Avalanche Photodiodes

  1. What is an Avalanche Photodiode? What are the differences between APDs and PIN devices?
  2. Where does the word "Avalanche" come from?


1. What is an Avalanche Photodiode ? What are the differences between APDs and PIN devices?
In very low light level applications, since a very large value feedback resistor is needed, there is Johnson current noise associated with the resistor, which reduces signal to noise ratio, which is undesirable. Silicon Avalanche Photodiodes (APDs) make use of internal multiplication instead to achieve gain. The gain is due to impact ionization. APDs are also useful when the amplifier noise is limiting.

2. Where does the word "Avalanche" come from?
TPDs internal gain is triggered by a high reverse bias (~200-300V usually). Due to high bias the resulting strong electric field energize the electrons with enough kinetic energy to generate more electrons by collision ionization and starting a ''chain reaction'', or ''avalanche'' process.