This document discusses energy bands in solids and semiconductors. It explains that in solids, the close arrangement of atoms results in the intermixing of electron orbitals between neighboring atoms. This leads to the formation of continuous and allowed energy bands separated by forbidden gaps. In insulators, the forbidden gap is large, while in semiconductors it is much smaller, around 0.7-1.1 eV. Semiconductors have electrical resistivity between conductors and insulators. The document also describes how doping semiconductors with group 3 or 5 atoms creates an excess or deficiency of electrons, forming n-type or p-type materials respectively.

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2. ENERGY BANDS IN SOLIDS
There are discrete energy levels in the
case of an isolated atom.
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3. Arrangement of electrons in an isolated Silicon
atom
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4. 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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5. Insulators
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Valence Band
Conduction Band
FORBIDDEN GAP
Energy
In an insulator, the forbidden
gap is very large and in
general is more than 3eV.
No electron is available for
conduction.
Large amount of energy is
needed to move electron
from valance band to
conduction band.Filled Band

6. Semiconductors
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Valence Band
Conduction Band
FORBIDDEN GAP
Around 0.7eV (Ge) and
1.1 eV (Si)
Energy
In the case of semiconductors the
forbidden gap is very small.
At 0K the conduction band is
empty and the valence band is
completely filled.
When a small amount of energy is
supplied, the electrons can easily
jump the forbidden gap.
The conductivity of a
semiconductor is of the order of
10 2mho m-1
Filled Band

7. 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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8. Conductors
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Valence Band
Conduction Band
Energy
In conductors there is no forbidden
gap.
The valence band and the
conduction band overlap.
The electrons from valence band
freely enter into the conduction
band due to overlapping of bands.
Therefore very low potential
difference can cause continuous
flow of current.
No forbidden gap

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Electrons and holes in semiconductors
At absolute 0 temperature,
in a pure semiconductor
the valence band is
completely filled and the
conduction band is vacant.
At 0 K
Conduction Band
Valence Band Electron
Energy

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Electrons and holes in semiconductors
At room temperature
some of the electrons get
energy to break the
covalent bond and moves
in to the conduction band.
At Room
temperature
Conduction Band
Valence Band
Hole
Electron
Energy

11. FORMATION OF N - TYPE MATERIAL
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12. 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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13. FORMATION OF P - TYPE MATERIAL
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20. A Bipolar Junction Transistor (BJT) has
three terminals connected to three
doped semiconductor regions. In an NPN
transistor, a thin and lightly doped P-
type base is sandwiched between a
heavily doped N-type emitter and
another N-type collector; while in a PNP
transistor, a thin and lightly doped N-
type base is sandwiched between a
heavily doped P-type emitter and
another P-type collector. In the following
we will only consider NPN BJTs.
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