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A diode is a semiconductor device, which allows current to flow through it in only one direction.
Types of diode
Diodes are categorized based on their working. Different types of diode are Thermionic diodes, semiconductor diodes & Shockley diode.
Thermionic diodes
Thermionic diodes are vacuum tube devices (or thermionic valves) having electrodes surrounded by a vacuum within a glass envelope.
Working
In Thermionic diodes, initially, current is made to pass through the cathode, which comprises of filament treated with a mixture of barium and strontium oxides. The passing of the current through the filament causes filament heating, resulting in the thermionic emission of electrons into the vacuum envelope. In forward operation, anode-- a surrounding metal electrode, is positively charged, to electrostatically attract the emitted electrons. However when the voltage polarity is reversed, electrons are not easily emitted by the unheated anode surface due to which any reverse flow constitues a very tiny current.
Applications
-Analog signal applications, amplifiers and as rectifiers in power supplies.
Semiconductor diodes
The working of semiconductor diode is based on P-N junction consisting of holes and electrons. In a p-n diode, conventional current can flow from the p-type side (the anode) to the n-type side (the cathode), however the current is not capable of flowing in reverse direction. There are several types of semiconductor junction diodes, which are as follows:
1. P-N diodes
2. Zener diodes
3. Avalanche diodes
4. Photodiodes
5. Light-emitting diodes (LEDs)
6. Laser diodes
7. Schottky diodes
8. Current-limiting field-effect diodes
Forward biasing of diode:
When voltage is applied to semiconductor diode with positive voltage applied to p-side and negative voltage applied to n-side, holes from p-side region combine with electrons in n-side region, resulting in narrowing of depletion layer and flow of current from p-side to n-side.
Reverse biasing of diode:
When voltage is applied to semiconductor diode with negative voltage applied to n-side and positive voltage applied to p-side, holes from p-side region move towards in a direction opposite to n-side region and electrons in n-side region move in a direction opposite p-side region resulting in widening of depletion layer.
Another type of semiconductor diode, the Schottky diode, made of the contact between a metal and a semiconductor rather uses both PNP and NPN transistor characteristics to conduct.
Shockley diodes
The working of shockley diodes is based on working of a pair of interconnected PNP and NPN transistors. When shockley diodes is turned on, they continue to remain in the same state i.e. turned on state(latched), and stay off once they have been turned off. There are two ways to latch a Shockley diode, which are as follows:
1. Exceeding the anode-to-cathode breakover voltage
2. Exceeding the anode-to-cathode critical rate of voltage rise
The only way to stop Shockley diode from conducting is to reduce the current going through it to a level below its low-current dropout threshold.
Applications
Diodes are used in various electronic devices, such as radio, television, computer, lasers, alarm, temperature measurement, charge coupled devices, logic gates, detectors, power conversion and over voltage protection, etc.
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