The two type materials namely p-type and n-type are chemically combine  with  a special fabrication  technique to form a p-n junction.  the p-n junction  form a popular semiconductor device called diode.  Diode is a basic element  of electro-nics circuits.   its understanding is very important  for  operation  of electronics circuits , applications and devices.    

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.


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.
         1. It conducts current equally in both the directions.
         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. 
     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.   
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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