Avalanche photodiodes are semiconductor light detectors (photodiodes) that operate at relatively high reverse voltages (usually in the tens or even hundreds of volts), sometimes only slightly below threshold. In this range, the carriers (electrons and holes) excited by the absorbing photons are accelerated by a strong internal electric field and then generate secondary carriers, which often happens in photomultiplier tubes. The avalanche process occurs only over a distance of a few micrometers, and the photocurrent can be amplified many times. Therefore, avalanche photodiodes can be used as very sensitive detectors, requiring less electronic signal amplification and therefore less electronic noise. However, the quantum noise and amplifier noise inherent in the avalanche process negate the previously mentioned advantages. The additive noise can be quantitatively described by the additive noise figure, F, which is a factor that characterizes the increase in electronic noise power compared to an ideal photodetector.

It should be noted that the amplification factor and the effective responsivity of the APD are very related to the reverse voltage, and the corresponding values of different devices are different. Therefore, it is common practice to characterize a voltage range in which all devices achieve a certain responsivity.

The detection bandwidth of avalanche diodes can be very high, mainly due to their high sensitivity, allowing the use of smaller shunt resistors than in normal photodiodes.

Generally speaking, when the detection bandwidth is high, the noise characteristics of the APD are better than the ordinary PIN photodiode, and then when the detection bandwidth is lower, the PIN photodiode and a low noise narrowband amplifier perform better. The higher the amplification factor, the higher the additional noise figure, which is obtained by increasing the reverse voltage. Therefore, the reverse voltage is usually chosen so that the multiplication process noise is approximately equal to that of the electronic amplifier, as this will minimize the overall noise. The magnitude of the additive noise is related to many factors: the magnitude of the reverse voltage, the material properties (in particular, the ionization coefficient ratio) and the device design.

Silicon-based avalanche diodes are more sensitive in the wavelength region of 450-1000 nm (sometimes can reach 1100 nm), and the highest responsivity is in the range of 600-800 nm, that is, the wavelength in this wavelength region is slightly smaller than that of Si p-i-n diodes. The multiplication factor (also called gain) of Si APDs varies between 50 and 1000 depending on the device design and the applied reverse voltage. For longer wavelengths, APDs require germanium or indium gallium arsenide materials. They have smaller current multiplication factors, between 10 and 40. InGaAs APDs are more expensive than Ge APDs, but have better noise characteristics and higher detection bandwidth.

Typical applications of avalanche photodiodes include receivers in fiber optic communications, ranging, imaging, high-speed laser scanners, laser microscopes, and optical time domain reflectometry (OTDR).