Properties of p-n junction,junction barrier

What is potential barrier or junction barrier ?
Answer is :
A potential difference built up across the p-n junction which restricts
further movement of charge carriers across the junction is known as potential barrier.

Lets see how,
To understand the properties of a p-n junction, consider two types of extrinsic semiconductor :
1) p-type
2) n-type
The p-type semiconductor is having  negative acceptor ions and positively charged holes.
note that, the acceptor impurity atom is short of one electron and has the ability to accept one electron to fill the covalent bond therefore,it becomes a negative ion.
Shown below,

Whereas, n-type semiconductor is having positive donor ions and negatively charged electrons .
here , the donor impurity atom donates one electron to the semiconductor crystal, therefore it becomes a positive ion.
Shown below,

When the two pieces are joined together and suitably treated they form a p-n junction.

The moment they form a p-n junction some of the conduction electrons from n-type material diffuse over to the p-type material and undergo electron-hole recombination with the holes available in the valence band.


Simultaneously, holes from p-type material diffuses over the n-type material and undergo hole-electron recombination with the electrons available in the conduction band.


This process is called diffusion.

In this process, some of the free electrons move across the junction from n-type to p-type, leaving behind positive donor ions as they are robbed of the free electrons.
This establish a positive charge on the n-side of the junction.


Simultaneously , the free electrons which cross over the junction recombine with the holes of p-type and uncover some of the negative acceptor ions.
This establishes a negative charge on the p-side of the junction.

This process of diffusion continues till a sufficient number of donor and acceptor impurity ions are uncovered and establish a requisite potential difference.
(.i.e. is nearly 0.3 volt in case the p-n junction is formed of Germanium semiconductor and 0.7 volt for Silicon p-n junction).


After this, further diffusion is prevented because now positive charge on n-side repels holes to cross from p-type to n-type and negative charge on p-side repels free electrons to enter from n-type to p-type .


Thus, a potential difference created across the junction act as a barrier which restrict further movement of charge Carriers i.e. holes and electrons.
This is called a potential barrier or junction barrier Vo.



what is depletion layer ?
A region around the junction from which the charge carriers ( free electron and holes ) are depleted is called depletion layer.

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