In the photodiode, convert the modulated light back into an electrical signal. As the intensity of an optical signal at the receiver is very low, the detector has to meet high-performance specifications.
The conversion efficiency must be very high at the operating wavelength. The speed of the response must be high enough to ensure that the signal distortion does not occur. It must be possible to operated continuously over a wide range of temperature for many years.
At present, these requirements are met by a reverse biased p-n photodiode. In these devices, the semiconductor material will absorbs a photon of light, which excites an electron from the valence band to the conduction band.
The photogenerated electron leaves behind it a hole, and so each photon generates two change carriers. This increases the material conductivity so-called photo conductivity resulting in an increase in the diode current.
V-I characteristics of photodiode :
The characteristics of the photodiode are shown in clearly in the following figure. A photodiode is operated in a reverse bias mode. To show in figure photocurrent is nearly independent of reverse bias voltage which is applied. For zero luminance, the photocurrent is almost zero excluding for small dark current to shown in the figure.
 |
| V-I characteristics of a photodiode |