6. Semi Conductor Physics
Topics:- 6.1 Types of materials (insulator,conductor, conductor), intrinsic and extrinsic
semi-semi
conductor, p-n junction diode and its characteristics
6.2 Diode as rectifier-half wave and full wave rectifier,
semi conductor transistor pnp and npn (introduction only)
Conductors, Insulators and Semiconductors
Conductors: are materials which allow electric
current to pass through them easily.
They have low resistance. e.g. All metals
are good conductors i.e. Copper, iron silver
etc.
Insulators: are materials which do not allow
electric current to pass through them. They
have high resistance. e.g. Rubber, Plastic,
air, paper etc.
Semiconductors: are materials which have
conductivity higher than insulator but less than
conductors. e.g. Silicon(Si), Germanium(Ge).
M-Shell electron
Sodium atom(Na), Z = 11
Sodium (Na), Z = 11; 1s2 2s2 2p6 3s1
Atomic Structure of an atom
11p
12n
Nucleus
Electron
Orbit
K-Shell electron
L-Shell electron
Energy level of one atom
In a solid there are about 1023 atoms/cm3 .
Therefore each energy level of an
isolated atom gets split into 1023 levels,
closely spaced together.
Formation of Energy Bands in Solids
Energy band of solidTwo atom
Valence band: The highest energy band occupied
by the valence electrons is called the
valence band.
Conduction band: An unfilled band (Range of
energies) above the valence band is called the
conduction band.
Forbidden energy gap (EG) : The separation
between conduction band and valence band on
the energy level diagram is known as forbidden
energy gap.
Classification of materials on the basis of
Energy Bands
(A) Conductors
In case of conductors the forbidden energy gap (EG) is zero i.e.
valence band and the conduction band overlap.
Some electrons from valence band can easily pass
into the conduction band.
This free movement of electrons is responsible for
the conductivity of metals.
(B) Insulators
(i)The valence band is completely filled with
electrons.
(ii)Conduction band is
completely empty.
(iii)The energy gap between valence band and
conduction band is very large.
(C) Semiconductors
(i)The valence band is completely filled with electrons.
(ii)Conduction band is
completely empty.
(iii)There is small energy gap between valence band and
conduction band.
The best known examples are Silicon, Si (EG ~ 1.17 eV) and
Germanium, Ge (EG ~
0.74 eV).
Germanium(Ge),Z=32 are two important examples of
intrinsic semiconductors.
A semiconductor dopedwith
impurity is called
extrinsic semiconductor.
Doping: The process ofadding
impurities is called as doping.
The doping of a
semiconductor increasesits
conductivity.
A semiconductor in the pure form is called an
andintrinsic semiconductor. Silicon(Si),Z=14
Intrinsic and Extrinsic Semiconductors
Impurity can be of two kinds:
(a) Pentavalent : Each atom has five valence
electrons. Examples are phosphorous(P), z =
15 ; arsenic(As), z = 33; antimony(Sb), z =
51 etc.)
(b) Trivalent : Each
electrons. Examples
atom has 3 valence
are boron(B),z=5;
aluminium(Al), z = 13 ; gallium(Ga), z = 31 etc.)
n–Type Semiconductor :
(i)When a small amount of pentavalent impurity is
added to an intrinsic semiconductor (Si or Ge),
it provides a large number of free electrons.
The semiconductor is then, called
n-type semiconductor
(ii)The impurity atom donates one electron
to conduction band. It is therefore, also called
donor impurity.
semiconductor.
(ii) The fourth bond has a deficiency of
electron. Therefore, trivalent impurity is
also called acceptor impurity.
p–Type Semiconductor :
(i) When a small amount of
trivalent impurity is added to intrinsic
semiconductor, it creates a largenumber of
semiconductor
holes
is,
in valence band. The
then, called a p-type
p-n Junction DiodeA single crystal of Si or Ge which has
been doped in such a way that half of it is a p-
type and the other half an n-type semi-
conductor is known as a p-n junction diode.
A B
Symbol
Depletion layer : the region AB in the vicinity of
the junction which has been depleted of free
charge carriers (electrons and holes) and
has only immobile ions is called depletion
region.
The potential difference between the two sides
of the depletion layer is known as potential
barrier (VB) at the junction.
Biasing Of the p-n Junction Diode
(i) Forward bias : The p-n junction diode is said
to be forward biased if the positive terminal of
the battery is connected to the p-type
semiconductor and the negative terminal
is connected to the n-type semiconductor
-
terminal of the battery is connected to the
p- type semiconductor and the positive
t e r m i n a l i s c o n n e c t e d t o t h e n - t y p e
semiconductor.
-
(ii) Reverse bias : The p-n junction diode is
said to be reverse biased if the negative
Characteristics of a p-n Junction Diode
(i) Forward bias characteristics
(ii) Reverse bias characteristics
(i) Forward bias characteristics
(ii) Reverse bias characteristics
•Rectifier: An electronic device used to
convert alternating current (AC) into direct
current (DC) is known as rectifier.
(i)Half wave rectifier•Full wave rectifier
Junction Diode As A rectifier
(i) The p-n junction diode as half wave rectifier:
+
-
(ii) Full wave rectifier :
The transformer used is a ‘‘Centre tap’’ transformer.
During the first half of the ac cycle, a current flows
through the diode D1 along AD1XYTA.
During the other half of ac cycle,
current flows through the diode D2 along
BD2 XYTB.
Junction Transistorof threeThe transistor is composed
semiconductor elements.
p-n-p transistorIn the circuit symbols of a transistor,
only emitter has an arrow to indicate that it
is the supplier electrode.
Thickness of base is small so that it only controls the flow
of electrons from the emitter to the collector. .
IE = IB + IC
In normal operation of a transistor,
the emitter- base junction is
always forward-biased whereas
the collector-base junction is
reverse- biased.
n-p-n transistor
Working of p-n-p transistor
6. Modern Physics
7.1 Lasers: concept of energy levels, ionizations and
excitation
emission;
potentials; spontaneous
population inversion,Laser,
and stimulated
types of lasers,
ruby laser and applications of laser.
7.2
7.3 Super conductivity: Phenomenon
Fiber optics: Introduction and applications.
of super
conductivity, Type I
and applications.
Type II super conductor and its
Energy level diagram of Na Atom3p 3s
2p
2s1s
Ground state
The energy
absorbed or emitted
is in the form
of photons.
A photon is particle
of electro magnetic
wave.
Energy
When an electron jumps from inner to outer
orbit i.e. lower to higher energy level it
absorb the energy equal to difference
between these two energy levels. Similarly if
electron jumps down, it emits that energy
difference.
E2 – E1 = hv
Where h = Plank’s constant = 6.62 × 10-34 J-s andv = frequency
Also c = v , where = wavelength
E2 – E1 = E =hv
hcE = hv =
1 eV = 1.6 × 10–19
J
EXCITATION POTENTIAL :
The minimum energy required to raisean atom from one energy level toanother is called an excitation Energy orpotential.
E1 = –13.6eV E2 =– 3.4 eV
In case of H2 atom
E = E2 – E1 = [–3.4] – [–13.6] eV
E = 10.2 eV
IONISATION POTENTIAL
The minimum energy required to remove an
electron from outermost orbit of an atom is
ionisationcalled ionisation energy OR
potential.
E = E2 – E1 = [0] – [–13.6] eV
Hence the ionisation potential of
hydrogen atom is 13.6 eV.
In case of H2 atom
SPONTANEOUS EMISSION
An atom in the excited state is unstable
then it emits energy in form of photon in all
directions and comes back to groundstate or lower energy state. This
emission is called spontaneous.
STIMUL ATED EMISSIONThe process of forcing an atom to emit a
photon of same energy,
direction by another
frequency and
photon is called
stimulated emission.
The process of increasing the number of
atoms in upper state than in lower state is
called population inversion.
OPTICAL PUMPING
It is a method to achieve population inversion
in a sample. A flash lamp is used to increase
number of excited atoms by supplying
energy to ground state atoms.
POPULATION INVERSION
LASERLightAmplificationby StimulatedEmissionof Radiation
Types ofLaser*
*
*
Solid State Laser ( Ruby Laser)
Gas Laser (He- Ne Laser)
Semiconductor Laser(Galium Arsenide
laser is a semiconductor laser.)
RUBY LASERPrinciple: Laser action takes place in three steps.
E1
E2
E3
Ground State
Excited State
Construction:The laser element ruby rod which is a crystal ofaluminum oxide (Al2O3) doped with somepercentage (0.5%) of chromium atoms. The rodis about 2 cm to 30 cm in length and 0.5 cm to 2cm in diameter.
Working:When the external light is flashed, some
energy in the form of blue and green radiation
(5500 Å) is absorbed by the ruby rod.
Thus populationinversion results and
proper
condition for induced emission is created. The
highly populated state E3 is now exposed to
ruby red light of wavelength 6943 Å.
APPLICATIONS OF LASER(1)LASER WELDING
(2)LASER CUTTING
(3)LASER DRILLING
(4)MEASUREMENT OF DISTANCE
(5)LASER BASED ALIGNMENT
(6)LASERS IN CHEMISTRY
(7)COMMUNICATION BY LASER
(8)LASERS IN COMPUTER AND AUDIO, VIDEOSYSTEMS
OPTICAL FIBREAn optical fibre is a very thin fibre made
of glass or silica having a radius of the order
of a micrometer (10–6 m).
This is cladded by a thin layer of material
of lower refractive index.
µ2
µ1
>
µ2
µ1
Light pipe
A bundle of such thin fibres is called a light pipe.
TYPES OF FIBRE
1.Step-index optical fibre.
2. Graded index optical fibre.
1. Step-index optical fibre.Core material and cladding material has
uniform refractive index. But refractive index of
cladding material is slightly
material.
lower than core
8-12 µm( Core)
50-200 µm (core)
Only one wave to passthrough
Allow differentwave- mode topass through
2. Graded index optical fibre.In this type of optical fibre the refractive index of thecore is non uniform which increasestowards the claddingThe cladding has uniform refractive index.
from axis
Core
50-200 µm (core)
The rays entering at differentdifferent paths with same period.
angles follow
APPLICATIONS OF OPTICAL FIBRE
1. Light pipe is used to examine theinaccessible parts of the human body.
2. Optical Fibre are also used for transmittingand receiving electrical signals which areconverted to light by transducers.
3. They are used to transmit communication signalsthrough light pipes.4. Optical fibres are also being extensively used forlocal area networks (LAN)1. The clarity of the signals transmitted with optical
fibres is much better than otherconventional methods.
SUPER CONDUCTIVITY
The property of zero electrical resistance insome substances at very low absolutetemperatures. This phenomenon is calledsuper conductivity.
Superconductor
A superconductor is an element or metallic alloywhich, when cooled to near absolutezero, dramatically lose all electrical resistance.
Critical temperature TcThe critical temperature for
superconductors is the temperature at
which the electrical resistivity of a metal
drops to zero.
Tc
The critical temperature for mercury is 4.2 K.
EFFECT OF MAGNETIC FIELD
In the presence of strong magnetic field asuper conductor loses its super conductivityi.e. restoration to normal conducting state.
(a) T > Tc (b) T < Tc
Meissner effect
The effect, when a super conductor never hasa magnetic flux density even when in appliedmagnetic field, is called Meissner effect.
Type I and Type II Super ConductorsType I Super Conductors : They
are completely diamagnetic and hence the
flux is completely excluded. Type I
super conductors are also known as soft
super conductors. These exist in two states.
Examples
Super Conductors Tc, K Bc (0), Tesla
Al 1.18 0.0105Hg 4.15 0.0411Zn 0.85 0.0054
Type II Super Conductors :These Super Conductors are usually alloys and havean intermediate state also between normal &superconducting state.Type II super conductors (hard super conductors) areused to make high-field magnets, fusion reactors,and super conducting wires.
Examples
Super Conductor Tc, K Bc2 (0), Tesla
Nb3 Sn 18.0 24.5
Nb3 Ge 23.2 38.0
Nb3 Al 18.7 32.4
Applications of super conductors
1. Super conductors are used for
producing very strong magnetic field of
about 20-30 Tesla.
2. Power can be
super-conducting
losses.
transmitted through the
cables without any
3. They can be used to perform logic and
storage functions in computers.
4. A super conductor material can be
suspended in air against repulsive
force from permanent magnet. This
l e v i t a t i o n e f f e c t c a n b e u s e d i n
transportation.
5. Electrical machines and transformers
developed using super conductors
will have small size and high efficiency.
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