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प्रश्न
Write the important considerations which are to be taken into account while fabricating a p-n junction diode to be used as a Light Emitting Diode (LED). What should be the order of the band gap of an LED, if it is required to emit light in the visible range? Draw a circuit diagram and explain its action.
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उत्तर
For the formation of LED direct bandgap semiconductors like GaAs, CdS, etc should be used which emits light. The p-n junction should be operated in forwarding bias. Most of the Light is produced in p-area of the device so while fabricating LED this area is kept close to the surface of the diode so that the minimum amount of energy is absorbed. One can use different materials for producing different colored Lights. The semiconductor used for the fabrication of visible LEDs must at least have a bandgap of 1.8 eV. The order of the band gap if it is required to emit light in the visible range is about 3 eV to 1.8 eV.
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LED is a heavily doped P-N junction which under forward bias emits spontaneous radiation. When it is forward-biased, due to recombination of holes and electrons at the junction, energy is released in the form of photons. In the case of Si and Ge diode, the energy released in recombination lies in the infrared region. LEDs that can emit red, yellow, orange, green and blue light are commercially available. The semiconductor used for fabrication of visible LEDs must at least have a band gap of 1.8 eV. The compound semiconductor Gallium Arsenide – Phosphide is used for making LEDs of different colours.
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OR
iii. Draw V-I characteristic of a p-n junction diode in
(i) forward bias and (ii) reverse bias
Draw the circuit diagram of an illuminated photodiode and its I-V characteristics.
