p-n junction

Defined as :
When a p-type semiconductor is suitably joined to an n-type semiconductor, the contact surface so formed is called a p-n junction.
or
In other words,
If we join a piece of p-type material to the piece of N-type material such that the crystal structure remains continuous at the boundary, a p-n junction is formed.
Such a p-n junction makes a very useful device,which is called a semiconductor diode.

Furthermore, All the Semiconductor devices contain at least one p-n junction. So to understand the the working of various Semiconductor devices, it is essential for the readers to have a knowledge of p-n junction and its properties.This will help us to understand even those devices which have more than one p-n junction.

In actual practice, the p-n junction is not formed by just bringing a p-type semiconductor block near to an n-type semiconductor block as it looks.

Actually , p-n junctions are made by special techniques namely:
1.Growing
2.Alloying
3.Diffusion methods.

In which, growing and diffusion methods are the most common method of making a p-n junction .

First method,
1) Alloying :
An alloyed junction is made from an n-type slice of semiconductor by melting a pellet of trivalent indium placed on the slice. Step-wise, shown in figure,
This is done by heating the system to about 500 degree centigrade.
Then the Indium is absorbed into the Germanium or Silicon to produce a p-region.
Hence a p-n junction is formed.

Second method,
2) Diffusion :
However, diffusion process is also used in formation of a p-n junction. In this process , the semiconductor wafers of  one conductivity i.e. either p or n type are placed in a vessel that contains an oxide of the impurity to be added. The combination is passed slowly through a furnace with accurately known and controlled temperature of 800 degree centigrade to 1200 degree centigrade depending upon the type of junction desired .
At such a high temperature, a gas of impurity atom diffuses into the semiconductor material and forms a thin layer of opposite conductivity .

Thus , resulting in the formation of p-n junction .

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