ANALOG ELECTRONICS.pptx

B.L.D.E.Association’s
SHREE SANGANABASAVA MAHASWAMIJI POLYTECHNIC VIJAYAPUR – 03
MECHANICAL ENGINEERING DEPARTMENT
ELECTRICAL AND ELECTRONICS DEPARTMENT
Course : Analog Electronics
Semiconductors and Diodes
According to Bohr
i) An atom consists of a positively charged nucleus around which
negatively charged electrons revolve in different circular orbits.
ii) The electrons can revolve around the nucleus only in certain permitted
orbits i.e orbits of certain radii are allowed.
iii) The electrons in each permitted orbit have a certain fixed amount of
energy. The larger the orbit (i.e. larger radius), the greater is the energy
of electrons.
iv)If an electron is given additional energy(e.g. heat ,light etc),it is lifted
to the higher orbit .The atom is said to be in a state of excitation. This
state does not last long, because the electron soon falls back the acquired
energy in the form of heat, light or other radiation.

ELECTRICAL AND ELECTRONICS ENGINEERING DEPARTMENT
Structure of silicon atom
Valence Electrons:
The electrons in the outermost orbit of an atom are known as valence electrons. The valence electrons determines the
physical (metal or non –metal), chemical (gas or solid) and electrical properties of a material.
The valence electrons of different materials possess different energies. The greater the energy of a valence electrons, the
lesser it is bound to the nucleus.
Free Electrons:
The valence electrons which are very loosely attached to the nucleus are known as “free electrons”.
Thus free electrons moves under various fields and conduct electricity. More the number of free electrons better is the
conductivity.

DEPARTMENT OF MECHANICAL ENGINEERING
Energy levels:
First figure shows energy levels diagrams .The first orbit
represents the first energy level; the second orbit indicates
the second energy level and so on. The larger the orbit of an
electron, the greater is its energy and higher is the energy
level.
Second figure shows the energy levels of a single isolated
atom. Each orbit of an atom has a single energy. Therefore,
an electron can have only single energy corresponding to
the orbit in which its exist.
An electron possesses two types of energies i.e. kinetic energy due to its motion and potentials energy due to the
charge on the nucleus. The total energy of electrons are the sum of these two energies. The energy of an electron
increases as its distance from the nucleus increases. Thus it is clear that electrons in the last orbit possess very high
energy as compared to electron in the inner orbits.

Energy band
“The range of energies possessed by an electron in a solid is known as energy band”.
All valence electrons cannot become free. Thus there is energy band associated with
valence electrons called valence band.
The energy levels of free electrons also merge into each other to form an energy
band called conduction band.
when the atom is in solid, the electrons in any orbit can have a range of energies.
An energy band which separates the conduction band and the valence band
on energy band level diagram is called forbidden band or forbidden
energy gap denoted as EG.

Based on energy gap Eg, the materials are classified as conductors, insulators
and semiconductors.
Conductors
In terms of energy band ,the valence
and conduction bands are overlapped.
Insulators
In terms of energy band, the valence
band is full while the conduction
band is empty.
Semiconductors
In terms of energy band, the
valence band is almost filled and
conduction band is almost empty.

Properties of Semiconductors:
i) The resistivity of a semiconductor is less than an insulator but more than a conductor.
ii) Semiconductor have negative temperature coefficient of resistance i.e the resistance of a
semiconductor decreases with the increase in temperature and vice-versa.
Ex:Gremanium is actually an insulator at low temperature.
iii) When a suitable metallic impurity (E.g arsenic,gallium etc)is added to a semiconductor to a
semiconductor, its current conducting properties change appreciably.
iv) Semiconducting exhibit a rise in conductivity with increase in temperature and falls off at low
temperature.

Intrinsic Semiconductor
A semiconductor in an extremely pure form is known as an intrinsic semiconductor.
The conductivity of such intrinsic semiconductor is very poor and practically cannot be
used for manufacturing of the semiconductor devices.
Extrinsic Semiconductor
To increase the conducting properties of intrinsic semiconductor, it must be altered
.This is achieved by adding a small amount of suitable impurity to a semiconductor. Such
impure semiconductor is called an extrinsic semiconductor.
Doping: The process of adding impurities to an intrinsic semiconductor is known as
doping .The impurity added is called dopant.

p-type and n-type extrinsic semiconductor
There are two types of impurities used to obtain two different types of extrinsic semiconductors called p-
type and n-type.
The impurity having five valence electrons is called pentavalent impurity. When this is added, its each atom
donates one free electron and such doping is called donor doping. This creates n-type extrinsic
semiconductor.E.x:Arsenic ,bismuth,phosphoraous etc.

The impurity which has three valence electrons is called trivalent impurity .When this is
added, its each atom creates one hole which is ready to accept an electron .Hence this is
called acceptor an electron .Hence this is called acceptor doping. This creates p type
extrinsic semiconductor.
E.x: Gallium, Indium and Boron

Majority Charge Carriers and Minority Charge Carriers:
In n-type materials, the free electrons are large in number than Holes. Hence current is mainly
due to the free electrons. Hence free electrons are majority carriers while Holes are minority
carries in n-type materials.
In p-type material, the holes are large in number then free electrons .Hence current is mainly
due to holes while free electrons are minority carriers in p-type material.

Diffusion:
In given materials, if doping is not uniform then at one side there are large number of charge carriers (either free
electrons or holes) while there are less number of charge carriers on other side .As charge carriers are of similar type,
they start repelling in high concentration area .Thus charge carriers starts moving from high concentration area
towards low concentration area till uniform concentration is achieved all over the material .This process is called
“diffusion”.
Diffusion: Diffusion is a process in which charges move from a region of higher concentration to lower
concentration.

Diode (p-n junction)
When a sample of silicon is doped with both donor and acceptor semiconductor impurities so as to form a region of p-
type and second of n-type material in the same crystal lattice as shown in figure, the boundary where the regions meet is
called a p-n junction i.e when a p-type semiconductor, is suitably joined to an n-type semiconductor. The contact surface
is called p-n junction.
When the two pieces are joined together, they form pnjunction.The
resulting semiconductor devices called p-n junction diode. A diode is so
called because it has two terminal .One is anode, which is the positive
terminal and cathode which is the negative terminal. In case of a p-n
junction diode, the anode is the p-type and cathode is the n-type.

Definition: It is a two layer, two terminal, pn semiconductor device. A p-n junction diode is an
unidirectional device. It can conduct current only in one direction of the conventional current which can
flow when an external voltage is applied.

Formation of depletion region in an unbiased p-n junction
When p-n junction is formed, some of the conduction electrons from n-
type material diffuse into p-type and recombine with holes of acceptor
atoms. This acceptor atom becomes negative immobile ions and
accumulates just near the junction in p-type.
Simultaneously, holes from p-type material diffuse over to the n-
type material and recombine with electrons in donor atoms. This donor
atom becomes positive immobile ions and accumulates near the
junction in n-region.
When a sufficient number of donor and acceptor ions are
uncovered, further diffusion is prevented. Hence this diffusion process
establishes a wall of negative immobile charge on p-side and positive
immobile charge on n side. The region is depleted with immobile charge
carriers, hence it is called depletion region ,depletion layer or space
charge region.

Biasing: Applying external voltage to p-n junction diode is called biasing.
Forward biasing: When an external d.c voltage is connected in such a way that p region is connected to
positive and n region to negative of the d.c voltage then the biasing is called forward biasing.
Or
When external voltage applied to the junction is in such a direction that it cancels the potential barrier,
thus permitting current flow it is called forward biasing.
Figure shows p-n junction under forward biased condition
.When the voltage is increased, electrons and holes move
towards the junction . So that the depletion region
decreases and becomes zero when the applied voltage is
equal to V.Since depletion width is zero and the diode offers
zero resistance .Thus a forward biased p-n junction behaves
like a closed switch.

Reverse biased condition
When an external dc voltage is connected in such a way that p-region is connected to negative and n-region
to positive terminal of the d.c voltage then the biasing is called reverse biasing.
Or
When the external voltage applied to the junction is in direction that potential barrier is increased, it is
called reverse biasing.
Figure shows a p-n junction under reverse biased condition
.When reverse voltage is applied, majority carriers move
away from the junction. So that the depletion width start
increasing. The diode is known offers very high resistance
behaves like an open switch.
Due to the presence of a small number of minority
carriers a very small reverse saturation current flows through
the diode (1-2µA).This is also called the leakage current.

V_ I Characteristics of p-n junction
a) Zero external voltage: With zero external voltage the potential circuit behaves like a open circuit and not allowed
the current to flow as indicated by ‘O’ in figure (a).

b) Forward bias: With forward bias to the p-n junction, the potential barrier is start
reducing .At some forward voltage (0.7v for Si and 0.3V for Ge) the potential barrier is
altogether eliminated and current starts flowing in the circuit .From this, the current
increases with the increase in forward voltage .Thus rising curve OB is obtained with
forward bias as shown in figure. In region OA, the current increases very slowly and the
curve is non-linear. In region AB, the current increases very sharply and curve is almost
linear and diode behaves an ordinary conductor.
c) Reverse bias: With reverse bias to pn junction potential barrier start increasing at
the junction .Therefore junction resistance becomes very high and practically no
current flows through the circuit .However in the reverse characteristics a very small
(in µA) flows in the circuit with reverse bias .This is called reverse saturation current ‘Is’
and it is due to minority carriers.

P-N Junction Diode Applications
• It is used in clipping circuits as wave shaping circuits in computers, radios, radars etc.
• It is used as switches in digital logic designs.
• It is used in detector and demodulator circuits.
• It is used in clamping circuits in TV receivers as well as voltage multipliers.
• It is used as rectifiers in DC power supply manufacturing.

ANALOG ELECTRONICS.pptx

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