Special Purpose Diodes:
Apart from rectification, P-N Junction diodes have other uses also. The diodes used for some special purposes are different form the diodes used for some other special purpose in properties, used semiconductor, quantity of impurities and construction, etc. Now we will study about some special purpose diodes.
Zener Diode:
In reverse biasing arrangement of P-N junction diode we saw that in the state of breakdown voltage the reverse current of the diode is not less but increases rapidly; but the potential at the ends of the diode ** approximately equal to the breakdown voltage meaning ** constant.
For any diode; breakdown voltage depends on the quantity of impurities mixed. Therefore, according to the quantity of the impure element any special breakdown voltage diode’* fabrication can be done. When any diode is so formed for any special breakdown voltage that in reverse biasing also at breakdown voltage it works without being damaged; then such type of diode is called zener diode. With the help of various quantities of impurity mixed today the diodes are available of less than 1 volt and in hundreds volt also. The sign of zener diode is shown in the figure.
Photo Diode:
The light of desired frequency when is falling on any semiconductor (electromagnetic radiation), then due to the energy obtained by the absorption, the electrons of the valence band of the semiconductor do make transition to the conduction band. In the such types of electrons a is generated corresponding to each electron in the valence band and hence in this way electron pair is generated. These electron pairs are different to those electron pairs generated by thermal energy. Clearly, in this process the conductivity of the semiconductor increases. Due to the incident light the increase in the conductivity of the semiconductor so observed is called photoconductivity and their effect is called photo-conductive effect. If the frequency of the photons of the incident light is v, and its energy hv is equal or more than the forbidden energy gap of the semiconductor Eg (hv ≥ Eg) then these photons shown above effect.
Photo diode is used in following devices:
Light detection.
Light operated switch.
Reproduction of sound in films.
For reading computer types and computer cards.
Light Emitting Diode (LED):
Light emitting diodes are those P-N junction diodes which in forward biasing emit light. When an electron present in the conduction band of semiconductor do transition make the present in the valence band recombination meaning, there is recombination of electron and then, in this process energy is released. Generally the electrons are at the minimum energy level of the conduction band (EC) and the holes are at the maximum energy level of the valence band (EV). Hence, energy released in the process is EC – EV = Eg meaning equal to the forbidden energy gap. In many semiconductor like silicon and germanium, this energy is released in the form of heat (thermal) but in gallium arsenide, phosphide (GaAsP) and gallium phosphide (GaP) like semiconductors, this energy is released in the form of visible light energy.
The intensity of light should be more for this. It is necessary that this type of recombinations are large in number. But for intrinsic and extrinsic both types of semiconductors such types of recombinations are in limited number. This is due to the reason that in intrinsic semiconductors the number of electrons pairs is less and in extrinsic semiconductors one type of charge carrier are in majority and the other type of charge carriers are in ****.
In semiconductors like GaP if the P and N region of the junction area the impurity is mixed in large quantity then the depletion region will be very thin. If the junction is in forward biasing then the large number of holes of P region move towards N region and the large number of electrons of N region move towards P region. Hence, near the depletion layer there would be large recombination of electrons and holes. As a result the emitted light will be of high intensity. The figure shows a light emitting diode.
The semiconductor for LED is chosen as per the desired colour light. At present the LED lights of red, green, yellow, orange and blue colour are available. The LED is used for following purposes:
In the form of indicator light.
In the form of seven segment display unit.
LED generating high intensity light are used in optical fibre communication.
High intensity light giving LED bulbs.
