3. Content
1.Introduction
2.Semiconductors
3.Energy band in solids
4. Conduction band, Valance band and Forbidden gap
5.Insulators, Conductors and Semiconductors
6.Electrons and holes in semi conductors
7.Intrinsic semiconductor
8.Doping of a semiconductor
9.Extrinsic semi conductors
10.N-type semi conductor
11.P-Type semiconductor
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4. In the year 1947, INTRODUCTION
Transistor was
invented by three
scientists.
William Shockley, John
Bardeen and Walter
Brattain in 1956 ,
shared Nobel Prize in
physics for creating the
transistor.
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5. SEMICONDUCTORS
Semiconductors have resistivity
between good conductors and
insulators.
The resistivity of semiconductor
lies approximately in between 10 -2
ohm m and 10 4 ohm m at room
temperature.
.
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6. ENERGY BANDS IN SOLIDS
There are discrete energy levels in the
case of an isolated atom.
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8. In solids , the atoms are arranged in a
systematic space lattice and each atom is
influenced by neighbouring atoms.
The closeness of atoms results in the
intermixing of electrons of neighbourring
atoms.
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9. Insulators
Conduction Band In an insulator, the forbidden
gap is very large and in
general is more than 3eV.
FORBIDDEN GAP
No electron is available for
conduction.
Energy
Valence Band Large amount of energy is
needed to move electron
from valance band to
Filled Band conduction band.
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10. In the case of semiconductors the
Semiconductors forbidden gap is very small.
At 0K the conduction band is
empty and the valence band is
Conduction Band
completely filled.
FORBIDDEN GAP
Around 0.7eV (Ge) and When a small amount of energy is
1.1 eV (Si) supplied, the electrons can easily
jump the forbidden gap.
Energy
Valence Band
The conductivity of a
semiconductor is of the order of
10 2mho m-1
Filled Band
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11. Conductors
In conductors there is no forbidden
gap.
The valence band and the
conduction band overlap.
Conduction Band The electrons from valence band
freely enter into the conduction
band due to overlapping of bands.
Energy
No forbidden gap
Therefore very low potential
Valence Band difference can cause continuous
flow of current.
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12. Electrons and holes in semiconductors
At absolute 0 temperature,
At 0 K
in a pure semiconductor
the valence band is
Conduction Band completely filled and the
Energy
conduction band is vacant.
Valence Band Electron
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13. Electrons and holes in semiconductors
At Room At room temperature
temperature
some of the electrons get
Conduction Band
energy to break the
covalent bond and moves
Energy
in to the conduction band.
Valence Band Electron
Hole
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15. Group 3 atoms have one less electron
than silicon so when they are introduced
into the crystal, there is a ‘hole' where
one extra electron should be.
Electrons from neighbouring atoms can
move into this hole, leaving a hole where
they used to be which is in turn filled by
another neighbouring electron.
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