This document discusses energy bands in solids and classifications of materials based on their band structure. It explains that in solids, electron energy levels form bands of allowed energies separated by forbidden bands. Materials are classified as conductors, insulators or semiconductors depending on their band gap. Conductors have overlapping bands resulting in no band gap, insulators have a large band gap, and semiconductors have a narrow band gap that allows excitation of electrons with small amounts of energy. Intrinsic and extrinsic semiconductors are also described based on their pure or doped material composition and charge carrier types.

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2. ENERGY BANDS IN SOLIDS
• In solid materials, electron energy levels form
bands of allowed energies, separated by forbidden
bands.
• Valence band = Outermost (highest) band filled
with electrons (“filled” = all states occupied).
• Conduction band = Next highest band to
valence band (empty or partly filled).
• Gap = Energy difference between valence and
conduction bands, = width of the forbidden band.
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3. Points to Understanding
• Electrons in a completely filled band cannot
move, since all states occupied (Pauli
principle).
• Only way to move would be to “jump” into
next higher band - needs energy.
• Electrons in partly filled band can move, since
there are free states to move to.
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4. Classification of solids into three types,
according to their band structure
Insulators semiconductor conductor
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5. conductor (metal)
• Material capable of carrying electric current, i.e.
material which has “mobile charge carriers”. (e.g.
electrons, ions,..).
• A metal which is very good carrier of electricity is
called conductor.
• In a conductor (metal) - The valence and conduction
bands overlap, so practically the energy gap is zero.
Thus, electrons need very little energy to stay in the
conduction band, and conduct electricity.
• valence band only partially filled, or (if it is filled), the
next allowed empty band overlaps with it.
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6. conductor (metal)
• If the electron to become free to conduct
means, it must be promoted into an empty
available energy state.
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7. conductor (metal)
• For metals, these empty states are adjacent to
the filled states. Generally, energy supplied by
an electric field is enough to stimulate
electrons into an empty state.
• e.g. metals, liquids with ions (water, molten
ionic compounds), plasma, copper and
aluminium are good examples of a conductor.
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8. insulators
• Materials with no or very few free charge carriers
• Gap = forbidden region between highest filled band
(valence band) and lowest empty or partly filled band
(conduction band) is very wide, about 3 to 6 eV;
• In an insulator the valence band is filled with electrons, so
electrons can not move within the valence band.
• In order to produce conduction of electricity, the electrons
from the valence band must go into the conduction band.
Thus, energy of more than the energy gap must be supplied
to the electrons in the valence band, in order to transfer
them into the conduction band. Because the energy gap in
insulator is large, it prevents this change in energy by the
electrons. Thus, insulators are poor conductors.
• e.g. quartz, most covalent and ionic solids, plastics, glass,
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9. insulators
Practically it is impossible for an electron to jump from the
valence band to the conduction band. Hence such
materials cannot conduct and called insulators.
Such materials may conduct only at very high
temperatures or if they are subjected to high voltage. Such
conduction is rare and is called breakdown of an insulator.
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10. Semiconductor
• A semiconductor material is one whose electrical
properties lie in between those of insulators and good
conductors.
• In terms of energy bands, semiconductors can be
defined as those materials which have almost an
empty conduction band and almost filled valence
band with a very narrow energy gap (0.1eV to1 eV ).
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11. Semiconductor
• The energy gap is very small, and very little energy is needed
to transfer electrons from the valence band into the
conduction band.
• Even the thermal energy at room temperature is enough.
• By raising the temperature, more and more electrons will be
transferred to the conduction band.
• This process results in an increase in conductivity with
increase in temperature.
• Examples are: germanium and silicon.
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12. Semiconductor IN PERIODIC TABLE
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13. SEMICONDUCTOR MATERIALS
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14. Types of Semiconductors
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15. Intrinsic Semiconductors
• An intrinsic semiconductor is one which is made of the
semiconductor material in its extremely pure form.
• Examples of such semiconductors are: pure germanium
and silicon which have forbidden energy gaps of 0.72
eV and 1.1 eV respectively.
• The energy gap is so small that even at ordinary room
temperature; there are many electrons which possess
sufficient energy to jump across the small energy gap
between the valence and the conduction bands.
• Alternatively, an intrinsic semiconductor may be
defined as one in which the number of conduction
electrons is equal to the number of holes.
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16. Intrinsic Semiconductors
• Electrons moving to conduction band leave
“hole” (covalent bond with missing electron)
behind; under influence of applied electric
field, Neighboring electrons can jump into the
hole, thus creating a new hole, etc.
• Holes can move under the influence of an
applied electric field, just like electrons;
both contribute to conduction.
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17. Intrinsic Semiconductors
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18. Extrinsic Semiconductors
(“doped semiconductor”)
• semiconductor with small admixture of
trivalent or pentavalent atoms.
• Those intrinsic semiconductors to which some
suitable impurity or doping agent or doping
has been added in extremely small amounts
(about 1 part in 108) are called “Extrinsic or
Impurity semiconductors”.
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19. TYPES OF EXTRINSIC
SEMICONDUCTOR
• Depending on the type of doping material
used, extrinsic semiconductors can be sub-
divided into two classes:
N-type semiconductors (donor)
P-type semiconductors (acceptor)
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20. N-type Extrinsic Semiconductor
• This type of semiconductor is obtained when a
Pentavalent material like antimony (Sb) is added to
pure silicon crystal.
• dopant with 5 valence electrons. 4 electrons used for
covalent bonds with surrounding Si atoms.
• each antimony atom forms covalent bonds with the
surrounding four silicon atoms with the help of four of
its five electrons.
• The fifth electron is loosely bound to the antimony
atom. it is mobile charge carrier. This electron needed
only small amount of energy to lift it into conduction
band (0.05 eV in Si). which improves the conduction
ability to some extent.
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21. N-type Extrinsic Semiconductor
• Hence, it can be easily excited from the valence
band to the conduction band by the application
of electric field or increase in thermal energy.
• The resultant material is known as an n-type
semiconductor.
• It is seen from the above description that in N-
type semiconductors, electrons are the majority
carriers while holes constitute the minority
carriers. has conduction electrons, no holes.
• e.g.of dopant with 5 valence electrons P, As, Sb
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22. N-type Extrinsic Semiconductor
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24. P-type Extrinsic Semiconductor
• This type of semiconductor is obtained when traces of a
trivalent like boron (B) are added to a pure
germanium crystal.
• dopant with 3 valence electrons (e.g. B, Al, Ga, In)
• only 3 of the 4 covalent bonds filled and vacancy in
the fourth covalent bond is hole.
• In this case, the three valence electrons of boron atom
form covalent bonds with four surrounding germanium
atoms but one bond is left incomplete and gives rise to
a hole.
• hole is left free as a mobile charge carrier, which
improves the conduction ability to some extent.
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25. P-type Extrinsic Semiconductor
• Thus, boron which is called an acceptor impurity
causes as many positive holes in a germanium
crystal as there are boron atoms there by
producing a P-type (P for positive) extrinsic
semiconductor.
• In this type of semiconductor, conduction is by
the movement of holes in the valence band.
• The resultant material is known as a p-type
semiconductor.
• Examples for trivalent dopant B, Al, Ga, In
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26. P-type Extrinsic Semiconductor
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28. P and N-type Extrinsic
Semiconductor
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29. P and N-type Extrinsic
Semiconductor
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30. p-n JUNCTION
• It is two terminal devices consisting of a P-N junction formed
either in Ge or Si crystal.
• It is circuit symbol is shown in fig. (1-a). The P and N type
regions are referred to as anode and cathode respectively. In
fig. (1-b) arrowhead indicates the conventional direction of
current flow when forward biased. It is the same direction in
which hole flow takes place.
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32. • http://mediatoget.blogspot.in/2011/06/classification-
of -materials based-on-energy-band -theory.html.
• HorstWahl,QuarkNet presentation, June 2001
Semiconductors, diodes, transistors.
• http://engineering-electrical-equipment.org/electrical-
distribution/introduction-to-semiconductors..html.
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