(i) Zener diode: A properly doped junction diode which has a sharp reverse breakdown voltage is called a zener diode.
A zener diode is a special type of junction diode that is designed to operate in the reverse breakdown region. An ordinary junction diode operated in this regin will usually be destroyed due to excessive current. This is not the case for the zener diode we have seen that when reverse Noltage on a junction diode is increased, a critical voltage called reverse breakdown voltage is reached when the reverse current increases sharmly to high value.
(ii) LED (Light emitting diode): A light emitting diode (LED) is a diode that given off visible light when forward biased. Light emitting diodes are made from silicon or germanium but.are made by using elements like gallium, phosphorous and arsevic. By varying the quantities of these elements, it is possible to produce light of different wavelengths with colours that include red, green yellow and blue. For example, a LED manufactured with gallium arsenide it will produce a red light. If the LED is made with gallium phosphide, it will produce a green light.
