Type of semiconductor

Type of semiconductor

From the previous topic we are well aware of  semiconductor, conductor and insulator and how they differ in terms of conductivity. In this topic we are going to see about semiconductor in details.

Semiconductors can be classified into two types viz. 

a) Intrinsic semiconductor :Extremely pure semiconductor are called as Intrinsic semiconductor. Eg: pure silicon, germanium. 

Let us understand Intrinsic semiconductor in details using diagram.

Intrinsic semiconductor at absolute zero temperature .Fig A

Silicon atom has 4 electron in its outermost orbit. As shown in the fig A each silicon atom share each electron with neighbouring silicon atom to complete their octet. Thus every silicon atom in a silicon lattice has a covalent bond between each other. At absolute zero temperature there is no free electron available for conduction and thus intrinsic semiconductor behaves as a insulator.

Intrinsic semiconductor at higher temperature. Fig B

As seen in fig B, as soon as the temperature is heated ,this covalent bond are broken and thus the valence electron becomes conduction electron. This removal of valence electron from orbit creates a vacancy in the band called as holes and it behaves as if it a positive charge.Thus every broken bond has equal number of free electrons and holes. n_e = n_h. As soon as potential difference is applied ,these electrons and holes contribute to electric current.

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b) Extrinsic semiconductor : When intrinsic semiconductor is added with pentavalent or trivalent impurity we get extrinsic semiconductor. 

Extrinsic semiconductor (Lattice structure) 

Thus we can obtain extrinsic semiconductor by adding impurities to intrinsic semiconductors.

# The process of adding impurities to intrinsic semiconductor is called as doping.

# The impurities added are called as dopants.

Extrinsic semiconductor can be further classified into two types viz.

a) P type semiconductor : When Si or Ge crystal is doped with trivalent impurity such as indium(In), boron(B), aluminium(Al),we get P-type semiconductor.

p type semiconductor with indium as impurity

In above fig, we can see indium is added as a trivalent impurity. As indium has 3 electro in it's outermost orbit, 3 covalent bond are formed between indium and silicon atom. But the bond between indium and 4th neighbouring silicon atom has a vacancy which is called as hole. Thus this vacancy can accept one electron and called as acceptor impurity. At room temperature due to thermal energy this a electron can jump into holes thus leaving a hole at the place from where it jumps and contribute to current. Thus each hole is created for each impurity added. As p type semiconductor has large number of holes it is called as P type semiconductor(positive). Thus holes act as a majority charge carrier and electrons act as a negative charge carrier. Mathematically, n_e << n_h.

b) N type semiconductor :When Si or Ge crystal is doped with pentavalent impurity such as Arsenic(As), Antimony(Sb), Phosphorus(P),we get P-type semiconductor.

n type semiconductor with antimony as impurity

In above fig, we can see Antimony (Sb) is added as a pentavalent impurity. As antimony has 5 electro in it's outermost orbit, 4 covalent bond are formed between antimony and silicon atom. But the bond between 5th electron is with core which is broken at normal room temperature. Thus 1 electron each from antimony is available as free electron for conduction and thus it is called as donor impurity. Thus each free electron is created for each impurity added which contribute to electric current. As n type semiconductor has large number of electrons it is called as N type semiconductor(negative). Thus holes act as a minority charge carrier and electrons act as a majority charge carrier. Mathematically, n_e>> n_h.

_{│<<<Semiconductor│    │P-N Junction>>>│}

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