Formation of p-n junction

Generally junction refers to a point where two or more

things are joined. For example, when one or more
railway tracks are joined a railway junction is formed.
The region where the tracks meet or joined is called
railway junction.
In the similar way, when an n-type semiconductor is
joined with the p-type semiconductor, a p-n junction is
formed. The region where the p-type and n-type
semiconductors are joined is called p-n junction. It is
also defined as the boundary between p-type and n-type
semiconductor. This p-n junction forms a most popular
semiconductor device known as diode.
 P-N Junction

• The credit of discovery of the p-n junction

goes to American physicist Russel Ohi of
Bell Laboratories.
• p-n junction is also a fundamental building
block of many other semiconductor
electronic devices such as transistors,
solar cells, light emitting diodes, and
integrated circuits
Zero bias P-N junction
 Zero bias P-N junction
• The p-n Junction in which no external voltage is applied is
called zero bias p-n junction. Zero bias p-n Junction is also
called as unbiased p-n junction.
• In n-type semiconductor large number of free electrons is
present while at p-type semiconductor small number of free
electrons is present. Hence, the concentration of electrons at
n-type semiconductor is high while the concentration of
electrons at p-type semiconductor is low.

• Due to this high concentration of electrons at n-side, they get

repelled from each other. Hence they try to move towards the
low concentration region.
 Zero bias P-N junction
• According to coulomb’s law there exist an electrostatic
force of attraction between the opposite charges.
• Hence, the free electrons from the n-side are attracted
towards the holes at the p-side. Thus, the free electrons
move from n-region (high concentration region) to p-
region (low concentration region).
• Similarly the concentration of holes at p-type
semiconductor is high while the concentration of holes at
n-type semiconductor is low.
• Hence, the holes from the p-side are attracted towards
the free electrons at the n-side. Thus, the holes move
from p-region (high concentration region) to n-region (low
concentration region).
 Formation of positive and negative ion

• Each free electron that crosses the junction

to fill the holes in the p-side creates negative
ions on the p-side. Negative ions are also
called as acceptors because they accept
extra electrons from outside atoms.
• Each free electron that left the n-side
parent atom and crosses the junction to fill
the holes in the p-side atom creates positive
ion at n-side. Positive ions are also called as
donors because they donate extra electrons
to the outside atoms.
 Barrier voltage
• Thus, a net positive charge is built at the n-side of the p-n
junction due to the positive ions at the n-side; similarly a
net negative charge is built at the p-side of the p-n junction
due to the negative ions at the p-side after diffusion.
• This net negative charge at the p-side of the p-n junction
prevents the further flow of free electrons crossing from n-
side to p-side because the negative charge present at the
p-side of p-n junction repels the free electrons.
• Similarly, the net positive charge at n-side of the p-n
junction prevents the further flow of holes crossing from p-
side to n-side. Hence, positive charge present at n-side
and negative charge present at p-side of p-n junction acts
as barrier between p-type and n-type semiconductor.
 Barrier voltage
• Thus, a barrier is build near the junction
which prevents the further movement of
electrons and holes.
 Barrier voltage
• The negative charge formed at the p-side of the p-n
junction is called negative barrier voltage while the
positive charge formed at the n-side of the p-n junction
is called positive barrier voltage. The total charge
formed at the p-n junction is called barrier voltage,
barrier potential or junction barrier.
• The size of the barrier voltage at the p-n junction is
depends on, the amount of doping, junction
temperature and type of material used. The barrier
voltage for silicon diode is 0.7 volts and for
germanium is 0.3 volts.
• The barrier voltage at the p-n junction opposes only
the flow of majority charge carriers but allows the flow
of minority carriers (.I.e. free electrons at p-side and
holes at n-side) to cross the junction.
 What is depletion region?
• Generally, depletion refers to reduction or decrease or
deficiency in quantity of something.
• The region near the junction where flow of charges carriers
are decreased over a given time and finally results in empty
charge carriers or full of immobile charge carriers is called
depletion region.
• The depletion region is also called as depletion zone,
depletion layer, space charge region, or space charge layer.
The depletion region acts like a wall between p-type and n-
type semiconductor and prevents further flow of free
electrons and holes.
• The width of depletion region depends on the amount of
impurities added to the semiconductor. , the width of
depletion region is more in the lightly doped semiconductors
and is less in the heavily doped semiconductor
 P-N junction semiconductor diode
• A p-n junction diode is two-terminal or two-
electrode semiconductor device, which allows the
electric current in only one direction while blocks the
electric current in opposite or reverse direction. If the
diode is forward biased, it allows the electric current
flow. On the other hand, if the diode is reverse biased,
it blocks the electric current flow.
• The p-n junction diode is made from the
semiconductor materials such as silicon, germanium,
and gallium arsenide. For designing the diodes, silicon
is more preferred over germanium. The p-n junction
diodes made from silicon semiconductors works at
higher temperature when compared with the p-n
junction diodes made from germanium
semiconductors.
 P-N junction semiconductor diode
• The basic symbol of p-n junction diode
under forward bias and reverse bias is
shown in the below figure.
 Ideal diode
• The ideal diode or perfect diode is a two terminal device,
which completely allows the electric current without any loss
under forward bias and completely blocks the electric current
with infinite loss under reverse bias.

• Ideal diodes actually do not exist. However, the V-I

characteristics of ideal diodes is used to study the diode
circuits. In other words, it is used to study the quality of a real
diode by comparing it with the ideal diode.

• Under forward biased condition, ideal diode acts as a perfect

conductor with zero resistance whereas under reverse biased
condition, it acts as a perfect insulator with infinite resistance.
In other words, ideal diodes acts as closed circuit or short
circuit under forward biased condition and acts as an open
circuit or open switch under reverse biased condition.
Ideal diode symbol
 Biasing of p-n junction semiconductor diode

• The process of applying the external voltage to a p-n junction

semiconductor diode is called biasing.

• External voltage to the p-n junction diode is applied in any of the two
methods: forward biasing or reverse biasing.

• If the p-n junction diode is forward biased, it allows the electric current
flow. Under forward biased condition, the p-type semiconductor is
connected to the positive terminal of battery whereas; the n-type
semiconductor is connected to the negative terminal of battery.

• If the p-n junction diode is reverse biased, it blocks the electric current
flow. Under reverse biased condition, the p-type semiconductor is
connected to the negative terminal of battery whereas; the n-type
semiconductor is connected to the positive terminal of battery.
 Forward biased p-n junction diode
• The process by which, a p-n junction diode
allows the electric current in the presence
of applied voltage is called forward biased
p-n junction diode.
• In forward biased p-n junction diode, the
positive terminal of the battery is
connected to the p-type
semiconductor material and the negative
terminal of the battery is connected to
the n-type semiconductor material.
• Under no voltage or unbiased condition, the
p-n junction diode does not allow the electric
current. If the external forward voltage
applied on the p-n junction diode is increased
from zero to 0.1 volts, the depletion
region slightly decreases. Hence, very small
electric current flows in the p-n junction
diode. However, this small electric current in
the p-n junction diode is considered as
negligible. Hence, they not used for any
practical applications
• If the voltage applied on the p-n junction diode is
further increased, then even more number of free
electrons and holes are generated in the p-n
junction diode. This large number of free electrons
and holes further reduces the depletion region
(positive and negative ions). Hence, the electric
current in the p-n junction diode increases. Thus,
the depletion region of a p-n junction diode
decreases with increase in voltage. In other words,
the electric current in the p-n junction diode
increases with the increase in voltage.
• If the p-n junction diode is forward biased with
approximately 0.7 volts for silicon diode or 0.3 volts for
germanium diode, the p-n junction diode starts
allowing the electric current. Under this condition, the
negative terminal of the battery supplies large number
of free electrons to the n-type semiconductor and
attracts or accepts large number of holes from the p-
type semiconductor. In other words, the large number
of free electrons begins their journey at the negative
terminal whereas the large number of holes finishes
their journey at the negative terminal.
• The free electrons, which begin their journey from the negative terminal, produce a large
negative electric field. The direction of this negative electric field is apposite to the direction of
positive electric field of depletion region (positive ions) near the p-n junction.

• Due to the large number of free electrons at n-type semiconductor, they get repelled from
each other and try to move from higher concentration region (n-type semiconductor) to a
lower concentration region (p-type semiconductor). However, before crossing the depletion
region, free electrons finds the positive ions and fills the holes. The free electrons, which fills
the holes in positive ions becomes valence electrons. Thus, the free electrons are
disappeared.

• The positive ions, which gain the electrons, become neutral atoms. Thus, the depletion region
(positive electric field) at n-type semiconductor near the p-n junction decreases until it
disappears.

• The remaining free electrons will cross the depletion region and then enters into the p-
semiconductor. The free electrons, which cross the depletion region finds the large number of
holes or vacancies in the p-type semiconductor and fills them with electrons. The free
electrons which occupy the holes or vacancies will becomes valence electrons and then these
electrons get attracted towards the positive terminal of battery or terminates at the positive
terminal of battery. Thus, the negative charge carriers (free electrons) that are crossing the
depletion region carry the electric current from one point to another point in the p-n junction
diode.
• The positive terminal of the battery supplies large number of holes to the
p-type semiconductor and attracts or accepts large number of free
electrons from the n-type semiconductor. In other words, the large
number of holes begins their journey at the positive terminal whereas the
large number of free electrons finishes their journey at the positive
terminal.

• The holes, which begin their journey from the positive terminal, produce a
large positive electric field at p-type semiconductor. The direction this
positive electric field is opposite to the direction of negative electric field of
depletion region (negative ions) near the p-n junction.

• Due to the large number of positive charge carriers (holes) at p-type

semiconductor, they get repelled from each other and try to move from
higher concentration region (p-type semiconductor) to a lower
concentration region (n-type semiconductor). However, before crossing
the depletion region, some of the holes finds the negative ions and
replaces the electrons position with holes. Thus, the holes are
disappeared.
• The negative ions, which lose the electrons, become
neutral atoms. Thus, the depletion region or negative
ions (negative electric field) at p-type semiconductor
near the p-n junction decreases until it disappears.
• The remaining holes will cross the depletion region
and attracted to the negative terminal of battery or
terminate at the negative terminal of battery. Thus, the
positive charge carriers (holes) that are crossing the
depletion region carry the electric current from one
point to another point in the p-n junction diode.
 Reverse biased p-n junction diode
• The process by which, a p-n junction diode blocks the electric
current in the presence of applied voltage is called reverse biased p-
n junction diode.

• In reverse biased p-n junction diode, the positive terminal of the

battery is connected to the n-type semiconductor material and the
negative terminal of the battery is connected to the p-type
semiconductor material.

• When the external voltage is applied to the p-n junction diode in

such a way that, negative terminal is connected to the p-type
semiconductor and positive terminal is connected to the n-type
semiconductor, holes from the p-side are attracted towards the
negative terminal whereas free electrons from the n-side are
attracted towards the positive terminal.
• In reverse biased p-n junction diode, the free electrons begin their
journey at the negative terminal whereas holes begin their journey at the
positive terminal. Free electrons, which begin their journey at the
negative terminal, find large number of holes at the p-type semiconductor
and fill them with electrons. The atom, which gains an extra electron,
becomes a charged atom or negative ion or motionless charge. These
negative ions at p-n junction (p-side) oppose the flow of free electrons
from n-side.

• On the other hand, holes or positive charges, which begin their journey at
the positive terminal, find large of free electrons at the n-type
semiconductor and replace the electrons position with holes. The atom,
which loses an electron, becomes a charged atom or positive ion. These
positive ions at p-n junction (n-side) oppose the flow of positive charge
carriers (holes) from p-side.
• If the reverse biased voltage applied on the p-n junction diode is further
increased, then even more number of free electrons and holes are
pulled away from the p-n junction. This increases the width of depletion
region. Hence, the width of the depletion region increases with increase
in voltage. The wide depletion region of the p-n junction diode
completely blocks the majority charge carriers. Hence, majority charge
carriers cannot carry the electric current.

• However, p-n junction diode allows the minority charge carriers. The
positive terminal of the battery pushes the holes (minority carriers)
towards the p-type semiconductor. In the similar way, negative terminal
of the battery pushes the free electrons (minority carriers) towards the
n-type semiconductor.
• The positive charge carriers (holes) which cross the p-
n junction are attracted towards the negative terminal
of the battery. On the other hand, the negative charge
carriers (free electrons) which cross the p-n junction
are attracted towards the positive terminal of the
battery. Thus, the minority charge carriers carry the
electric current in reverse biased p-n junction diode.
• The electric current carried by the minority charge
carriers is very small. Hence, minority carrier current is
considered as negligible.
V-I characteristics of p-n junction diode
 V-I characteristics of p-n junction diode

• The V-I characteristics or voltage-current

characteristics of the p-n junction diode is
shown in the figure. The horizontal line in
the figure represents the amount
of voltage applied across the p-n junction
diode whereas the vertical line represents
the amount of current flows in the p-n
junction diode.
 Forward V-I characteristics of silicon
diode

VF represents the forward voltage whereas IF represents

the forward current.
 Forward V-I characteristics of silicon diode

• If the external voltage applied on the silicon diode is less than

0.7 volts, the silicon diode allows only a small electric current.
However, this small electric current is considered as
negligible.
• When the external voltage applied on the silicon diode
reaches 0.7 volts, the p-n junction diode starts allowing large
electric current through it. At this point, a small increase in
voltage increases the electric current rapidly. The forward
voltage at which the silicon diode starts allowing large electric
current is called cut-in voltage. The cut-in voltage for silicon
diode is approximately 0.7 volts.
Forward V-I characteristics of germanium diode
Reverse V-I characteristics of p-n junction
diode

VR represents the reverse voltage

whereas IR represents the reverse
current.
 Reverse V-I characteristics of p-n junction diode
• The reverse saturation current is depends on the temperature. If
temperature increases the generation of minority charge carriers
increases. Hence, the reverse current increases with the increase in
temperature. However, the reverse saturation current is independent of
the external reverse voltage. Hence, the reverse saturation current
remains constant with the increase in voltage. However, if the voltage
applied on the diode is increased continuously, the p-n junction diode
reaches to a state where junction breakdown occurs and reverse current
increases rapidly.

• In germanium diodes, a small increase in temperature generates large

number of minority charge carriers. The number of minority charge
carriers generated in the germanium diodes is greater than the silicon
diodes. Hence, the reverse saturation current in the germanium diodes is
greater than the silicon diodes.
V-I Characteristics of Ideal diode
 Diode resistance
• The two types of resistance found in forward biased
diode are
A)Static resistance or DC resistance
B)Dynamic resistance or AC resistance
 Static resistance or DC resistance
• The resistance offered by a p-n junction diode
when it is connected to a DC circuit is called
static resistance.
• Static resistance is also defined as the ratio of
DC voltage applied across diode to the DC
current or direct current flowing through the
diode.
• The resistance offered by the p-n junction diode
under forward biased condition is denoted as Rf.
Dynamic resistance or AC resistance
• The dynamic resistance is the resistance offered
by the p-n junction diode when AC voltage is
applied.
• When forward biased voltage is applied to a diode
that is connected to AC circuit, an AC or
alternating current flows though the diode.
• Dynamic resistance is also defined as the ratio of
change in voltage to the change in current. It is
denoted as rf.
 Diode equivalent circuit
• An equivalent circuit is nothing but a combination of elements that
best represents the actual terminal characteristics of the device. In
simple language, it simply means the diode in the circuit can be
replaced by other elements without severely affecting the behavior of
circuit.
• The diodequivalent circuits are classified as
• Piece-wise linear model
• Simplified model
 Silicon vs Germanium diode
• Silicon and germanium semiconductor diodes
 For designing the diodes, silicon is more preferred over germanium.
 The p-n junction diodes made from silicon semiconductors works at high
temperature than the germanium semiconductor diodes.
 Forward bias voltage for silicon semiconductor diode is approximately 0.7
volts whereas for germanium semiconductor diode is approximately 0.3
volts.
 Silicon semiconductor diodes do not allow the electric current flow, if the
voltage applied on the silicon diode is less than 0.7 volts.
 Silicon semiconductor diodes start allowing the current flow, if the voltage
applied on the diode reaches 0.7 volts.
 Germanium semiconductor diodes do not allow the electric current flow, if
the voltage applied on the germanium diode is less than 0.3 volts.
 Germanium semiconductor diodes start allowing the current flow, if the
voltage applied on the germanium diode reaches 0.3 volts.
 The cost of silicon semiconductors is low when compared with the
 Breakdown in diode
• There are two types of reverse
breakdown regions in a diode:
avalanche breakdown and zener
breakdown.
Avalanche breakdown
• The avalanche breakdown occurs
in both normal diodes and zener
diodes at high reverse voltage.
When high reverse voltage is
applied to the p-n junction diode,
the free electrons (minority
carriers) gains large amount of
energy and accelerated to greater
velocities.
 CONTD..
• The free electrons moving at high speed will
collides with the atoms and knock off more
electrons. These electrons are again
accelerated and collide with other atoms.
Because of this continuous collision with the
atoms, a large number of free electrons are
generated. As a result, electric current in the
diode increases rapidly. This sudden increase
in electric current may permanently destroys
the normal diode.
 Zener breakdown
• The zener breakdown occurs in heavily doped p-n junction diodes
because of their narrow depletion region. When reverse biased
voltage applied to the diode is increased, the narrow depletion
region generates strong electric field.
• When reverse biased voltage applied to the diode reaches close to
zener voltage, the electric field in the depletion region is strong
enough to pull electrons from their valence band. The valence
electrons which gains sufficient energy from the strong electric field
of depletion region will breaks bonding with the parent atom. The
valance electrons which break bonding with parent atom will
become free electrons. This free electrons carry electric current from
one place to another place. At zener breakdown region, a small
increase in voltage will rapidly increases the electric current.
 CONTD..
•Zener breakdown occurs at low
reverse voltage whereas
avalanche breakdown occurs at
high reverse voltage.
•Zener breakdown occurs in
zener diodes because they have
very thin depletion region.
•Breakdown region is the normal
operating region for a zener
diode.
•Zener breakdown occurs in
zener diodes with zener voltage
(Vz) less than 6V.
 Difference between Zener and Avalanche
Breakdown
• The depletion region of the Zener is thin whereas the avalanche is
thick.
• The connection of the Zener is not-destroy whereas the avalanche is
destroyed.
• The electric field of the Zener is strong whereas the avalanche is
weak.
• The Zener breakdown generates electrons whereas the avalanche
generates holes as well as electrons.
• The doping of the Zener is heavy whereas the avalanche is low.
• The reverse potential of the Zener is low whereas the avalanche is
high.
• The temperature coefficient of the Zener is negative whereas the
avalanche is positive.
• The Ionization of the Zener is due to Electric field whereas the
avalanche is the collision.
 CONTD..
• The temperature coefficient of
the Zener is negative whereas
the avalanche is positive.
• The breakdown voltage (Vz) of
the Zener is inversely
proportional to temperature
(ranges from 5v to 8v)
whereas the avalanche is
directly proportional to
temperature (Vz > 8V).
• After the breakdown of the
Zener is voltage remains
constant whereas the
avalanche is voltage vary.
 What is zener diode?
• A zener diode is a special type of device designed to operate in the
zener breakdown region. Zener diodes acts like normal p-n junction
diodes under forward biased condition. When forward biased
voltage is applied to the zener diode it allows large amount of
electric current and blocks only a small amount of electric current.
• Zener diode is heavily doped than the normal p-n junction diode.
Hence, it has very thin depletion region. Therefore, zener diodes
allow more electric current than the normal p-n junction diodes.
• Zener diode allows electric current in forward direction like a normal
diode but also allows electric current in the reverse direction if the
applied reverse voltage is greater than the zener voltage. Zener
diode is always connected in reverse direction because it is
specifically designed to work in reverse direction.
 Zener diode definition
• A zener diode is a p-n junction semiconductor device
designed to operate in the reverse breakdown region.
The breakdown voltage of a zener diode is carefully set
by controlling the doping level during manufacture.
• The name zener diode was named after the American
physicist Clarance Melvin Zener who discovered the
zener effect. Zener diodes are the basic building blocks
of electronic circuits. They are widely used in all kinds of
electronic equipments. Zener diodes are mainly used to
protect electronic circuits from over voltage.
 Symbol of zener diode
• The symbol of zener diode is
shown in figure. Zener diode
consists of two terminals:
cathode and anode.
• In zener diode, electric current
flows from both anode to
cathode and cathode to anode.
• The symbol of zener diode is
similar to the normal p-n
junction diode, but with bend
edges on the vertical bar.
VI characteristics of zener diode
• The VI characteristics of a zener diode is shown
in the below figure. When forward biased voltage
is applied to the zener diode, it works like a
normal diode. However, when reverse biased
voltage is applied to the zener diode, it works in
different manner.
 CONTD..
• When reverse biased voltage is applied to a zener diode, it allows
only a small amount of leakage current until the voltage is less than
zener voltage. When reverse biased voltage applied to the zener
diode reaches zener voltage, it starts allowing large amount of
electric current. At this point, a small increase in reverse voltage will
rapidly increases the electric current. Because of this sudden rise in
electric current, breakdown occurs called zener breakdown.
However, zener diode exhibits a controlled breakdown that does
damage the device.
• The zener breakdown voltage of the zener diode is depends on the
amount of doping applied. If the diode is heavily doped, zener
breakdown occurs at low reverse voltages. On the other hand, if the
diode is lightly doped, the zener breakdown occurs at high reverse
voltages. Zener diodes are available with zener voltages in the
range of 1.8V to 400V.
Advantages of zener
diode
•Power dissipation capacity is
very high
•High accuracy
•Small size
•Low cost
Zenerdiode as voltage regulator.
 Photodiode
• A photodiode is a p-n junction or pin semiconductor device that
consumes light energy to generate electric current. It is also
sometimes referred as photo-detector, photo-sensor, or light
detector.
• Photodiodes are specially designed to operate in reverse bias
condition. Reverse bias means that the p-side of the photodiode is
connected to the negative terminal of the battery and n-side is
connected to the positive terminal of the battery.
• Photodiode is very sensitive to light so when light or photons falls on
the photodiode it easily converts light into electric current. Solar cell
is also known as large area photodiode because it converts solar
energy or light energy into electric energy. However, solar cell works
only at bright light.
• The construction and working of photodiode is almost
similar to the normal p-n junction diode. PIN (p-type,
intrinsic and n-type) structure is mostly used for
constructing the photodiode instead of p-n (p-type and n-
type) junction structure because PIN structure provide
fast response time. PIN photodiodes are mostly used in
high-speed applications.
• In a normal p-n junction diode, voltage is used as the
energy source to generate electric current whereas in
photodiodes, both voltage and light are used as energy
source to generate electric current.
 Photodiode symbol
• The symbol of
photodiode is similar to
the normal p-n junction
diode except that it
contains arrows striking
the diode. The arrows
striking the diode
represent light or
photons.
• A photodiode has two
terminals: a cathode and
an anode.
 Objectives of photodiode

1.Photodiode should be always operated in reverse bias

condition.
2.Applied reverse bias voltage should be low.
3.Generate low noise
4.High gain
5.High response speed
6.High sensitivity to light
7.Low sensitivity to temperature
8.Low cost
9.Small size
10.Long lifetime
 How photodiode works?
• A normal p-n junction diode allows a small amount of electric current
under reverse bias condition. To increase the electric current under
reverse bias condition, we need to generate more minority carriers.
• The external reverse voltage applied to the p-n junction diode will
supply energy to the minority carriers but not increase the population
of minority carriers.
• However, a small number of minority carriers are generated due to
external reverse bias voltage. The minority carriers generated at n-
side or p-side will recombine in the same material before they cross
the junction. As a result, no electric current flows due to these
charge carriers. For example, the minority carriers generated in the
p-type material experience a repulsive force from the external
voltage and try to move towards n-side. However, before crossing
the junction, the free electrons recombine with the holes within the
same material. As a result, no electric current flows.
 CONTD..
• To overcome this problem, we need to
apply external energy directly to the
depletion region to generate more charge
carriers.
• A special type of diode called photodiode
is designed to generate more number of
charge carriers in depletion region. In
photodiodes, we use light or photons as
the external energy to generate charge
carriers in depletion region.
 Light Emitting Diodes
• Light Emitting Diodes (LEDs) are the most widely used
semiconductor diodes among all the different types of
semiconductor diodes available today. Light emitting diodes emit
either visible light or invisible infrared light when forward biased. The
LEDs which emit invisible infrared light are used for remote controls.
• A light Emitting Diode (LED) is an optical semiconductor device that
emits light when voltage is applied. In other words, LED is an optical
semiconductor device that converts electrical energy into light
energy.
• When Light Emitting Diode (LED) is forward biased, free electrons in
the conduction band recombines with the holes in the valence band
and releases energy in the form of light.
• The process of emitting light in response to the strong electric field
or flow of electric current is called electroluminescence.
• Like the normal p-n junction diodes,
LEDs also operates only in forward
bias condition. To create an LED,
the n-type material should be
connected to the negative terminal
of the battery and p-type material
should be connected to the positive
terminal of the battery. In other
words, the n-type material should be
negatively charged and the p-type
material should be positively
charged.
• The construction of LED is similar to
the normal p-n junction diode except
that gallium, phosphorus and
arsenic materials are used for
construction instead of silicon or
germanium materials.
 How Light Emitting Diode (LED)
works?
• Light Emitting Diode (LED) works only in forward bias condition.
When Light Emitting Diode (LED) is forward biased, the free
electrons from n-side and the holes from p-side are pushed towards
the junction.
• When free electrons reach the junction or depletion region, some of
the free electrons recombine with the holes in the positive ions. We
know that positive ions have less number of electrons than protons.
Therefore, they are ready to accept electrons. Thus, free electrons
recombine with holes in the depletion region. In the similar way,
holes from p-side recombine with electrons in the depletion region.
• Because of the recombination of free electrons and holes in the
depletion region, the width of depletion region decreases. As a
result, more charge carriers will cross the p-n junction
 CONTD..
• Some of the charge carriers from p-side and n-side will
cross the p-n junction before they recombine in the
depletion region. For example, some free electrons
from n-type semiconductor cross the p-n junction and
recombines with holes in p-type semiconductor. In the
similar way, holes from p-type semiconductor cross
the p-n junction and recombines with free electrons in
the n-type semiconductor.
• Thus, recombination takes place in depletion region as
well as in p-type and n-type semiconductor.
• The free electrons in the conduction band releases
energy in the form of light before they recombine with
holes in the valence band.
 Light emitting diode (LED)
symbol
• The symbol of LED is similar to
the normal p-n junction diode
except that it contains arrows
pointing away from the diode
indicating that light is being
emitted by the diode.LEDs are
available in different colors.
The most common colors of
LEDs are orange, yellow,
green and red.
• The schematic symbol of LED
does not represent the color of
light. The schematic symbol is
same for all colors of LEDs.
Hence, it is not possible to
identify the color of LED by
seeing its symbol.
 What determines the color of an
LED?
• The material used for constructing LED determines its
color. In other words, the wavelength or color of the
emitted light depends on the forbidden gap or energy
gap of the material.
• Different materials emit different colors of light.
• Gallium arsenide LEDs emit red and infrared light.
• Gallium nitride LEDs emit bright blue light.
• Yttrium aluminium garnet LEDs emit white light.
• Gallium phosphide LEDs emit red, yellow and green
light.
• Aluminium gallium nitride LEDs emit ultraviolet light.
• Aluminum gallium phosphide LEDs emit green light.
Advantages of LED
1.The brightness of light emitted by LED is depends on the current
flowing through the LED. Hence, the brightness of LED can be easily
controlled by varying the current. This makes possible to operate LED
displays under different ambient lighting conditions.
2.Light emitting diodes consume low energy.
3.LEDs are very cheap and readily available.
4.LEDs are light in weight.
5.Smaller size.
6.LEDs have longer lifetime.
7.LEDs operates very fast. They can be turned on and off in very less
time.
8.LEDs do not contain toxic material like mercury which is used in
fluorescent lamps.
9.LEDs can emit different colors of light.
Disadvantages of LED
1.LEDs need more power to operate than normal p-n junction diodes.
2.Luminous efficiency of LEDs is low.
 Applications of LED
1.Burglar alarms systems
2.Calculators
3.Picture phones
4.Traffic signals
5.Digital computers
6.Multimeters
7.Microprocessors
8.Digital watches
9.Automotive heat lamps
10.Camera flashes
11.Aviation lighting
APPLICATION OF P-N JUNCTION
DIODE
• RECTIFIER

• CLIPPER

• CLAMPER
 Rectifier
• In a large number of electronic circuits, we require DC voltage for
operation. We can easily convert the AC voltage or AC current into
DC voltage or DC current by using a device called P-N junction
diode.
• One of the most important applications of a P-N junction diode is the
rectification of Alternating Current (AC) into Direct Current (DC). A P-
N junction diode allows electric current in only forward bias condition
and blocks electric current in reverse bias condition. In simple
words, a diode allows electric current in one direction. This unique
property of the diode allows it to acts like a rectifier.
 Rectifier definition
• A rectifier is an electrical
device that converts an
Alternating Current (AC) into a
Direct Current (DC) by using
one or more P-N junction
diodes.

Types of rectifiers
The rectifiers are mainly classified
into two types:

1. Half wave rectifier

2. Full wave rectifier
 Half wave rectifier definition

• A half wave rectifier is a type of

rectifier which allows only half
cycle (either positive half cycle
or negative half cycle) of the
input AC signal while the
another half cycle is blocked.
• The half wave rectifier is made
up of an AC source,
transformer (step-down),
diode, and resistor (load). The
diode is placed between the
transformer and resistor (load).
• Advantages of half wave rectifier
• We use very few components to construct the half wave rectifier. So
the cost is very low.
• Easy to construct
• Disadvantages of half wave rectifier
• Power loss
• The half wave rectifier either allows the positive half cycle or
negative half cycle. So the remaining half cycle is wasted.
Approximately half of the applied voltage is wasted in half wave
rectifier.

• Pulsating direct current

• The direct current produced by the half wave rectifier is not a pure
direct current; it is a pulsating direct current which is not much
useful.

• Produces low output voltage.

 Full wave rectifier definition
• A full wave rectifier is a type of
rectifier which converts both half
cycles of the AC signal into
pulsating DC signal.
• The full wave rectifier is further
classified into two types: center
tapped full wave rectifier and full
wave bridge rectifier.
Center tapped full wave
rectifier
• A center tapped full wave rectifier
is a type of rectifier which uses a
center tapped transformer and two
diodes to convert the complete AC
signal into DC signal.
• The center tapped full wave
rectifier is made up of an AC
source, a center tapped
transformer, two diodes, and a
load resistor.
 Output waveforms of full wave
rectifier
• The first waveform represents
an input AC signal. The
second waveform and third
waveform represents the DC
signals or DC current
produced by diode D1 and
diode D2. The last waveform
represents the total output DC
current produced by diodes
D1and D2. From the above
waveforms, we can conclude
that the output current
produced at the load resistor is
not a pure DC but a pulsating
DC.
Advantages of full wave rectifier with center tapped transformer
• High rectifier efficiency

• Full wave rectifier has high rectifier efficiency than the half wave rectifier. That
means the full wave rectifier converts AC to DC more efficiently than the half
wave rectifier.

• Low power loss

• In a half wave rectifier, only half cycle (positive or negative half cycle) is allowed
and the remaining half cycle is blocked. As a result, more than half of the
voltage is wasted. But in full wave rectifier, both half cycles (positive and
negative half cycles) are allowed at the same time. So no signal is wasted in a
full wave rectifier.

• Low ripples

• The output DC signal in full wave rectifier has fewer ripples than the half wave
rectifier.

Disadvantages of full wave rectifier with center tapped transformer

• High cost
 Full wave rectifier with filter
• The filter is an electronic device that
converts the pulsating Direct Current
into pure Direct Current.
• The filter is made up of a
combination of electronic
components such as resistors,
capacitors, and inductors. The
property of inductor is that it allows
the DC components and blocks the
AC components. The property of a
capacitor is that it allows the AC
components and blocks the DC
components.
• Here, a center tapped full wave
rectifier with a filter made up of
capacitor and resistor is explained.
The filter made up of capacitor and
resistor is known as capacitor filter.
• In the circuit diagram, the capacitor C
is placed across the load resistor RL.
DIFFERENT TYPES OF
FILTER CIRCUIT
• The load resistance of a center tapped full-
wave rectifier is 1450Ω and the necessary
end to end voltage is 100 sin(100πt).
Calculate the (i)peak, rms and average
value of current (ii) Efficiency of rectifier.
Assume that each diode has an idealized
V-I characteristics having slope
corresponding to a resistance of 50Ω.