Photodiode
A Photodiode is a special purpose p-n junction diode which used for detecting optical signal (photo-detectors). It is the semiconductor equivalent of ‘photocell’. It works on the same principle of ‘photovoltaic effect’.
Construction of Photodiode:
A Photodiode is a pn-jnction diode, constructed with a transparent window to allow light to fall on the diode (at its charge depletion region). It is operated under reverse bias.
Working of Photodiode:
The photodiode works when it is illuminated with light (photons) with energy (hν) greater than the energy gap (Eg) of the semiconductor. When an electron, originally present in the covalent bond, receives a photon of energy more than Eg , it breaks the bond and cause the generation of e-h pairs takes place in or near the depletion region of the diode of electron-hole pair.
Due to electric field of the junction, electrons and holes are separated before they recombine. This causes the generation of extra charge carriers in the material and thereby increases the current. The magnitude of the photocurrent depends on the intensity of incident light (photocurrent is proportional to incident light intensity).
Question: What is the reason to operate the photodiodes in reverse bias?
Answer:
The reason is, for a given intensity of light, the percentage change in the number of minority charge carrier is more significant than that for the majority charge carriers. As, in case of reverse bias current is determined by minority charge carriers, the change in current is more apparent in reverse bias condition. Hence, photodiodes are preferably used in the reverse bias condition for measuring light intensity.
- It is easier to observe the change in the current with change in the light intensity, if a reverse bias is applied.
- Thus photodiode can be used as a photo-detector to detect optical signals.