This is another type of diode, it
is also a semiconductor diode designed in such a way which, when exposed to
light, generates a potential difference or change its electrical resistance. In
its operation, when photons of light falls on the semiconductor diode,
electrons absorbs energy from the photons, become photo-excited and jump into the conduction band, also leaving holes
in the valence band, transport of the free electrons and holes increases the
electrical conductivity of the material, when an external voltage is applied it
results in a current flow. This is known as Electron-hole
photo-generation in the semiconductor diode.
Figure 3.13: Electron-hole
photo-generation in Semiconductor diode.
Figure 3.14: Symbol of a photo-diode.
The circuit symbol of a photo-diode
is similar to that of an LED just that the arrows points inward in the case of a
photo-diode.
The photo-diode operates in the
reverse bias mode, even though is a semiconductor diode and we know it will
impose a very high resistance to the flow of current in reversed biased, this
is to ensure that the current observed in a circuit involving a photo-diode is
solely due to the intensity of the incident light, i.e. photons energy.
Photo-diodes has many applications
in electronics design; an example of which is an Opto Isolator.
In some circuit we may need to
electrically isolate two different parts of the circuit, i.e. there should be
no connections at all but we may still want what happens in one of the circuit
to have an effect on the other.
Figure 3.13: Opto
Isolated Circuits
For example the circuit in figure 3.13,
when voltage Vs changes, the intensity of LED L also varies and in essence
varies the intensity of light falling on the photo-diode P, which in-turn
changes resistance of the photo-diode and this can be detected at V0.
Therefore, though the circuit is perfectly isolated with reference to electrical connections,
but through light beam they are connected, this is called and Opto-isolator.
See Also:
Varactor Diode
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