Ch.

14 Semiconductors
Introduction
Applications - a cell phone, a smart watch, a computer or even an
LED lamp.
Electrical Conductivity –
Metals – 6.25 x 107 Sm-1
Insulators – 10-10 Sm-1
Semiconductors - 1.56 x 10-3 Sm-1
customized to have its electrical conductivity as per our
requirement.
Temperature dependence of electrical conductivity of a
semiconductor can be controlled.
 Electrical conduction in solids
It depends on
- temperature,
- the number of charge carriers,
- crystal structure,
- types and the nature of defects present in a solid.
Conductors Insulators Semiconductors

Eg . Any metals Eg. Glass, wood or rubber Eg. Silicon, germanium, gallium
arsenide, gallium nitride,
cadmium sulphide
Large number of free electrons very small number of free charge carriers in a
electrons. semiconductor can be controlled
as per requirement.
Temperature dependence of electrical
conductivity of (a) metals and (b)semiconductors.
Classification of Semiconductors
Elemental semiconductors: Silicon, germanium

Compound Semiconductors: Cadmium sulphide, zinc

sulphide, etc.

Organic Semiconductors: Anthracene, doped pthalocyanines,

polyaniline
Band theory of solids, a brief introduction
• The electrons in atoms are arranged in different and
discrete energy levels.
• According to Pauli’s exclusion principle, no two
electrons can have the same set of quantum numbers,
or, no two electrons with similar spin can occupy the
same energy level. Any energy level can accommodate
only two electrons (one with spin up state and the
other with spin down state).
• the topmost occupied energy level is called the valence level.
Corresponding energy band is called the valence band.
• When sufficient energy is provided to electrons from the valence band
they are raised to higher levels. The immediately next energy level that
electrons from valence band can occupy is called conduction level. The
band formed by conduction levels is called conduction band.
• In a semiconductor or an insulator, their is a gap between the bottom of the
conduction band and the top of the valence band. This is called the energy
gap or the band gap.
• Only electrons from the valence band can be excited to the empty
conduction band, if the thermal energy gained by these electrons is greater
than the band gap.
• Electrons can also gain energy when an external electric field is applied to a
solid.
• Metals – the valence band and the
conduction band overlap and there is no band
gap. Electrons, find it easy to gain electrical
energy. They are easily available for
conduction.
• Semiconductors - the band gap is fairly small.
When excited, electrons gain energy and
occupy energy levels in conduction band
easily and can take part in electric conduction.
• Insulators - wide gap between valence band
and conduction band. Therefore, electrons
find it very difficult to gain sufficient energy
and occupy energy levels in the conduction
band.
 Intrinsic Semiconductor
• A pure semiconductor such as pure silicon or pure germanium is called an
intrinsic semiconductor.
• At absolute zero temperature, all valence electrons are tightly bound to
respective atoms and the covalent bonds are complete. No conduction.
• At room temperature, a few covalent bonds are broken due to thermal
agitation and some valence electrons can gain energy. A valence electron is
moved to the conduction band.
• It creates a vacancy in the valence band.
• These vacancies of electrons in the valence band are called holes. The holes
are thus absence of electrons in the valence band and they carry an effective
positive charge.

• For an intrinsic semiconductor, the number of holes per unit volume, (the
number density, nh) and the number of free electrons per unit volume, (the
number density, ne) is the same. nh = ne
• Electrical conduction takes place by transportation of both carriers or any
one of the two carriers in a semiconductor. When a semiconductor is
connected in a circuit, electrons, being negatively charged, move towards
positive terminal of the battery. Holes have an effective positive charge, and
move towards negative terminal of the battery. Thus, the current through a
semiconductor is carried by two types of charge carriers which move in
opposite directions.
 Extrinsic semiconductors
• Addition of a small amount of a suitable impurity to an intrinsic
semiconductor increases its conductivity appreciably. The process of adding
impurities to an intrinsic semiconductor is called doping.
• The semiconductor with impurity is called a doped semiconductor or an
extrinsic semiconductor. The impurity is called the dopant. The parent atoms
are called hosts.
• Silicon or germanium can be doped with a pentavalent impurity such as
phosphorus (P) arsenic (As) or antimony (Sb) . They can also be doped with a
trivalent impurity such as boron (B) aluminium (Al) or indium (In).
• Extrinsic semiconductors can be of two types

a) n-type semiconductor or

b) p-type semiconductor.
 n-type semiconductor

These are materials doped with pentavalent impurity (donors) atoms . Electrical
conduction in these materials is due to electrons as majority charge carriers.
1. The donor atom lose electrons and become positively charged ions.
2. Number of free electrons is very large compared to the number of holes,
ne>> nh
Electrons are majority charge carriers.
3. When energy is supplied externally, negatively charged free electrons
(majority charges carries) and positively charged holes (minority charge carriers)
are available for conduction.
 p-type semiconductor

These are materials doped with trivalent impurity atoms (acceptors). Electrical
conduction in these materials is due to holes as majority charge carriers.
1. The acceptor atoms acquire electron and become negatively charged-ions.
2. Number of holes is very large compared to the number of free electrons.
nh >> ne
Holes are majority charge carriers.
3. When energy is supplied externally, positively charged holes (majority charge
carriers) and negatively charged free electrons (minority charge carriers) are
available for conduction.
Charge neutrality of extrinsic semiconductors:
• n-type as well as p-type semiconductors are electrically neutral.
• Always remember, for a semiconductor,

ne.nh = ni 2
 p-n junction
• When n-type and p-type semiconductor materials are fused together, a p-n
junction is formed.
Diffusion -
• the number of carriers on both sides is different and a large density gradient
exists on both sides of the p-n junction. This density gradient causes
migration of electrons from the n-side to the p-side of the junction. They fill
up the holes in the p-type material and produce negative ions.
• As a result, in the p-type region near the junction there are negatively
charged acceptor ions, and in the n-type region near the junction there are
positively charged donor ions.
• The transfer of electrons and holes across the p-n junction is called diffusion.
Depletion region:
• The diffusion of carriers across the junction and
resultant accumulation of positive and negative
charges across the junction builds a potential
difference across the junction. This potential
difference is called the potential barrier.
• It prevents continuous diffusion of carriers across
the junction.
• Free charge carriers cannot be present in a region
where there is a potential barrier. The regions on
either side of a junction, therefore, becomes
completely devoid of any charge carriers. This
region across the p-n junction where there are no
charges is called the depletion layer or the depletion
region.
Biasing a p-n junction:
p-n junction diode
Forward biased
Reverse biased
Static and dynamic resistance of a diode
(i) Static (DC) resistance

(ii) Dynamic (AC) resistance

 Semiconductor devices
• Advantages:
1. Electronic properties of semiconductors can be controlled to suit our
requirement.
2. They are smaller in size and light weight.
3. They can operate at smaller voltages (of the order of few mV) and require
less current (of the order of μA or mA), therefore, consume lesser power.
4. Almost no heating effects occur, therefore these devices are thermally
stable.
5. Faster speed of operation due to smaller size.
6. Fabrication of ICs is possible.
 Semiconductor devices
• Disadvantages:
1. They are sensitive to electrostatic charges.
2. Not vary useful for controlling high power.
3. They are sensitive to radiation.
4. They are sensitive to fluctuations in temperature.
5. They need controlled conditions for their manufacturing.
6. Very few materials are semiconductors.
Applications of semiconductors and p-n
junction diode
1. Solar cell
2. Photo resistor
3. Bi-polar junction transistor
4. Photodiode
5. LED
6. Solid State Laser
7. Integrated Circuits (ICs)
Thermistor
• Thermistor is a temperature sensitive resistor. Its resistance changes
with change in its temperature. There are two types of thermistors,
• Negative Temperature Coefficient (NTC) - Resistance of a NTC
thermistor decreases with increase in its temperature. Its
temperature coefficient is negative. Use - temperature sensors,
temperature control circuits.
• Positive Temperature Coefficient (PTC) - Resistance of a PTC
thermistor increases with increase in its temperature. Use - reusable
fuse to limit current passing through a circuit to protect against over
current conditions, as resettable fuses.