Explain about semiconductors
Semiconductor
Material whose conductivity lies between that of conductor and insulator is called semiconductor
Germanium and silicon have resistivity of the order of 100 ohms. Some important semiconductor are silicon,Germanium, gallium,arsenide.
Classification of semiconductor
Semiconductor are classified into two types
1.intrinsic semiconductor
2.extrinsic semiconductor
Intrinsic semiconductor or pure semiconductor
Germanium and silicon combine to form one crystal. Covalent bonds are formed by the sharing of the valence electrons by the neighboring atoms.at absolute zero temperature 0k ,as no energy is imparted,all the electron in the outer orbits remain in the valence bands.conseqzently,the semiconductor behaves as an insulator
However, at room temperature, some pf the electrons in the outer orbits may acquire sufficient to break the Covalent bonds in ge is 072ev and si is 1.1ev.si and ge have the same crystal structure and identical characteristics.
When the energy is imparted to an atom,an electron is removed from the valence band of an atom ,in which case a hole is said to be created. In pure si and Ge,there will be as many holes as there are electrons and these combinations are called as electron-hole pairs. Electrons and holes are present in equal number and this concentration is called intrinsic concentration.
n=p=ni
Where
P= number of free electrons
ni=Intrinsic semiconductor
Conduction in Intrinsic semiconductor
An Intrinsic semiconductor at room temperature has fewer number of electrons in the conduction band and an identical number of holes in the valence electron.on the application of an Intrinsic semiconductor external electric field,free electrons move towards the positive terminal of the battery and holes towards the negative terminal, there by consisting an electric current.
Extrinsic semiconductor
When Intrinsic semiconductor at room temperature the electron-holes pairs generated account for a small electroncurrent practically zero holes current on account of recombination. The electrical conductivity is increasing by adding impurities. Th semiconductor is called extrinsic semiconductor.
Adding impurities atoms to a pure semiconductor is called doping.
This are two types
1.n -type
2.p-type
1.n type semiconductor: when a small number of fifth group impurities such as bismuth,antimony,arsenic and phosphorus are added to pure semiconductor during crystal growth the results is n-type semiconductor.
Fig 3 : Covalent bonds with fifth group impurity
The fifth group atoms have five valance electrons and pure have only four valence electrons. The four valence electrons of Ge are covalent bonds with four valence electrons of impurity atoms.the fifth valence electron are not covalent bonded. It is loosely bound to the parent impurity atom. The amount of energy that needed to remove the electrons from the valence bands.an impurity atom donates its excess electron it is called the donar atom.
Fig 4 :Conduction in N type semiconductor
When the donar electron an impossible positive ion. N type having large number of electrons and small number of holes are there.hence majority carrier are electrons and holes are minority carrier.
2.p type semiconductor:when a small number of third group impurities such as
Fig 5 :Covalent bond with third group impurity
gallium,indium,aluminum, and boron is added to pure semiconductor during crystal growth,the resulting semiconductor is called a p-type semiconductor. The third group atoms have three valence electron and pure Ge and si have four valence electrons. The three valence electrons are covalent bond with four pure valence electrons. If fourth electrons are not bonded. This is accepts electron the atom becomes a negatively charged ion, which is immobile. The hole moves from one atom to the other. Hence p type are Majority is holes and electrons are mininority. Conduction in this type of material essentially through holes.
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