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Zener diode

It is a highly doped p–n junction which is not damaged by high reverse current. It can operate continuously, without being damaged in the region of reverse background voltage. In the forward bias, zener diode acts as ordinary diode. It can be used as voltage regulator (Fig. 17).
Fig. 17

Light emitting diode (LED)

Specially designed diodes, which give out light radiations when forward biased (Fig. 18). LED are made of GaAsP, GaP, etc.
These are forward-biased P–N-junctions which emit spontaneous radiation.

Fig. 18

Photo diode

Photo diode is a special type of photo-detector. Suppose an optical photon of frequency ν is incident on a semiconductor, such that its energy is greater than the band gap of the semiconductor (i.e. hν > Eg). This photon will excite an electron from the valence band to the conduction band leaving a vacancy or hole in the valence band which obviously increases the conductivity of the semiconductor. Therefore, by measuring the change in the conductance (or resistance) of the semiconductor, one can measure the intensity of the optical signal (Fig. 19).
Fig. 19

Solar cells

A solar cell is based on the photovoltaic effect. One of the semiconductor region is made so thin that the light incident on it reaches the P–N-junction and gets absorbed. Solar cell converts solar energy into electrical energy.

Fig. 20

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