(a) N-type extrinsic semiconductor :A small amount of pentavalent impurity is added to a pure semiconductor to result in n type extrinsic semiconductor. The added impunity has 5 valence electron.
For example, if pentavalent atom is added to the semiconductor atom , four of the valence electrons get attached with the Ge atom while one electron remains as a free electron.
This is as shown in the following figure.
All of these free electrons constitute electron current. Hence the impurity when added to pure semiconductor , provide electrons for conduction.
• Key points :-
• in n-type extensive semiconductor is the conduction takes place through electrons , the electron are majority carriers and the holes are minority carriers.
• As there is no addition of positive or negative charge that is the electrons are electrically neutral.
• The minimum energy required to conduction in n type germanium is 0.01 ev.
• The minimum energy required to conduction in n-type silicon is 0.05 ev.
• N-type semiconductor at 0 k is a perfect insulator.
• Fermi level is near to conduction band.
(b) P- type extrinsic semiconductor:- A small amount of trivalent impurity is added to a pure semiconductor to result in p type extrinsic semiconductor. The added impurity has 3 valence electrons.
For example, if trivalent atom is added to the semiconductor atom 3 of the valence electron get attached with the semiconductor atoms,to form three covalent bonds.
But, one more electron in semiconductor remains without forming any bond. As there is no electron in trivalent atom remaining to form a covalent bond, the space is treated as a hole.
this is as shown in the following figure.
The boron impurity when added in a small amount, provides a number of holes which helps in the conduction. All of these holes constitutes hole current.
• key points:-
• In p-type extrinsic semiconductor, as the conduction takes place through holes , the holes are majority carriers while the electrons are minority charge carriers.
• The impurity added here provides holes which are add called as acceptors, because they accept electrons from the germanium atoms.
• As the number of mobile holes remains equal to the number of acceptors, the p-type semiconductor remains electrically neutral.
• The minimum energy required to conduction in p type Ge is 0.01 ev.
• The minimum energy required to conduction in p type Si is 0.05 ev.
• P type semiconductor at 0 k is a perfect insulator.
• Fermi level is near to valence band.
• Note :-
Why silicon is preferred in semiconductor ?
Among the semiconductor materials like germanium and silicon, the extensively used material for manufacturing various electronic components is silicon. Silicon is prepared over germanium for many reasons such as
• The energy band gap is 0.7 ev for Si , whereas it is 0.2 ev for Ge.
• The thermal power generation is smaller.
• The formation of sio2 layer is easy for silicon, which helps in the manufacture of many components along with integration technology.
• Si is easily found in nature than Ge.
• Noise is less in components made up of Si than in Ge.
• So i.e these advantages over germanium then silicon is preferred in semiconductor.
