P-type semiconductor

 

Defined as:
When a small amount of a trivalent impurity is added to a pure semiconductor providing a large number of holes in it , the extrinsic semiconductor thus formed is known as p-type semiconductor.

Let us see how,
For making an n-type semiconductor ,we add a pentavalent impurity to an intrinsic semiconductor. If we add a trivalent impurity such as (Boron, Aluminium,gallium or Indium) to the intrinsic semiconductor,the result is a p-type semiconductor .
Let us consider a sample of intrinsic silicon to which a very small amount of Boron is added. 

The boron atom in the crystal has only three valence electrons which form covalent bond with three neighbouring Silicon atom. The fourth neighbouring silicon atom is unable to form a covalent bond with the Boron atoms because the Boron atom does not have the fourth electron in its valence Orbit.There is a deficiency of an electron around the Boron atom.

Acceptor impurities :
The addition of trivalent impurity such as gallium, indium (atomic number 31,49 respectively), provide a large number of holes in the semiconductor.Such impurities which produce p-type semiconductor are known as acceptor impurities because each atom of them create one hole ,which can accept one electron .

Other examples:
When a small amount of trivalent impurity like  gallium (atomic number 31 electronic configuration 2 8 18 32) having 3 valence Electrons is added to a Germanium crystal, each atom of the impurity fits in the Germanium crystal in such a way that 3 Valence Electrons form covalent bonds with three surrounding Germanium atoms .In the fourth covalent bond only Germanium atom contribute one electron while gallium atom has no valence electron to contribute ,this missing electron is called a hole.

Thus, each gallium atom provides one hole in the Germanium crystal.Since an extremely small amount of gallium impurity has a large number of atom therefore it provides millions of  holes in the semiconductor .

note: The single electron in the incomplete bond has a greater tendency to snatch an electron from the neighbouring atom.This tendency is so great that an electron in an adjacent covalent bond,having very small additional energy, can jump to occupy the vacant position.This electron then completes the covalent bond around gallium atom.

lets see energy band diagram:
With the addition of  trivalent impurity a large number of holes are made available in the crystal .However a minute quantity of free electrons are also available in the conduction band which are produced when thermal energy at room temperature is imparted to the Germanium crystal forming hole-electron pairs.But the holes are much more in the number than the conduction band electrons.A large number of holes are made available by the addition of trivalent impurity.

A minute quantity of hole-electron pair are formed at room temperature because of heat energy imparted to the semiconductor crystal.The free electrons thus formed are lifted to the conduction band leaving behind holes in the valence band.The number of holes provided by the trivalent impurities is far exceeding the number of free electrons.It is due to this predominance of holes over electrons that the material is called p-type semiconductor (where “p” stands for positive).

Conduction through p-type semiconductor :

In a p-type semiconductor, large number of holes are created by the trivalent impurity. When a potential difference is applied across this type of semiconductor,the holes available in the valence valence band are directed towards the negative terminal constituting electric current.As the current flow through the crystal is by holes ,which are carrier of positive charge, therefore this type of conductivity is called positive or p-type conductivity. In fact, in p-type conductivity the valence electron move from one covalent bond to another this gives a look as if holes are moving, unlike the n-type semiconductor where current conduction is by free electrons available in the conduction band.
It is important to note that conductivity of n-type semiconductor is nearly double to that of p-type semiconductor ,as the electrons available in the conduction band in n-type semiconductor are much more mobile then the holes available in the valence band in p-type semiconductor ,the mobility of the holes is poor because they are more bound to the nucleus as they are available in the valence band ,which is nearer to the nucleus as compared to the conduction band.
It may be added here that even at room at room temperature electron-hole pair are formed.These free electron which are available in the minute quantity also constituent a little current in the p-type semiconductor .However for all practical purposes this current is neglected.So this is about p-type semiconductor.

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