Photodiode

A photodiode is a semiconductor diode that functions as a photodetector. Photodiodes
are packaged with either a window or optical fibre connection, in order to let in the light
to the sensitive part of the device. They may also be used without a window to detect
vacuum UV or X-rays.A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent case so that light can reach the base-collector junction. The phototransistor works like a photodiode, but with a much higher sensitivity for light, because the
electrons that are generated by photons in the base-collector junction are injected into the
base, and this current is then amplified by the transistor operation.

However, a phototransistor has a slower response time than a photodiode.A photodiode is a p-n junction or p-i-n structure. When light of sufficient photon energy strikes the diode, it excites an electron thereby creating a mobile electron and a positively charged electron hole. If the absorption occurs in the junction’s depletion region, these carriers are swept from the junction by the built-in field of the depletion region, producing a photocurrent.Photodiodes can be used under either zero bias (photovoltaic mode) or reverse bias.

In zero bias, light falling on the diode causes a voltage to develop across the device, leading to a current in the forward bias direction. This is called the photovoltaic effect, and is the basis for solar cells — in fact, a solar cell is just a large number of big, cheap photodiodes.Diodes usually have extremely high resistance when reverse-biased. This resistance is reduced when light of an appropriate frequency shines on the junction. Hence, a reversebiased diode can be used as a detector by monitoring the current running through it.

Circuits based on this effect are more sensitive to light than ones based on the
photovoltaic effect.Avalanche photodiodes have a similar structure, but they are operated with much higher reverse bias. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.