UNIT-I_Energy_band_theory_of_solids.ppt

ANIL NEERUKONDA INSTITUTE OF TECHNOLOGY &
SCIENCES(A)
Department of Electronics and Communication Engineering
ECE 125 Basic Electronics Engineering
 Academic year : 2022-23
 Class & Section : 1/4 ECE-A
 Name of the Faculty : Mr.D.Anil Prasad
ANIL PRASAD DADI/ECE/ANITS

Contents
• Introduction
• Band Theory of solids
• Classification of materials based on Energy bands
• Mobility and Conductivity

Introduction
• There are three types of materials, based on their conducting
properties:
– conductors
– semiconductors
– insulators

Introduction
• At room temperature the conductivity
and resistivity of selected materials is
shown in table.
• Metals and alloys have the highest
conductivities followed by
semiconductors and then by
insulators

Introduction
• Resistivity vs. Temperature

Band Theory of Solids
• Energy band theory of solids indicate that the allowed energy states
for electrons are nearly continuous over certain ranges, called
energy bands, with forbidden energy gaps between the bands.

• When atoms are far apart electron
energies in atom1 and atom2 are same.
• The force of attraction of electron in
atom1 and its nucleus is same as the
force of attraction of electron in atom2
and its nucleus.

• When atoms are brought closer to
each other the columbic force of
attraction of electrons changes in
atom1 and atom2.
• The force of attraction between
electron in atom1 and nucleus in
atom2 changes. Similarly the force of
attraction between electron in atom2
and nucleus in atom1 changes.

• When force of attraction changes the
electron energy changes.
• Thus when atoms are brought closer
to each other the energies of electron
in each of the hydrogen atom changes
or splits into two energy levels.

• Si-1s22s22p63s23p2
• Consider N atom system which has
2N 1s states,
2N 2s states,
6N 2p states,
2N 3s states,
6N 3p states

Band Theory of Silicon
• As the atoms are brought closer
coupling of various atoms take place.
The energy levels split into bands
beginning with outer shell(n=3)
• The 3s states will tend to spread into
range of energies. Because there are
2N 3s states there should be 2N
separate energy states represented
by lines

• Similarly 6N 3p states are shown by
6N separate energy states
represented by lines. Since N is very
large (5x1022 atoms/cm3)for si the
various levels are so close to each
other such that it represents a
continuous band

• If further atomic separation is
reduced we can have a single
continuous band. As the distance
between atoms approaches the
equilibrium interatomic spacing of
Si (lattice constant of si, r0=5.43A)
this band splits into two bands
separated by an energy gap, Eg

• The number of states in the
lower band and upper band
are exactly 4N states each.
The total number of
electrons are 4N electrons
in 8N states. So all 4N
electrons will occupy the
lower energies available.
The upper band will be
empty at T=00K

• Valence Band:
Valence Band refers to the lower band in which valence electrons are
found in energy levels. It may have all the electrons it needs or only some
of them.
• Conduction Band:
Conduction band refers to the upper band where free electrons are not
present in any energy levels. The following allowed energy band is the
conduction band. It could be devoid of electrons entirely or only partially.

• Forbidden energy Gap:
The Forbidden Energy Gap, Forbidden Band Gap, or Band Gap is the
name given to the energy gap between the valence band and conduction
band.
The amount of energy required by an electron to move from the
valence band into the conduction band is known as the energy gap, or
Eg. It's expressed in eV.

Classification of materials based
on Energy Bands

on Energy Bands
•Insulator:
Material which do not conduct current is called insulator.
 The energy band gap in insulator is above 5 eV.
• Conductor:
 The energy band gap in a conductor, a material that conducts
electricity, is zero.

on Energy Bands
•Semiconductor:
 Semiconductor refers to materials with conductivity that is
between insulator and conductor.
 The energy band gap is quite narrow, around 1 eV.

Mobility and conductivity
• In any material along with atoms, ions are
also present.
• In any material impurities are present.
• When two different elements are present
then their electronic configuration is
different. The electrons can be taken or
given by each of the material and hence
ions are present.

• The +ve sign within circle represents the net
positive charge of nucleus and tightly bound
inner electrons.
• The dots represent the outer or valence
electrons in atom. These electrons doesn’t
belong to any particular atom. They have
lost their individuality and can wander freely
about from atom to atom in metal.

• The electrons in a metal are in random motion and on an
average there will be as many electrons passing through unit
area in any direction as in the opposite direction in a given time.
Hence the average current is zero.

• In a metal, electrons are in continuous
motion and collide with atoms and scattered
by the heavy ions and maintain different
velocities.
• When a constant electric field E is applied
to the metal, due to electrostatic force
electrons would be accelerated and the
velocity would increase indefinitely with
time. (The increase in electron velocity is
not due to collisions with the ions)

• At each inelastic collision with an ion an
electron loses energy and a steady state
condition is reached where a finite value of
drift speed vd is attained
• Drift velocity: It is average velocity
maintained by electrons while moving inside
material . Drift velocity is in the direction
opposite to that of the E.
• vdαE
• Vd=µE

References
• Robert L Boylestad, Electronic Devices And Circuit Theory, Prentice
Hall, seventh edition,2021
• Jacob Millman and Christos Halkias, Electronics Devices and
Circuits, Black edition, October,2017

UNIT-I_Energy_band_theory_of_solids.ppt

More Related Content

What's hot

Similar to UNIT-I_Energy_band_theory_of_solids.ppt

More from AnilPrasadDadi

Recently uploaded

UNIT-I_Energy_band_theory_of_solids.ppt