are generated due to the absorption of light photons near the junction. Then the charge carriers are separated due to the electric field of the depletion region. Electrons move towards n –type silicon layer and holes move towards p -type silicon layer. The electrons reaching the n -side are collected by the front contact (metal finger contact) and holes reaching p -side are collected by the back of photodiode is exactly inverse to that of an LED.
Photodiode works in reverse bias condition. Its circuit symbol is shown in Figure . (a). The direction of arrows indicates that the light is incident on the photodiode.
The device consists of a p-n junction semiconductor made of photosensitive material kept safely inside a plastic case as shown in Figure . (b). It has a small transparent window that allows light to be incident on the p-n junction. Photodiodes can generate current when the p-n junction is exposed to light and hence are called as light sensors.
(a) p (b) Figure . (a) Circuit symbol (b) Schematic view of photodiode When a photon of sufficient energy ( hv ) strikes the depletion region of the diode, some of the valence band electrons are elevated into conduction band, in turn holes are developed in the valence band. This creates electron-hole pairs. The amount of electron-hole pairs generated depends on the intensity of light incident on the p-n junction.
These electrons and holes are swept across the p-n junction by the electric field created by reverse voltage before recombination takes place. Thus, holes move towards the p -side and electrons towards the n -side. When the external circuit is made, the electrons flow through the external circuit and constitute the photocurrent. When there is no incident light, there exists a reverse current which is negligible.
Figure . Cross-sectional view of a solar cell Sunlight Front electrical contacts Anti-reflective coating n -side Back electrical contact p -side Depletion region – – – – – – – - - - - - - - - Unit electronics and Communication a p -type material sandwiched between