Semiconductors by Shiny , Assistant Professor, Department of ECE, Sri Ramakrishna Institute of Technology, Coimbatore

1. By
Ms.J.Shiny Christobel
Assistant Professor/ ECE
Sri Ramakrishna Institute of Technology, Coimbatore
1

2.  Electronics Engineering is a branch of
engineering which deals with the flow of
electrons in vacuum tubes, gas and
semiconductor.
 Applications of Electronics:
Home Appliances, Medical Applications,
Robotics, Mobile Communication, Computer
Communication etc.
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3.  Atom is the basic building block of all the
elements.
 It consists of the central nucleus of positive
charge around which small negatively
charged particles called electrons revolve in
different paths or orbits.
 An Electrostatic force of attraction between
electrons and the nucleus holds up electrons
in different orbits.
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4.  Nucleus is the central part of an atom and
contains protons and neutrons. A proton is
positively charged particle, while the neutron has
the same mass as the proton, but has no charge.
Therefore,nucleus of an atom is positively
charged
 Positive and negative ions:
 Protons and electrons are equal in number hence
if an atom loses an electron it has lost negative
charge therefore it becomes positively charged
and is referred as positive ion.
 If an atom gains an electron it becomes
negatively charged and is referred to as negative
ion.
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5.  Valence electrons:
The electrons in the outermost orbit of an atom are known
as valence electrons.
 Free electrons:
 The valence electrons of different material possess
different energies. The greater the energy of a valence
electron, the lesser it is bound to the nucleus.
 In certain substances, particularly metals, the valence
electrons possess so much energy that they are very
loosely attached to the nucleus.
 The loosely attached valence electrons move at random
within the material and are called free electrons.
 The valence electrons, which are loosely attached to the
nucleus, are known as free electrons.
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6.  The range of energies possessed by an electron in a
solid is known as energy band.
 Three important theory of energy band. They are
 Valence Band
 Conduction Band
 Forbidden Band
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7.  The lowermost band is the valence band.
 A band which occupied by the valence electrons or a
band having highest energy is defined as Valence band.
 The valence band may be partially or completely filled.
This band can never empty.
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8.  The upper most band is the conduction band.
 All electrons in the conduction band are free electrons
and can be easily removed by the application of
external voltage
 If as substance has empty conduction band, it means
current conduction is not possible in that substance.
 Conduction band may be empty or partially filled
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9. External
energy
Valence band
Conduction band
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10.  By applying external energy some electrons in valence
band moves to conduction band
 Therefore, Holes in valence band and electrons in
conduction band are equal.
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11.  The separation between valence band and conduction
band is known as forbidden energy gap.
 If an electron is to be transferred from valence band to
conduction band, external energy is required, which is
equal to the forbidden energy gap.
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12. Forbidden energy gap
External
energy Valence band
Conduction band
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13.  The extent of forbidden band(i.e., separation between
conduction and valence bands) will determine whether
a substance is an insulator or conductor or
semiconductor.
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14.  In an insulator, the energy gap between valence and
conduction band is very large.
 Therefore, a very high electric field is required to push
the valence electrons to the conduction band.
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15.  In a conductor, the valence and conduction bands
overlap.
 Due to this overlapping, a slight potential difference
across a conductor causes the free electrons to
constitute electric current
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16.  In a semiconductor, the energy gap between valence
and conduction band is very small.
 Therefore, Comparatively smaller electric field(Smaller
than insulator but greater than conductors) is required
to push the electrons from valence band to the
conduction band.
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17.  The resistivity of a semiconductor is less than that of
an insulator but more than that of a conductor
 A semiconductor has almost filled valence band and
nearly empty conduction band with a small energy gap
separating the two
 A semiconductor is formed by covalent bonds.
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18.  Many semiconductors are available but most
frequently used materials are germanium(Ge) and
silicon(Si)
 In germanium it contains 32 electrons and silicon
contains 14 electrons
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19.  Forbidden energy gap for germanium is 0.7eV
 Forbidden energy gap for Silicon is 1.1eV
0.7eV 1.1eV
Conduction Band
Valence Band
Conduction Band
Valence Band
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20.  Intrinsic (Pure form)
 Extrinsic (Impure form)
NType(Semiconductor+Pentavalent imp)
PType(Semiconductor+Trivalent imp)
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21.  A semiconductor which pure and contains no impurity is
known as an intrinsic semiconductor.
 The electrons in an intrinsic semiconductor, which move in
to the conduction band at high temperature are called as
intrinsic carriers.
 In the valence band, a vacancy is created at the place
where the electron was present, before it had moved in to
the conduction band.
 This vacancy is called hole
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22.  In an Intrinsic semiconductor, the number of free
electrons and holes are equal.
Conduction band
Valence band
Conduction band
Valence band
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23.  The process of addition of a very small amount of impurity into a
intrinsic semiconductor is called as doping
 The impurity atoms are called dopants.
 The doping material is either pentavalent or trivalent.
 Pentavalent atoms (Arsenic , antimony, phosphorous, Bismuth
which have five valence electrons)
 The pentavalent doping atom is known as donor atom, since it
donates one electron to the conduction band of pure
semiconductor
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24.  Trivalent atoms(Boron, aluminium, gallium, indium
which have three valence electrons).
 The trivalent atom is called an acceptor atom, because
it accepts one electron from the pure semiconductor
atom.
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25.  An extrinsic semiconductor is one in which an impurity
with a valency higher or lower than the valency of the
pure semiconductor is added, so as to increase the
electrical conductivity of the semiconductor.
 Two types of extrinsic semiconductor
N-type semiconductor
P-type semiconductor
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26.  When a small amount of pentavalent impurity such as
arsenic is added to a pure germanium semiconductor
crystal, the resulting crystal is called N-type
semiconductor.
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Covalent
bond

27.  The four valence electrons of arsenic atom form
covalent bonds with electrons of neighbouring four
germanium atoms.
 The fifth electron of arsenic atom is loosely bound, this
electron can move about almost as freely as an
electron in a conductor and hence it will be the carrier
of current.
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28.  In energy band picture, the energy state
corresponding to the fifth valence electron is in the
forbidden gap and lies slightly below the conduction
band. This level is called as donor level
Conduction band
Valence band
Donor level
Energy band diagram for N-type
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29.  When the fifth valence electron is transferred to the
conduction band, the arsenic atom becomes positively
charged immobile ion.
 Each impurity atom donates one free electron to the
semiconductor. These impurity atoms are called
donors.
 In N-type semiconductor Free electron are the majority
charge carriers and holes are the minority charge
carriers. 3/16/2022 29

30.  When a small amount of trivalent impurity (such as
boron or gallium) is added to a pure semiconductor
crystal, the resulting semiconductor crystal is called P-
type semiconductor
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31.  The three valence electrons of the boron atom form
covalent bonds with valence electrons of three
neighborhood germanium atoms.
 In the fourth covalent bond, only one valence electron
is available from germanium atom and there is
deficiency of one electron which is called as hole.
 Hence for each boron atom added, one hole is created.
 The holes can accept electrons from neighbourhood,
the impurity is called as acceptor.
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32. Conduction Band
Valence Band
Acceptor level
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33.  In the P-type semiconductor, the acceptor impurity
produces an energy level just above the valence band.
 The energy difference between acceptor energy level and
the valence band is much smaller, electrons from the
valence band can easily jump into acceptor level by
thermal agitation.
 In p-type semiconductors, holes are the majority charge
carriers and free electrons are the minority charge carriers.
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