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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