PHOTODIODE & SOLAR CELL


Photodiode

A photodiode is a special type of junction diode used for detecting optical signals.
It is a reverse biased p-n junction made from a photosensitive material.In photodiode,current carriers are generated by photons through photo excitation.Its symbol is



Construction
A photodiode is fabricated with a transparent cover to allow light to fall on the diode and operated under reverse bias.



Working
When the photodiode is illuminated with light (photons),with energy greater than the energy gap of the semiconductor,then electron-hole pairs are generated due to the absorption of photons.
These charge carriers contribute to the reverse current.

V-I Characteristics

Its V-I characteristics are shown in figure.We observe from the figure that current in photodiode changes with the change in light
intensity(I) when reverse bias is applied.





Solar cell
Solar cell is a p-n junction diode,which converts solar energy into electrical energy.

Its symbol is



Construction
It consists of a silicon or gallium-arsenide p-n junction diode packed in a can with glass window on the top.



Working
When photons of light (of energy hv>Eg) falls at the junction,electron-hole pairs are generated near the junction and they move in opposite diractions due to junction field.They will be collected at the two sides of the junction,giving rise to a photovoltage between the top and bottom metal electrodes.The top metal contact acts as a negative electrode.When an external load is connected across metal electrodes, a photocurrent flows.

I-V Characteristics
The I-V characteristics of solar cell are shown in the figure.We can see in the figure,that it is drawn in the fourth quadrant of the coordinate axes, because  a solar cell does not draw current but supplies the same to the load.


SANIKA PHADKE
HARSHITHA SUNKAVALLI

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