1where M is the gain, eta is the quantum efficiency, and nu is the frequency of light. This is simply the number of electrons that are ionized divided by the energy of the photon multiplied by a gain factor.
Photodiodes are popular detectors for optical instruments. To create one, a PN junction is created in a semiconductor. Light entering the semiconductor can be absorbed, creating electron-hole pairs.1 The electron is then swept to the cathode, creating a photocurrent.
When using photodiodes to measure light intensity one needs to know the responsivity of the photodiode. The responsivity is the ratio of the output current to the power of the light incident on the photodiode. It can be related to the gain by
1
where M is the gain, eta is the quantum efficiency, and nu is the frequency of light. This is simply the number of electrons that are ionized divided by the energy of the photon multiplied by a gain factor.
If a large enough electric field is applied to the semiconductor free electrons can aquire enough energy to free other electrons through impact ionization. A chain reaction can occur where new electrons free more electrons. This is known as the avalanche effect.1 Photodiodes that take advantage of this avalanche effect are known as Avalanche Photo Diodes.
The advantage APDs offer to light detection is their large gain. Photodiodes gave a gain of about 1 while it is possible to achieve gains of over 100 using APDs.1 APDs can also be used in a high bandwidth applications.
There are some disadvantaged that come with an APDs large gain. The avalanche effect is noisy and amplifies dark current. The APD's responsivity and breakdown voltage is also very temperature dependant.