong to a category of photodetectors that can sense incident light signals and convert them into enlarged electrical outputs via avalanche gain. Compared with traditional photodiodes, APDs can provide higher gain and sensitivity.
In APDs, the photogenerated carriers accelerated by strong electric fields collide with lattice atoms to create more electron-hole pairs, resulting in an avalanche multiplication effect that eventually transforms into a magnified electrical signal.
Owing to their high sensitivity, low noise, and fast response, APDs have been widely used in optical communications, lidars, and other sensing technologies. For instance, in self-driving vehicles, APDs enhance the accuracy and reliability of vehicular optical communication systems and LiDAR systems.
As an important optoelectronic device, APDs show great potential in various cutting-edge technologies with the development of material science and fabrication processes.
Figure 1. Image of an avalanche photodiode (APD). (Source: ams Technologies)
Figure 2. Schematic diagram of an APD structure. A typical APD has a similar structure to a PIN photodiode, consisting of two highly doped (p+ and n+) regions and two lightly doped (I region or intrinsic region and P region) areas. Compared with PIN photodiodes, the depletion width in the I region of APDs is relatively thin. The p+ region acts as the anode while the n+ region acts as the cathode. The reverse bias voltage is mainly applied across the pn+ junction.
Figure 3. Working principles of an APD. A reverse bias voltage is applied with the p+ region connected to the negative terminal and n+ region tied to the positive terminal of the battery. Photo-generated electron-hole pairs in the intrinsic region are driven by the electric field towards the high-field pn+ junction, where avalanche multiplication takes place through impact ionization.