In electronics , a diode is a two-terminal electronic component that conducts primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance to the current in one direction,and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is a crystalling piece of semiconductor material with a p–n junction connected to two electrical terminals. A vacuum tube diode has two electrodes, a plate (anode) and a heated cathode. Semiconductor diodes were the first semiconductor electronics devices. The discovery of crystals'rectifying abilities was made by German physicist Ferdinand Braun in 1874. The first semiconductor diodes, called cat's whisker diodes, developed around 1906, were made of mineral crystals such as galena.Today, most diodes are made of silicon, but other semiconductors such as selenium and germanium are sometimes used.
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The P-N Junction diode:
- The p-n junction forms a popular semiconductor device called p-n junction.
- The p-n junction has two terminals called electrodes, One each from p-region and n-region. due to two electrodes it called diode di + electrode.
- To connect the n and p-region to the external terminals, a metal is applied to the heavily doped n and p-type semiconductor region. such a contact between a metal and a heavily doped semiconductor is called ohmic contact.
- Such an ohmic contact has two important properties.
2. The voltage across the contact is very small , which do not effect the performance of the device. the arrowhead in the symbol indicates
- The p-region acts as anode while the n-region acts as cathode. the arrowhead in the symbol indicates the direction of the conventional Current.
- Applying external DC voltage to any electronics device is Called biasing.
- Depending upon the
polarity of the dc voltage externally applied to it , the biasing is classified
as forward biasing and reverse biasing.
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Depletion Layer:
When physical contact between p and n region is made, free electrons in n material diffuse across the junction into p material fig-a Diffuse of each electron from n to p,leaves a positive charge behind n region near the junction.Similarly, Diffusion of each hole from p to n , leaves a negative charge behind p region across the junction. as result of this diffusion , n region near the junction becomes positively and p region in the vicinity of junction becomes negatively charged, fig-b . these charge establish an electric field across the junction. When this Field grows strong enough, it stops further diffusion. Some electrons, as these diffusion from n to p recombine with hole in p-region and disappear. similar recombination occurs in n-region.
The width of depletion layer, is order of 0.0005 mm. In Equilibrium there is a potential difference of 0.7 across the depletion region in silicon and 0.3V across the depletion region in germanium.
The potential difference across the depletion Layer is Barrier potential.
Forward Biasing of P-N junction diode:
When positive terminal of a battery is connected to p-type material and negative terminal to n-type material fig-c the p-n junction is forward biased.positive terminal of battery sucks electrons from n material leaving holes travels through p material towards the negative charge at p-n junction and thus neutralize partly this negative charge. Similarly , negative terminal of battery injects electron into n layer. these electrons move through n material, reach the p-n junction thereby neutralizing partly the positive charge . as a result, width of depletion region reduced.
when electric field stops further diffusion, charge carrier don't move. as aconsequence , opposite charge on each side of junction produce immobile ions fig-b the region extending into both p and n semiconductor layers is called Depletion layer or space charge region.
The width of depletion layer, is order of 0.0005 mm. In Equilibrium there is a potential difference of 0.7 across the depletion region in silicon and 0.3V across the depletion region in germanium.
The potential difference across the depletion Layer is Barrier potential.
Forward Biasing of P-N junction diode:
When positive terminal of a battery is connected to p-type material and negative terminal to n-type material fig-c the p-n junction is forward biased.positive terminal of battery sucks electrons from n material leaving holes travels through p material towards the negative charge at p-n junction and thus neutralize partly this negative charge. Similarly , negative terminal of battery injects electron into n layer. these electrons move through n material, reach the p-n junction thereby neutralizing partly the positive charge . as a result, width of depletion region reduced.
When the p-n junction is forward biased as long as the applied voltage is less than the barrier potential, there con not be any conduction.when applied voltage is greater than barrier potential as well as diode will conduct.`
Reverse Biasing of P-N junction Diode:
In case p material is connected to negative terminal of the battery and n material to positive terminal of battery, then it can be deduced that width of depletion layer increase, fig-d. this biasing is called reverse biasing.
In reverse biasing ,negative terminal of battery attracts the holes in p-region and positive terminal of battery attracts the electrons in n region away from
the junction.
The resistance of the reverse biased diode is very high and the diode is said to be nonoperative in reverse biased .
However due to increase barrier potential, the free electrons on p side are dragged towards positive while holes on the n side are dragged towards negative of the battery.
This constitutes a current called reverse current. it flow due to minority charge carriers and hence its magnitude is very small. the reverse current
is of the few order microampers for Ge and nanoampers for Si diode.
Reverse Breakdown in diode:
If reverse biase voltage increase is beyond particular value, large reverse current can flow damaging the diode, this is called reverse breakdown of a diode.
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