SEMI CONDUCTOR MATERIAL
PRESENTATION BY:
Er. Satnam Singh
Lecturer(Electrical engg.)
GPC Mohali (Khunimajra)
Contents
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Introduction of different electrical materials
Atomic theory
Energy band
Classification of material on the basis of energy band
structure
– Crystal structure of semiconductor
– Semiconductor material
Germanium
Silicon
– Intrinsic semiconductor
– Extrinsic semiconductor
N- type semiconductor
P-type semiconductor
Classification of Electrical and
Electronics Engineering Material
The electrical and electronics material can be classified
into following types:
1. Conducting materials
2. Semi conducting material
3. Insulating material
4. Magnetic material
Conducting material
•The conducting material are those material in which the electric
current can easily flow. In other word, we can say that materialls
which offer very low resistance to electric current is called
conducting material.
•The resistivity of these materials is very low, its values lies between
10-8 -10-6 Ω-m.
•The most well known conducting materials are copper, aluminum,
brass bronze etc.
Conductor Atomic Structure
• The atomic structure of
good conductors usually
includes only one electron
in their outer shell.
– It is called a valence electron.
– It is easily striped from the
atom, producing current flow.
Copper
Atom
Insulating material
•The insulating material are those material which do not allow the
passage of electric current or we can say that material which offer
very high resistance to electric current is called insulating materials.
•The resistivity of these materials is very high, its values lies between
1012 -1018 Ω-m.
•The most well known insulating material are rubber, wood, glass
ceramic, mica etc
Most insulators are compounds of several elements.
The atoms are tightly bound to one another so electrons are difficult
to strip away for current flow.
Semi-conducting material
•The semi-conducting material are those material whose
resistivity is less than insulator but more than conductor. In
other word we can say that conductivity of these material is
less than conductor but more than insulator.
•The resistivity of these materials is medium, its values lies
between 100 -102Ω-m.
•The most well known insulating material are silicon,
germanium, gallium etc.
Semiconductor Valence Orbit
• The main characteristic
of a semiconductor
element is that it has
four electrons in its
outer or valence orbit.
Magnetic material
•The magnetic material are those
materials which can be magnetized and
which are attracted towards the magnet.
•Such material create a magnetic field
on the surrounding space
•Iron nickel and cobalt are most well
known used magnetic materials.
Semi - conductor
• Semiconductor material are those which possess the
conductivity higher than insulators but lesser than
conductor and are used for manufacturing most of the
active components.
Atomic Theory
• The atomic theory state that all the matter weather solid,
liquid or gases consist of minute particles called molecules
which can be further divided into atoms
• The atom is defined as tiny discrete particles which is
incapable of division.
• Those substance whose molecule consist of similar atoms
are called elements and the material whose molecules
consist of dissimilar atoms are called compounds
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Bohr’s Atomic Theory
Bohr’s theory tell us that the atom of an element consist of two main
parts
Nucleus
Extra nucleus
The central part of an atom is known as nucleus. It contain proton
and neutrons. The nucleus hold the entire mass of an atom.
The proton are the positive charge particles, while neutron are
chargeless particles. Therefore the net charge on nucleus remain
positive.
The outermost part of atom arround the nucleus is known as extranucleus. It contain electron only. As the number of electron is equal
to no. of protons, so an atom is therefore electrically neutral.
The no. of electrons in any orbit is given by 2n2,where n is the
number orbit from nucleus.
E.g. 1st orbit has 2(1)2 = 2 electron as n = 1
2nd orbit has 2(2)2 = 8 electron as n = 2
3rd orbit has 2(3)2 = 18 electrons as n = 3
4th orbit has 8 electron
Bohr’s Atomic Theory
• Bohr’s atomic theory can be summarized as follow:
a) The atomic no. of an atom is the no. of proton present
in the nucleus
b) The algebraic sum of proton and neutron of an atom
is called the atomic weight or mass of the atom.
c) The number of proton inside the nucleus is equal to
the number of electron outside of it.
d) The electron on various energy level are arranged in
definite shells named as K,L,M,N……………
e) The outermost orbit of an atom can contain maximum
8 electron
Classification of material on the
basis of atomic structure
a) Conducting material : On the basis of atomic
structure conductor is defined as a substance, whose
atom has its outermost orbit incomplete. Such a
substance act as good conductors of an electricity e.g.
aluminum or copper
Classification of material on the
basis of atomic structure
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Copper : It is denoted by Cu having atomic no. = 29 and
atomic weight = 64
no. of electrons = no. of protons
no. of electron = atomic no.
Hence no. of electron in Cu =29
no. of neutrons = atomic weight - atomic no.
Therefore no. of neutron = 64 – 29 = 35
no. of electron in K shell = 2(1)2 = 2 (n=1)
L shell = 2(2)2 = 8 (n=2)
M shell = 2(3)2 = 18 (n=3)
N shell = 1
So there is one electron valence on it, it is called free electron
(or valence electron)
Valence electron of an atom are defined as the electron which
are loosely attached to the nucleus at an atom and can be easily
detached
Classification of material on the
basis of atomic structure
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Aluminium (Al) : It is denoted by Al having atomic no. = 13
and atomic weight 27
no. of electrons = no. of protons
no. of electron = atomic no.
Hence no. of electron in Al =13
no. of neutrons = atomic weight - atomic no.
Therefore no. of neutron = 27 – 13 = 14
no. of electron in K shell = 2(1)2 = 2 (n=1)
L shell = 2(2)2 = 8 (n=2)
M shell = 3 (n=3)
So there is 3 electron valence on it, it is called free electron (or
valence electron)
Valence electron of an atom are defined as the electron which
are loosely attached to the nucleus at an atom and can be easily
detached
Classification of material on the
basis of atomic structure
b) Semiconducting material : In semiconducting
materials, if a potential difference is applied across
the end of the material, a partial flow of electron
takes place i.e. some of electron can be detached
from the atom and some of them cannot be detached,
hence the conduction is partial and therefore the
properties of semiconductor are in between those of
conductor and insulator e.g. silicon and germanium.
Classification of material on the
basis of atomic structure
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Silicon(Si) : it is denoted by Si having atomic no. = 14
And atomic weight = 28
no. of electrons = no. of protons
no. of electron = atomic no.
Hence no. of electron in Si =14
no. of neutrons = atomic weight - atomic no.
Therefore no. of neutron = 28-14 = 14
no. of electron in K shell = 2(1)2 = 2 (n=1)
L shell = 2(2)2 = 8 (n=2)
M shell = 4
So there is 4 electron valence in its outermost orbit
Classification of material on the
basis of atomic structure
II. Germanium (Ge) : it is denoted by Ge having atomic no.
= 32 And atomic weight = 73
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no. of electrons = no. of protons
• no. of electron = atomic no.
• Hence no. of electron in Si =32
• no. of neutrons = atomic weight - atomic no.
• Therefore no. of neutron = 73-32 = 41
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no. of electron in K shell = 2(1)2 = 2 (n=1)
L shell = 2(2)2 = 8 (n=2)
M shell = 2(3)2 = 18 (n=3)
N shell = 4
So there is 4 electron valence in its outermost orbit
Classification of material on the
basis of atomic structure
c) Insulator :
• In insulating material the electron are firmly held to
their atom and hence if a potential difference is
applied then a little or no electron flow occurs
• There fore no electron can be detached from
outermost orbit so it is not easy to pass the electric
current through them. e.g. neon
• Neon (Ne) atomic no. 10
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no. of electron = atomic no. =10
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no. of electron in K shell =2(1)2 = 2 n=1
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L shell =2(2)2 = 8 n=2
• It outer most orbit does not contain any free electron
as its outer most orbit is completely filled.
Energy band theory
• In an atom, the electron revolving in different orbits possesses
certain energy level.
• The amount of energy on each orbit is fixed. but when we talks
about solids, where the atoms are closely packed the electron on
various orbits are influenced by energy of electrons of
neighboring orbits. In that case, energies of various bands are
considered in a single band.
• So the range of energy possessed by various electrons of the same
orbit of different atoms in a solid is known as energy band.
Energy band theory
• Following are the important types of energy bands:
a) Valence band : The range of energy possessed by valence
electron (electron present in outer most orbit) is called
valence band. This band may be completely or partially
filled by valence electrons.
b) Conduction band : The range of energy possesed by free
electron (electrons responsible for conduction) is called
conduction band. The conduction in solid is possible only
when some electron are present in conduction band.
c) Forbidden energy gap: The energy gap existing between
a conduction band and valence band is known as
forbidden energy gap. There is no electron in this region.
The electrons can only jump to conduction band when
they receive extra energy from external source.
Classification of material on the
basis of energy band
• Insulators: Insulator are those material in which we
cannot pass the electric current easily. In insulator,
conduction band remain empty, while valence band
has many electrons.
• In this case forbidden energy gap between valence
band and conduction band is quite large approx 20eV)
so it is not easy to conduct electric current through
them.
Classification of material on the
basis of energy band
• Semi conductor : Semiconductor material are those
material whose resistivity lies between conductor and
insulator. In semiconductor valence band is full of
electron while conduction band is empty. But in this
case, energy gap is very small (1 eV) with the
application of small potential difference electron can
jump from valence band to conduction band.
Classification of material on the
basis of energy band
• Conductor : Conductor are those material which offer
least resistance to electric current. Here conduction
band is overlapped with valence band. So large no. of
electrons can jump from valence band to conduction
band with the application of small potential difference
Semiconductor material
• Semiconductor material are those which possess the
conductivity higher than insulators but lesser than
conductor and are used for manufacturing most of the
active components.
Semiconductor materials
 Silicon and germanium each have 4 electrons in
their outer orbital. To fill the outer most shell each
atom acquire four more electron by sharing one
electron each from the four adjacent atoms and
hence form the crystal.
 In a crystal the atom are bonded together in
cohesive manner.
 In the fig, the core represent the nucleus and all
other electron as valence electrons. The valence
electron take part in forming covalent bonds with
four neighbouring atoms.
 At absolute zero all four covalent bond are intact
and no electron is free to conduct.
A chip, an LED and a
transistor are all made
from semiconductor
material.
Germanium
• Germanium (Ge) is an earth element pure germanium
is obtained from coal ash in the form of germanium
dioxide, which is then reduced to pure germanium.
Germanium
Atomic structure :
• The atomic no. Ge is 32. therefore it has 32 protons in its
nucleus along with 32 neutrons and 32 electron which
revolves around the necleus in fixed paths known as orbits
• These 32 electrons can be distributed in various orbits or
shells according to the rule 2n2 where n is orbit no. except
that there are four electrons in the outermost orbit.
• This outermost orbit is also known as valence orbit. The
germanium atom has two electron in its first orbit four
electron in second orbit eighteen electron in its third orbit,
and only four electron in its valence shell.
Germanium
• Crystal structure : A substance in which the atoms or molecules
are arranged in an orderly pattern is known as a crystal
• All semiconductor have crystalline structure. Each atom is
surrounded by a neighbouring atoms in a repetitive manner.
Therefore in a germanium crystal a large number of atoms are
bonded together
• This bonding is due to the fact that each atom has tendency to
complete its outermost or valence orbit by acquiring eight
electron in it.
• However in most of the substance the last orbit is incomplete
i.e. last orbit does not have 8 electrons.
• This makes the atom active to bargain with the other atoms to
acquire 8 electron in the last orbit. To do so the atom may lose
gain or share valence electron with other atoms.
Germanium
• In semiconductor bonds are formed by sharing the valence
electrons of an atom with the neighboring atoms. This type
of bonding in which which bonds are formed by sharing
the valence electrons are called covalent bonds.
• The various germanium atom are held together through
covalent bond to form a germanium crystal as shown in fig
• The covalent bond in between the various Ge atoms are
formed by sharing their valence electron with the
neighboring atoms.
• Since germanium is tetravalent element each atom forms
four covalent bonds with the four neighboring atoms.
Silicon
Atomic structure :
• The atomic no. Si is 14. therefore it has 14 protons in its
nucleus along with 14 neutrons and 14 electron which
revolves around the nucleus in fixed paths known as orbits
• The no. of electrons in first, second and third orbit are 2,8
and 4 respectively as shown in fig.
• The silicon has 4 electrons in its valence shell so it is also
known as tetravalent.
Silicon
• Crystal structure : Like germanium silicon atoms are also
arranged in an orderly manner to form a silicon crystal.
• The crystal structure of Si is shown in fig it is quite similar
to that of germanium crystal.
Concept of electron and hole
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At absolute zero, the valence band in the intrinsic semiconductors is
totally filled and the conduction band is empty.
Since there are four covalent bond each bond is not so strong. When the
temperature is increased the electron get sufficient energy to make a
valence electron to move away from the influence of its nucleus.when this
happen the electron becomes free to conduct and hence appears in the
conduction band.
When an electron moves away to the conduction band, a vacancy is
created in the valence band. This vacancy is called hole. Whenever a free
electron is generated a hole is created simultaneously
Fig shows the generation of electron hole pair in semiconductor. The
amount of energy required to break a covalent bond in germanium is
0.72eV and that in silicon is 1.12eV.
When such energy is supplied, one of the covalent bond is broken and the
electron appears in conduction band. This result in generation of holes.
The hole does remain stable and is quickly filled by the electron from the
adjacent covalent bond. The hole is generated there.
This way electron and hole moves in opposite direction
Intrinsic semiconductor
• An extremely pure semiconductor is called intrinsic
semiconductor
INTRINSIC SEMICONDUCTOR
At zero Kelvin all of the four valence
electrons of each atom in the silicon crystal
form part of the covalent bond with the four
neighboring atoms.
The valence band is completely full and the
conduction band completely empty.
The semiconductor behaves as a
perfect insulator because there are
no conducting electrons present.
INTRINSIC SEMICONDUCTOR
At temperatures above zero Kelvin some of the
valence electrons are able to break free from their
bonds to become free conduction electrons.
The vacancy that is left behind is referred to as a
hole. This hole is treated as a positive carrier of
charge.
Conduction due solely to thermally
generated electron-hole pairs is
referred to as intrinsic conduction.
Extrinsic semiconductor
• An doped semiconductor is called extrinsic
semiconductor.
• The process by which the an impurity is added to a
semiconductor is known as doping.
• A semiconductor to which an impurity at controlled rate is
added to make it conductive is known as extrinsic
semiconductor.
Doping Silicon to Create n-Type and p-Type Silicon
 The process of doping, creates p-type and
n-type silicon.
 Materials introduce an atom of another
element into the silicon crystal to alter the
electrical properties.
 The "dopant” has either 3 or 5 valence
electrons; silicon has 4.
 Substituting a phosphorus atom with 5
valence electrons for a silicon atom in a
silicon crystal leaves an extra, unbonded
electron that is relatively free to move
around the crystal.
 Substituting a boron atom with 3 valence
electrons for a silicon atom in a silicon
crystal leaves a hole (a bond missing an
electron) that is relatively free to move
around the crystal.
When a dopant atom with a valence of less than four is substituted into the silicon
structure, a hole is created in the structure and an acceptor energy level is created just
above the valence band. Little energy is required to excite the holes into motion.
EXTRINSIC CONDUCTION
A pure or intrinsic conductor has thermally generated holes
and electrons. However these are relatively few in number.
An enormous increase in the number of charge carriers can
by achieved by introducing impurities into the
semiconductor in a controlled manner. The result is the
formation of an extrinsic semiconductor. This process is
referred to as doping. There are basically two types of
impurities: donor impurities and acceptor impurities. Donor
impurities are made up of atoms (arsenic for example)
which have five valence electrons. Acceptor impurities are
made up of atoms (gallium for example) which have three
valence electrons.
N-TYPE EXTRINSIC SEMICONDUCTOR
Arsenic has 5 valence
electrons, however, only
4 of them form part of
covalent bonds. The 5th
electron is then free to
take part in conduction.
The electrons are said to
be the majority carriers
and the holes are said to
be the minority carriers.
P-TYPE EXTRINSIC SEMICONDUCTOR
Gallium has 3 valence
electrons, however,
there are 4 covalent
bonds to fill. The 4th
bond therefore remains
vacant producing a hole.
The holes are said to be
the majority carriers and
the electrons are said to
be the minority carriers.
Revision
– Atomic Theory
– Energy band
– Classification of material on the basis of energy band
structure
– Semiconductor material
– Crystal structure of semiconductor
Germanium
Silicon
– Intrinsic semiconductor
– Extrinsic semiconductor
N- type semiconductor
P-type semiconductor