1. Class 12 th Physics
Priyanka Jakhar
Physics Lecturer
GGIC Vijay Nagar
Ghaziabad U.P.
Semiconductor Part 1

2. Electronics
Electronics is the combination of two words electrons and dynamics.
Electronics is a science of flow and control of electrons in vacuum and solids.
Modern electronics is a part of solid state Physics in which flow of electrons
is channelised in solid before the introduction of modern semiconductor
electronics. Electronics was based on vacuum tubes like that diode ,triode
and tetrode valves in vacuum tubes .
The flow of electron from heating cathode was directed by various
electrodes like grid in vacuum the device based on these tubes but bulk
consume high power less reliable and have limited life also they require
vacuum for electrons flow.
First type of semiconductor diode was introduced by Sir Jagdish Chandra
Bose and Indian brain the solid state electronics was first come into light
after the invention of first practical transistor by William Shockley and his
team at Bell Labs in 1947.
Semiconductor electronics devices were started referred as they are cheap
small in size operate at low power produce almost no heat and reliable as
compared to the vacuum tube .

4. Energy Band In Solid :--
According to Quantum Mechanical Laws, the energies of electrons in a free atom can not have
arbitrary values but only some definite (quantized) values.
However, if an atom belongs to a crystal, then the energy levels are modified.
This modification is not appreciable in the case of energy levels of electrons in the inner
shells (completely filled).
But in the outermost shells, modification is appreciable because the electrons are shared by
many neighbouring atoms.
Due to influence of high electric field between the core of the atoms and the shared electrons,
energy levels are split-up or spread out forming energy bands.
Consider a single crystal of silicon having N atoms. Each atom can be associated with a lattice
site.
Electronic configuration of Si is 1s2, 2s2, 2p6,3s2, 3p2.(Atomic No. is 14)
When all the atoms of the Crystal are well separated ( region A ) :-- There is no interaction between
the atoms so each of an atom has same energy levels .This is represented by straight line in region A. In this
region out of four an outermost electrons 2N electrons are in 2N impossible is 3s states all have same energy
(and remaining 2N are in 6N possible 3p states ,all have same energy .Only 2N levels of 3p states are filled so
some 3p e states are empty. There is a gap between 3s and 3p states.

5. O
• • • • • •
• •
• •
• •
• •
2p6
2s2
1s2
3p2
3s2
Energy
a b c d Inter atomic spacing (r)
Conduction Band
Forbidden Energy Gap
Valence Band
Ion
core
state
Formation of Energy Bands in Solids:
Region A
RegionB
RegionD
RegionC

6. When interatomic distance reduce (region B ):-- The interaction between the atom becomes significant
the 3s and 3p States which earlier have identical energies spread out and form energy band .The gap between the
bands decrease .The different energy levels of e with continuous energy variation are called energy bands.
When interatomic distance reduce further ( region C ) :--The bands merge with each other .No energy
gap is visible since, the upper and lower energy states are mixed together .
When interatomic distance reduce further (region D ):-- The energy band again split apart . As r
becomes equal to 𝒓 𝟎 the actual interatomic distance of the Crystal the energy band of 8 N states split is apart into
two 4N field and 4N empty states separated by a gap called energy gap or energy band gap 𝑬 𝒈 .
The lower band which is completely filled by 4N valence electrons at absolute zero temperature is called Valence
band .
The upper band which is completely empty at absolute zero temperature is called conduction band .
The collection of very closely spaced energy levels is called an energy band.
Note:
1. The exact energy band picture depends on the relative orientation of atoms in a crystal.
2. If the bands in a solid are completely filled, the electrons are not permitted to move about, because there
are no vacant energy levels available.

8. Valence band :-- This is the lower end of a material having
valence electron .
It may be completely filled or partially empty.
The electron lying in the valence band are not usually taking part in
electrical conduction as they cannot be agitated by external electric
field.
Conduction band :-- This is the upper band of the material
with no electron at absolute zero.
It may be completely empty or partially filled .
The electrons lying in conduction band can be agitated by external
electric field .
They impart electrical conduction .
The band gap or energy gap :-- The separation between
the valence band and conduction is known as the energy gap of the
two bands.
This is the minimum energy required to move an electron from the
valence band to conduction band.
It has no electron .
It is also called the forbidden gap .
Conduction Band
Forbidden Energy Gap
Valence Band
• • • • • •
≈6 eV

9. Conduction Band and Valence Band in Semiconductors:--
Valence Band:--
The energy band involving the energy levels of valence electrons is known as the valence band. It is
the highest occupied energy band. When compared with insulators, the bandgap in semiconductors
is smaller. It allows the electrons in the valence band to jump into the conduction band on receiving
any external energy.
Conduction Band:--
It is the lowest unoccupied band that includes the energy levels of positive (holes) or negative (free
electrons) charge carriers. It has conducting electrons resulting in the flow of current. The
conduction band possess high energy level and are generally empty. The conduction band in
semiconductors accepts the electrons from the valence band.

10. The first possible energy band diagram shows that the
conduction band is only partially filled with electrons.
With a little extra energy the electrons can easily reach the
empty energy levels above the filled ones and the conduction
is possible.
The second possible energy band diagram shows that the
conduction band is overlapping with the valence band.
This is because the lowest levels in the conduction band needs
less energy than the highest levels in the valence band.
The electrons in valence band overflow into conduction band
and are free to move about in the crystal for conduction.
Conductor / Metal
Conduction Band
• • • • • •
Valence Band
Partially filled
Conduction Band
• • • • • •
Conduction Band
Valence Band
Forbidden Energy Gap
• • • •• •

11. Semiconductor
At absolute zero temperature, no electron has energy to jump from
valence band to conduction band and hence the crystal is an insulator.
At room temperature, some valence electrons gain energy more than the
energy gap and move to conduction band to conduct even under the
influence of a weak electric field.
As an electron leaves the valence band, it leaves some energy level in
band as unfilled.
Such unfilled regions are termed as ‘holes’ in the valence band. They
are mathematically taken as positive charge carriers.
Any movement of this region is referred to a positive hole moving from
one position to another.
Conduction Band
Valence Band
Forbidden Energy Gap ≈1 eV
Eg-Si = 1.1 eV EgGe= 0.74 eV
• • • •• •
Since 𝑬 𝒈 is small, therefore, the
fraction is sizeable for
semiconductors.
Insulators
Electrons, however heated, can not practically jump to conduction band
from valence band due to a large energy gap. Therefore, conduction
is not possible in insulators.
Eg-Diamond = 7 eV
Conduction Band
Forbidden Energy Gap
Valence Band
• • • • • •
≈6 eV

12. Fermi Level :-- The highest energy level which can be occupied by electrons in a crystal, at absolute 0
temperature , is called Fermi Level.
If the electrons get enough energy to go beyond this level, then conduction takes place.
Fermi energy :-- The highest possessed of free electron in a material at absolute zero known as Fermi
energy .The energy corresponding to fermi energy level is called Fermi energy.
The electron cannot interact unless possessed the Fermi level of energy.
This was named after physicist Enrico fermi, the creator of world first nuclear reactor .
At absolute zero the Fermi energy and Fermi level are equal but different at other temperatures .
The Fermi level lies between valence and conduction bands of the material .

13. Difference between Fermi energy and Fermi level:--
The Fermi energy is only defined at absolute zero, while the Fermi level is defined for any temperature.
The Fermi energy is an energy difference (usually corresponding to a kinetic energy), whereas the Fermi level is a
total energy level including kinetic energy and potential energy.
Fermi Level in Semiconductors:--
Fermi level (denoted by EF) is present between the valence and conduction bands.
It is the highest occupied molecular orbital at absolute zero.
The charge carriers in this state have their own quantum states and generally do not interact with
each other.
When the temperature rises above absolute zero, these charge carriers will begin to occupy states
above
Fermi level.
In a p-type semiconductor, there is an increase in the density of unfilled states. Thus,
accommodating more electrons at the lower energy levels.

14. BASIS OF
COMPARISON
CONDUCTOR SEMICONDUCTOR INSULATOR
Description
Conductor is a material
which permits the
electric current or heat
to pass through it.
Semi-conductor is a
material or substance
that may act as a
conductor as well as
insulators under
different conditions.
Insulators are materials
or substances which do
not allow heat or
electricity to pass
through it.
Conductivity
Have very high
conductivity, (107 Ʊ/m)
thus they can conduct
electrical current
easily.
Have intermediate
conductivity (between
10-7 Ʊ/m to 10-13 Ʊ/m),
thus they can act as
insulator and
conductor at different
conditions.
They have very low
conductivity
(1013 Ʊ/m), thus they
do not allow current
flow.
Difference Between Conductor, Insulator And Semi-Conductor

15. Reason For
Conductivity Or
Otherwise
The conduction in
conductors is due to
the free electrons in
metal bonding.
The conduction in
semiconductors is due
to the movement of
electrons and holes.
Non conduction is due
to absence free
electrons or holes.
Resistance Vs
Temperature
The resistance of a
conductor increases
with an increase in
temperature.
The resistance of a
semiconductor
decreases with
increase in
temperature.
Insulators have very
high resistance but it
decreases with
temperature.
Charge Carrier
In conductors,
electrons are charge
carriers.
In semiconductors,
intrinsic charge carriers
are holes and
electrons.
Insulators do not have
any charge carriers.
Valence Electrons
They have only one
valence electron in the
outermost shell.
They have four valence
electrons in the
outermost shell.
They have eight
valence electrons in
the outermost shell.

16. Band Gap
In conductors, there is low energy
gap between the conduction and
valence band of a conductor. It
does not need extra energy for
the conduction state.
The band gap of
semiconductor is
greater than that of
conductor but smaller
than that of an
insulator.
The band gap in
insulators is huge,
which needs an
enormous amount of
energy like lightning to
push electrons into the
conduction band.
Coefficient
Of
Resistivity
Conductors have a positive
coefficient of resistivity. Its
resistance increases with increase
in temperature.
Semiconductors have a
negative coefficient of
resistivity.
The coefficient of
resistivity of an
insulator is also
negative but it has very
huge resistance.
Absolute
Zero
Some special conductors turn
into superconductors when
supercooled down while other
have finite resistance.
The semiconductors
turn into insulators at
absolute zero.
The insulator resistance
increases when cooled
down to absolute
zero.

18. Holes and Electrons in Semiconductors:--
1. Holes and electrons are the types of charge carriers accountable
for the flow of current in semiconductors.
2. Holes (valence electrons) are the positively charged electric
charge carrier whereas electrons are the negatively charged
particles.
3. Both electrons and holes are equal in magnitude but opposite in
polarity.
Mobility of Electrons and Holes:--
1. In a semiconductor, the mobility of electrons is higher than that of the holes.
2. It is mainly because of their different band structures and scattering mechanisms.
3. Electrons travel in the conduction band whereas holes travel in the valence band.
4. When an electric field is applied, holes cannot move as freely as electrons due to
their restricted movment.
5. The elevation of electrons from their inner shells to higher shells results in the
creation of holes in semiconductors.