Unit:-1Semi Conductor Physics

ATOMIC STRUCTURE:

There are two models of atomic structure in use today: the

Bohr model and the quantum mechanical model. Of these two

models, the Bohr model is simpler and relatively easy to

understand.

A model is useful because it helps you understand what’s

observed in nature. It’s not unusual to have more than one

model represent and help people understand a particular topic.

▪ The Bohr model shows that the electrons in atoms are in

orbits of differing energy around the nucleus (think of planets

orbiting around the sun).

▪ Bohr used the term energy levels (or shells) to describe

these orbits of differing energy. He said that the energy of an

electron is quantized

Covalent bonding in silicon and germaniumCovalent bonding in silicon:-

The outermost shell of atom is capable to hold up to eight electrons. The atom which has eight electrons in the outermost orbit is said to be completely filled and most stable. But the outermost orbit of silicon has only four electrons. Silicon atom needs four more electrons to become most stable. Silicon atom forms four covalent bonds with the four neighboring atoms. In covalent bonding each valence electron is shared by two atoms.

Covalent bonding in germanium:-The outermost

orbit of germanium has only four electrons. Germanium

atom needs four more electrons to become most stable.

Germanium atom forms four covalent bonds with the

four neighboring atoms. In covalent bonding each

valence electron is shared by two atoms.

Conductors:-Conductors are generally substances which

have the property to pass different types of energy. In the

following, the conductivity of electricity is the value of

interest.

Insulator:-An electrical insulator is a material whose

internal electric charges do not flow freely; very little

electric current will flow through it under the influence of an

electric field. This contrasts with other materials,

semiconductors and conductors, which conduct electric

current more easily.

Semiconductors:-Semiconductors are solids whose

conductivity lies between the conductivity of conductors and insulators

Intrinsic Semiconductor

A semiconductor, which is in its extremely pure form, is known

as an intrinsic semiconductor. Silicon and germanium are the

most widely used intrinsic semiconductors.

Both silicon and germanium are

tetravalent, i.e. each has four

electrons (valence electrons) in

their outermost shell.

Each atom shares its four

valence electrons with its four

immediate neighbours, so that

each atom is involved in four

covalent bonds.

Extrinsic Semiconductor

Pure semiconductors have negligible conductivity at room

temperature. To increase the conductivity of

semiconductor, some impurity is added. The

intrinsic

resulting

semiconductor is called impure or extrinsic semiconductor.

Impurities are added at the rate of ~ one atom per 106 to 1010

semiconductor atoms. The purpose of adding impurity is to

increase either the number of free electrons or holes in a

semiconductor.

Energy Band Diagram

Conduction

electrons

Energy Band Diagram

Forbidden energy band is small

for semiconductors.

Less energy is required for

electron to move from valence

to conduction band.

A vacancy (hole) remains when

an electron leaves the valence

band.

Hole acts as a positive charge

carrier.

Semiconductors

The materials whose electrical conductivity lies between those

of conductors and insulators, are known as semiconductors.

Silicon

Germanium

Cadmium Sulphide

0.7 eV

0.3 eV

2.4 eV

Silicon is the most widely used semiconductor.

Semiconductors have negative temperature coefficients of

resistance, i.e. as temperature increases resistivity deceases

Intrinsic Semiconductor

nductor is

number of

When

increase

electron

Since s so,

holes (p)Free e

= Intrin

the temperature of an intrinsic semico

d, beyond room temperature a large

-hole pairs are generated.

the electron and holes are generated in pair

lectron concentration (n) = concentration of

sic carrier concentration (ni)

Extrinsic Semiconductor

Two types of impurity atoms are added to the semiconductor

Atoms containing 5

valance electrons

(Pentavalent impurity atoms)

e.g. P, As, Sb, Bi

Atoms containing 3

valance electrons

(Trivalent impurity atoms)

e.g. Al, Ga, B, In

N-type semiconductor P-type semiconductor

N-type Semiconductor

The semiconductors which are obtained by introducing

pentavalent impurity atoms are known as N-type

semiconductors.

Examples are P, Sb, As and Bi. These elements have 5

electrons in their valance shell. Out of which 4 electrons will

form covalent bonds with the neighbouring atoms and the 5th

electron will be available as a current carrier. The impurity atom

is thus known as donor atom.

In N-type semiconductor current flows due to the movement of

electrons and holes but majority of through electrons. Thus

electrons in a N-type semiconductor are known as majority

charge carriers while holes as minority charge carriers.

P-type Semiconductor

The semiconductors which are obtained by introducing trivalent

impurity atoms are known as P-type semiconductors.

Examples are Ga, In, Al and B. These elements have 3

electrons in their valance shell which will form covalent bonds

with the neighbouring atoms.

The fourth covalent bond will remain incomplete. A vacancy,

which exists in the incomplete covalent bond constitute a hole.

The impurity atom is thus known as acceptor atom.

In P-type semiconductor current flows due to the movement of

electrons and holes but majority of through holes. Thus holes in

a P-type semiconductor are known as majority charge carriers

while electrons as minority charge carriers.

The free electron and hole concentrations are related by the

Law of Electrical Neutrality i.e.

Total positive charge density is equal to the total negative

charge density

Let ND = Concentration of donor atoms = no. of positive

charges/m3 contributed by donor ions

p = hole concentration

NA=Concentration of acceptor atoms

n = electron concentration

By the law of electrical neutrality

ND + p = NA + n

Charge carrier concentration in N-type and

P-type Semiconductors

For N-Type semiconductor

NA = 0 i.e. Concentration of acceptor atoms

And n>>p, then

ND + 0 = 0 + n

ND = n

i.e. in N-type, concentration of donor atoms is equal to the

concentration of free electrons.

i

According to Mass Action Law

n.p n2

Dp n2 / n n2 / N

i i

For P-Type semiconductor

ND = 0 i.e. Concentration of donor atoms

And p>>n, then

NA + 0 = 0 + p

NA = p

i.e. in P-type, concentration of acceptor atoms is equal to the

concentration of holes.

According to Mass Action Law

in.p n2

An n2 / p n2 / N

i i

Barrier Formation in P-N Junction Diode

The holes from p-side diffuses to the n-side while the free

electrons from n-side diffuses to the p-side.

This movement occurs because of charge density gradient.

This leaves the negative acceptor ions on the p-side and

positive donor ions on the n-side uncovered in the vicinity of the

junction.

.

Barrier Formation in P-N Junction Diode

Thus there is negative charge on p-side and positive on n-side.

This sets up a potential difference across the junction and hence

an internal Electric field directed from n-side to p-side..

Equilibrium is established when the field becomes large enough to

stop further diffusion of the majority charge carriers.

The region which becomes depleted (free) of the mobile charge

carriers is called the depletion region. The potential barrier

across the depletion region is called the potential barrier.

Width of depletion region depends upon the doping level. The

higher the doping level, thinner will be the depletion region.

Unit:-2Semi Conductor Diode

PN Junction Diode:-P-N junction diode is the most

fundamental and the simplest electronics device. When one side of an intrinsic semiconductor is doped with acceptor i.e, one side is made p-type by doping with n-type material, ap-n junction diode is formed. This is a two terminal device.

Diffusion current:-Diffusion current is a current in

a semiconductor caused by the diffusion of charge carriers (holes and/or electrons). This is the current which is due to the transport of charges occurring because of non uniform concentration of charged particles in a semiconductor.

Diffusion current Drift current

Diffusion current movement caused by variation in the carrier concentration.

Drift current movement caused by electric fields.

Direction of the diffusion current depends on the slope of the carrier concentration.

Direction of the drift current is always in the direction of the electric field.

Forward Bias P-N Junction

When an external voltage is applied to

the P-N junction making the P side

positive with respect to the N side the

diode is said to be forward biased.

The barrier potential difference is decreased by the external

applied voltage. The depletion band narrows which urges

majority carriers to flow across the junction.

A Forward biased diode has a very low resistance.

When an external voltage is applied

to the P-N junction making the P side

negative with respect to the N side

the diode is said to be Reverse Biased.

The barrier potential difference increases. The depletion bandwidens preventing the movement of majority carriers across thejunction.

A Reverse Bias diode has a very high resistance.

Reverse Bias P-N Junction

Conductivity of N and P-type

semiconductor

For intrinsic semiconductor

i q.(n.n p. p )

For N-Type semiconductor (n>>p)

q.n.n

For P-type semiconductor (p>>n)

q.p. p

Diode Applications —Half Wave Rectifier

•Diode converts ac input voltage to a pulsed dc

output voltage.

•Whenever the ac input becomes negative at

diode’s anode, the diode blocks current flow.

– o/p voltage become zero.

•Diode introduces a 0.6V drop so o/p peak is 0.6V

smaller than the i/p peak.

•The o/p frequency is same as the i/p frequency.

Vin

Diode Applications —Full Wave Rectifier• A full-wave rectifier does not block negative swings in the i/p voltage, rather

it transforms them into positive swings at the o/p.

• To gain an understanding of device operation, follow current flow through pairs ofdiodes in the bridge circuit.

• It is easily seen that one pair (D3-Rout-D2) allows current flow during the +ve halfcycle of Vin while the other pair (D4-Rout-D1) allows current flow during the -ve halfcycle of Vin.

– o/p voltage peak is 1.2V below the i/p voltage peak.

– The o/p frequency is twice the i/p frequency.

D1 D3

D2 D4

ParametersCenter tapped full wave

rectifierBridge rectifier

Number of diodes 2 4

Maximum efficiency 81.2% 81.2%

Peak inverse voltage 2Vm Vm

Vdc(no load) 2Vm/π 2Vm/π

Transformer utilization factor

0.693 0.812

Ripple factor 0.48 0.48

Form factor 1.11 1.11

Peak factor √2 √2

Average current Idc/2 Idc/2

Output frequency 2f 2f

The differences between center tapped fullwave rectifier and bridge rectifier:-

Working of Zener Diode

Zener Forward

Zener Reverse

Experimental Determination of Carrier concentration and

Mobility

▪ Consider a semiconductor (P-Type or N-types) in which

current I and Magnetic field B is applied, a force is act on

the charge carriers. This force pushing the charge carriers

towards the back of the semiconductor.

▪ When the mobile carriers (i.e. electrons or holes) are

pushed towards the back, the front becomes depleted and

the semiconductor loss it neutrality.

▪ Now there is an excess of mobile charge carriers at the

back and an excess of opposite charge due to impurity

atom at the front.

If the semiconductor is N-type,

The electron will be in excess at the back surface and the surface

becomes negatively charged with respect to front. This gives rise

to a potential difference called Hall voltage between front and

back

If the semiconductor is P-type,

The hole will be in excess at the back surface and the surface

becomes positively charged with respect to front. The polarity of

Hall voltage is in reverse direction

FILTER CIRCUIT:-

The filter is a device that allows passing the

dc component of the load and blocks the ac component of the

rectifier output. Thus the output of the filter circuit will be a

steady dc voltage.

Shunt Capacitor Filter:-

As the name suggests, a capacitor is used as the filter and this high value capacitor is shunted or placed across the load impedance.

LC or L Section Filter:-

L section filter connected to a full wave bridge rectifier

The inductor will operate as an inductor filter, attempting to

keep the current flowing through the load at a constant

rate. The

reduced ripple voltage from the inductor will then be

filtered by the action of the capacitor as illustrated

Pi Filter:-

AC sees the capacitors as a short circuit and the inductor as a block

DC sees the capacitors as an open circuit and the inductor as a short

circuit

The AC of the will flow through C1 and be blocked by the inductor the

DC will not pass through the capacitor but will pass through the

inductor. Any AC not passed through C1 will then be “cleaned up” by

C2

Types of Diodes

• Light Emitting Diode (LED)

• Avalanche Diode.

• Laser Diode.

• Schottky Diode.

• Zener Diode.

• Photo Diode.

• Varicap Diode or Varactor Diode.

• Rectifier Diode.

ZENER

DIODE It was named after Clarence Zener, who discovered this electrical

property.

It Permits current in the forward direction as well as in the reverse

direction if the voltage is larger than the breakdown voltage or “Zener

knee voltage” or “Zener voltage”.

▪ USES

▪ Zener Diode Shunt Regulator.

▪ Zener Diode as Peak Clipper.

▪ Switching operation.

USES OF ZENER DIODE:-1. Zener Diode Shunt Regulator

2. Meter Protection

3. Zener Diode as Peak Clipper

4. Switching operation.

Light-emitting diode:-

Emits visible light when an electric current passes

through it.

LED’s are essentially pn diodes operated in

forward bias.

Benefits Of LED:

• Low power requirement.

• High efficiency.

• Long life.

Applications:

• Indicator lights.

• LCD panel backlighting.

• Fiber optic data transmission.

• Remote control.

Photo Diode:-

A type of photo detector capable of converting light into either current or voltage.

Designed to operate in reverse bias.

USES :

Used in:

• CD Players.

• Smoke Detectors.

• Light sensors

• Various medical applications, such as detectors

for computer tomography and pulse oximeters.

Breakdown in P-N junction diode

In Electronics, the term “breakdown” stands for release of

electron-hole pairs in excess.

The critical value of the voltage, at which the breakdown of a

P-N junction diode occurs is called the breakdown voltage.

The breakdown voltage depends on the width of the depletion

region, which, in turn, depends on the doping level.

There are two mechanisms by which breakdown can occur at a

reverse biased P-N junction:

1. Avlanche Breakdown (uncontrolled)

2. Zener Breakdown (controlled)

Avalanche breakdown

If the reverse bias is made very high, the thermally generated

electrons and holes get sufficient K.E from applied voltage to

break the covalent bonds near the junction and a large no. of

electron-hole pairs are released. These new carriers, in turn,

produce additional carrier again by breaking bonds. Thus

reverse current then increase abruptly and may damage the

junction by the excessive heat generated.

The avalanche breakdown occurs in lightly doped junctions,

which produce wide depletion region.

The avalanche breakdown voltage increases as the temp. of

the junction increases due to the increased probability of

collisions of electron and holes with crystal atoms.

Zener breakdown

Zener Breakdown occurs at low voltage in heavily doped

reverse biased p-n junction.

Strong electric field directly (without impact of electron) pull out

the electrons from the covalent bond.

Zener breakdown voltage decreases as the temp. of the

junction increases. Since an increase in temp. increase the

energy of valence electron. So escape from covalent bond

become easier for these electrons. Thus a smaller reverse

voltage Is sufficient to pull the valence electron out of the

covalent bonds.

V-I Characterstics of Zener and

Avalanche breakdown

Unit:-3Introduction to Bipolar Transistor

Transistor:-Transistors are basically classified into two types; they are Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET). The BJTs are again classified into NPN and PNP transistors.NPN TransistorNPN is one of the two types of Bipolar Junction Transistors (BJT). The NPN transistor consists of two n-type semiconductor materials and they are separated by a thin layer of p-type semiconductor. Here the majority charge carriers are electrons and holes are the minority charge carriers.PNP TransistorThe PNP is another type of Bipolar Junction Transistors (BJT). The PNP transistors contain two p-type semiconductor materials and are separated by a thin layer of n-type semiconductor.