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Sunday, 27 October 2019

Properties of N type and P type Semi Conductors

N-type Semi Conductors :


Impurities like Arsenic and Antimony have five valence electrons and are called the pentavalent impurity. When a pentavalent impurity is added to the intrinsic semi conductor, only four of its five valence electrons lock into the covalent bond formation of the atomic structure, and the fifth electron of the impurity is free to move through the crystal. As long as the impurity atoms are far apart the single electron behaves as an isolated atom. The energy of this electron is just less than that of the conduction band of Germanium and it may easily be excited by thermal energy into the conduction band and so takes part in conduction. The conduction is mainly by the electrons in the conduction band. The lattice of the N-type of semi conductor is shown in Figure.


P-type Semi Conductor:

When materials like Aluminium, Boron, Indium etc., which has three valence electrons are added to intrinsic semiconductors is small quantities, the impurity atom replaces a Germanium (or Silicon) atom in the crystal lattice resulting in a positive hole available for electrons. Impurities with three valence electrons called trivalent group, and the impurities are called acceptor. The energy of the vacant level is just above the filled valence band of Germanium and an electron may be easily excited thermally into the vacancy leaving a positive hole in the valence band. Electrical conduction now takes place due to positive holes and we get P-type of semiconductor. In P-type materials holes are the majority carriers and electrons are the minority carriers. The crystal lattice of a P-type semiconductor is shown in figure above.


Properties of N type and P type Semi Conductors:

Electrical Characteristics:

Electrical Characteristics of P and N type semi conductors are given in table below.


N - Type
P - Type
Impurities
Elements of Group V
Phosphorous (P)
At. No. 15, At.Wt 30.97
Arsenic (As)
At. No. 33, At.Wt 74.92
Antimony (Sb)
At. No. 51, At.Wt 121.75
Bismuth (Bi)
At. No. 83, At.Wt 208.99
Elements of Group III
Boron (B)
At. No. 5, At.Wt 10.81
Aluminium (Al)
At. No. 13, At.Wt 26.98
Gallium (Ga)
At. No. 31, At.Wt 69.72
Indium (In)
At. No. 49, At.Wt 114.82
Function of Impurities
Donor
Acceptor
Valence Impurities
5
3
Nature of Majority Carriers
Electrons
Holes
Band in which conduction takes place
Conduction Band
Valence Band
Energy gap
0.01 ev (Germanium)
0.05 ev (Silicon)
0.01 ev (Germanium)
0.045 ev (Silicon)

Functional Characteristics:

The functional characteristics of Semi Conductors are given in below Table.

Substances
Melting Point oC
Forbidden Energy gap
eV
Electron mobility cm2/volt-sec
Hole mobility cm2/ volt-sec
B
2300
1.1
10
10
Si
1420
1.11
1200
250
Ge
940
0.72
3600
1700
Te
452
0.36
1700
1200
Hg Te
670
0.2
10,000
100
B2 Te3
585
0.2
600
150
Mg2 Sb
778
0.36
200
150
PbSo
1065
0.5
1400
1400
PbS
1114
1.2
650
800
Cd Te
1045
1.45
450
100



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