People often use an APD photodetector to detect light, especially optical powers. Essentially, users understand it to be a photon detector that uses electric carriers’ photo-excitation. Operators will also get an electronic output signal from photodetectors like an electric current or voltage proportional to the incident optical power. As such, these are in the category of optoelectronics.

Types of APD Photodetectors

APD photodetectors come in different types regarding application requirements, which can be suitable in a specific circumstance:


This type of APD photodetector is based on vacuum tubes as a unique type of phototubes. Even for photon counting, they can exhibit an extremely high sensitivity combined with a large active area and a high speed. As many of these types are based on multichannel plates, they are not similar to the traditional photomultipliers because of their powerful compact.


These are gas-filled, or tubes as the users can exploit the photoelectric effect.

Photoconductive Detectors

They are based on specific semiconductors like cadmium sulfide. They are not as expensive as photodiodes and come with competitive prices. However, they display a nonlinear response, not quite sensitive, and are relatively slow. Fortunately, they are capable of responding to long-wavelength infrared light.


They are identical to photodiodes. However, they exploit photocurrent’s internal amplification. People don’t typically use them, unlike photodiodes.

Metal-Semiconductor-Metal Photodetectors

These types of APD photodetectors don’t have a p-n junction and have two Schottky contacts. They have hundreds of gigahertz of bandwidths and relatively faster than photodiodes.


These photodetectors have a p-i-n structure or p-n junction as semiconductor devices. With this, they can generate a photocurrent and absorb light in a depletion area. They can exhibit a high quantum efficiency since they are quite linear, fast, and compact. When users operate them in combination with suitable electronics, they can display a high dynamic range and generate about one electron per incident photon. Avalanche photodiodes are one of such sensitive types, and people sometimes use them for photon counting.

Photodetectors’ Essential Properties

With regards to the application, an APD photodetector must fulfill several needs like:

  • The detector’s active region may be vital, especially when operating with laser diodes’ strongly divergent beams. Users may also find it hard to achieve all the light on the active space due to non-constant beam divergence or high light sources. They can then measure the total power by using an integrating sphere with suitable calibration.
  • It is critical to have a high quantum efficiency and a high responsivity in some situations with the introduction of added quantum noise. This case affects single-photon detectors and applies to the detection of light’s squeezed states.
  • Operators must ensure the appropriate use of the detector for some optical power ranges. A nonlinear response or damage issues can limit the maximum detected power. Thus, the noise will determine the minimum power. The dynamic range magnitude is quite crucial. People typically specify it as the ratio of minimum and maximum detectable power. Photodiodes are some of the detectors that can display significant linearity over a dynamic range of over 70 dB.
  • When in a specific spectral area, it needs to be sensitive. Users must define the responsivity within wavelength range and should be constant.