SOLIDS AND ITS CLASSIFICATIONS
CLASSIFICATION OF SOLIDS:
The solid materials are classified into three group conductors,
semiconductors and insulators.
Conductors- Conductors have abundance of
free electrons so it allow electric current to pass through. E.g., Metals
Insulators- Insulators have very low free
electrons so it do not allow the electric current to pass through. E.g., Glass,
Wood
Semiconductors- Semiconductors have electrical
conductivity that lies between conductor and insulator. E.g., Silicon,
Germanium
ATOMIC STRUCTURE:
An atom composed of protons (+ ve) neutrons (neutral) in the
nucleus and electrons (-ve) revolving in the orbit.
The electrons revolving round in various shells such as k,l,m,n,o.
Each have specific electrons to revolve k-2, l-8, m-18, n-32, o-50.
The electrons revolve in the outer most shell of an atom is called
valence electrons
Semiconductor should have 4 valence electrons.
Insulators should have 8 valence electrons.
Conductors should have 1 or 2 valence electrons.
SEMICONDUCTOR ELEMENTS:
SILICON-14: It has 4 valence electrons hence it is called
tetravalent element.
GERMANIUM-32: It also has 4 valence electrons hence it also
called as tetravalent electrons
ENERGY LEVELS:
As we know that electrons revolve around the nucleus in different
shells.
These electrons bound with nucleus with some energy.
So that each shell exhibits particular energy called discrete
energy levels.
It depends on 2 parameters
-momentum
of electrons
-distance
of electrons from nucleus
The electrons in different shells are bonded with nucleus
The strength of the bond in inversely proportional to the distance
between the nucleus and the shell where the electron is situated
External energy must be supplied to detach the electrons from the
shell.
If the distance is larger, then the energy required to free the
electrons is smaller.
Also energy required is higher if the distance is smaller.
The energy levels of various shells are calculated as,
En = -13.6/n^2 eV
The negative sign indicates that the electrons are bonded to
nucleus with an attractive force.
The energy gap between two consecutive shells is called the
“forbidden energy gap”.
ENERGY BAND:
A solid material contains large number of atoms.
Each atom has an energy level so that for large number of atoms
energy levels are closely spaced.
The closely spaced energy levels are called energy band.
TYPES OF ENERGY BAND:
VALENCE BAND:
The electrons in
the outermost shell are called valence electrons.
And the energy band associated with this is
called valence band.
CONDUCTION BAND:
When the
electrons are freed from atom they are responsible for the conduction and
therefore called conduction electrons.
The band associated with this is called conduction band.
ENERGY BAND:
The gap between
the valence band and the conduction band is called forbidden energy gap.
External supplied
to free the electron from outermost shell should be equal to the forbidden
energy gap.
Otherwise the
electron cannot jump from valence band to conduction band.
CONDUCTION OF ELECTRONS:
Conduction of electrons takes place if it jumps from valence band
to conduction band
But the size of the band gap energy determines the conductivity
FOR INSULATORS, The energy gap is very wide also the conduction
band is empty
E.g.:
Glass – 10eV
For conductors, there is no energy gap and the conduction band
& valence band overlap each other
E.g.:
Metals
For semi-conductor, the energy gap is very small and the conduction
is somehow better than insulator
E.g.:
Germanium- 0.78eV
Silicon – 1.21eV
COVALENT BOND:
As we know that semi conductors have 4 electrons in its outermost
shell, the valence band is incomplete
It is ready to bond with the neighboring atom so that the 4
electrons form 4 bonds with the neighboring atoms.
This bond is said to be covalent bond.
CRYSTAL STRUCTURE:
The three dimensional lattice is formed in the solid due to this
bonding.
This schematic arrangement of atoms is called crystal structure.
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