Basic Electronics for diploma students as per technical education Kerala Syllabus

Basic Electronics
COURSE CODE : 2041
Credits: 3
Program Core
Periods per semester: 45
PADHMAKUMAR P.K.
Lecturer in Electronics
GPTC Neyyattinkara
https://www.youtube.com/c/padhmakumarpk
padhmakumarpk@gmail.com

Course Outcomes

Module – 1 (CO1)
Explain Semiconductor theory, and working
principle of semiconductor diodes.

Module – I

Module – 1 Outcomes

Theory of Energy Bands
Every atom’s shell, according to Bohr’s theory, has a finite quantity of energy at
various levels. The interaction of electrons between the outermost and innermost
shells is explained by energy band theory. According to energy band theory, there
are three distinct energy bands:
• Valence band
• Forbidden energy gap
• Conduction band

Energy Bands Classification
• Valence Band
Valence electrons are the electrons in the outermost shell. The valence electrons form the
valence band, an energy band with a variety of energy levels. The valence band has the most
energy occupied.
• Conduction Band
Because the valence electrons are weakly connected to the nucleus, even at room temperature,
a few of them depart the outermost orbit and become free electrons. Free electrons are referred
to as conduction electrons due to their ability to conduct current in conductors. The
conduction band has the lowest occupied energy levels and includes conduction electrons.
• Forbidden Energy Gap
The forbidden gap is the region between the valence and conduction bands. As the name
indicates, the forbidden gap has no energy and no electrons remain in this band. If the
forbidden energy gap is larger, the valence band electrons are securely bound or firmly
connected to the nucleus. We’ll need a certain quantity of external energy to fill the restricted
energy gap.

Energy band diagrams of insulators, conductors and semiconductors

Conductors
• In conductors, there is no forbidden gap between the valence band and
conduction band which results in the overlapping of both the bands. The number
of free electrons available at room temperature is large.
• Gold, Aluminium, Silver, Copper, all these metals allow an electric current to flow
through them.

Insulators
• The materials that cannot conduct because the movement of the electrons from
the valence band to the conduction band is not possible, are called insulators
• Glass and wood are examples of the insulator. These substances do not allow
electricity to pass through them. They have high resistivity and very low
conductivity.
• The energy gap in the insulator is very high up to 7eV.

Semiconductors
The energy band diagram of semiconductors shows the conduction band is empty
and the valence band is completely filled but the forbidden gap between the two
bands is very small that is about 1eV.
Germanium and Silicon are the most preferable material whose electrical properties
lie in between semiconductors and insulators.
For Germanium, the forbidden gap is 0.72eV and for Silicon, it is 1.1eV. Thus,
semiconductor requires small conductivity.

Comparison between Conductor, Semiconductor and Insulator
Parameter Conductor Semiconductor Insulator
Forbidden energy gap Not exist Small (1 eV) Large (>5 eV)
Conductivity High (10-7 mho/m) Medium (10-7 to 10-13 mho/m)
Very Low (10-3 mho/m)
Almost negligible.
Resistivity Low Moderate High
Flow of current
Due to movement of free
electrons.
Due to movement of electrons
and holes.
Almost negligible but only due to
free electrons.
Temperature coefficient
of resistance
Positive Negative Negative
Charge carriers in
conduction band
Completely filled Partially filled Completely vacant
Charge carriers in
valence band
Almost vacant Partially filled Completely filled
Example
Copper, Aluminium, graphite
etc.
Silicon, Germanium, arsenic
etc.
Paper, rubber, glass, plastic etc.
Applications
Conducting wires,
Transformers, in electrical
cords etc.
Diodes, transistors,
optocouplers etc.
Sports equipment, home
appliances etc.

Intrinsic Semiconductors
A semiconductor is a material whose electrical
conductivity falls between that of a conductor and an
insulator. Based on the level of purity, semiconductors are
classified into intrinsic and extrinsic semiconductors.
Semiconductors that are chemically pure, in other words, free from impurities are
termed as intrinsic semiconductors. The number of holes and electrons is therefore
determined by the properties of the material itself.
In intrinsic semiconductors, the number of excited electrons is equal to the number of
holes; n = p.
They are also termed as undoped semiconductors or i-type semiconductors.
• Silicon and germanium are examples of i-type semiconductors.
• These elements belong to the IVth Group of the periodic table and their atomic
numbers are 14 and 32 respectively.

Electron-hole Pair in Semiconductors
• In semiconductors, free charge carriers are electrons and electron holes (holes).
• Electrons and holes are created by the excitation of an electron from the valence band to the
conduction band.
• An electron-hole (often simply called a hole) is the lack of an electron at a position where
one could exist in an atom or atomic lattice.
• Electrons and Holes are the two types of charge carriers that are responsible for creating an
electric current in semiconducting materials.
• Since, in a normal atom or crystal lattice, the negative charge of the electrons is balanced by
the positive charge of the atomic nuclei,
• The absence of an electron leaves a net positive charge at the hole’s location. As electrons
leave their positions, positively charged holes can move from atom to atom in
semiconducting materials.
• When an electron meets with a hole, they recombine, and these free carriers effectively
vanish.

Electron Excitation in Intrinsic Semiconductors
• Intrinsic semiconductors are materials such as silicon (Si) or germanium (Ge) that
have a half-filled valence band and an empty conduction band at 0 Kelvin. At this
temperature, there is no thermal excitation, and no electrons can move from the
valence band to the conduction band.
• However, as the temperature increases, some electrons in the valence band gain
enough energy to jump the band gap and move into the conduction band, leaving
behind a positively charged hole in the valence band. This process is called
"generation" and the resulting electron-hole pair is created through an excitation
process.
• These electron-hole pairs can also be created by absorbing light or other
electromagnetic radiation. When a photon with energy greater than the band gap is
absorbed by the semiconductor material, an electron in the valence band can be
excited into the conduction band, leaving behind a hole in the valence band.
• Once the electron-hole pair is formed, the electron and the hole can move around
the crystal lattice under the influence of an electric field, and contribute to the
electrical conductivity of the semiconductor.

Excitation of electron from covalent bond

Extrinsic Semiconductors
• Extrinsic semiconductors are semiconductors that are doped with specific
impurities. The impurity modifies the electrical properties of the
semiconductor and makes it more suitable for electronic devices such as
diodes and transistors.
• While adding impurities, a small amount of suitable impurity is added to pure
material, increasing its conductivity by many times.
Extrinsic semiconductors are also called impurity semiconductors or doped
semiconductors.
• The process of adding impurities deliberately is termed as doping and the
atoms that are used as an impurity are termed as dopants. The impurity
modifies the electrical properties of the semiconductor and makes it more
suitable for electronic devices such as diodes and transistors.

Properties of Dopant (Impurities)
• The dopant added to the material is chosen such that the original lattice of the
pure semiconductor is not distorted.
• The dopants occupy only a few of the sites in the crystal of the original
semiconductor
• The size of the dopant is nearly equal to the size of the semiconductor atoms.
• While doping tetravalent semi-conductor atoms such as Si or Ge, two types of
dopants are used, and they are:
• Pentavalent atoms: Atoms with valency 5; such as Arsenic (As), Phosphorous (Pi),
Antimony (Sb), etc.
• Trivalent atoms: Atoms with valency 3; such as Indium (In), Aluminium (Al), Boron
(B), etc.

Types of Extrinsic Semiconductors (N-Type and P-Type)

n-type Semiconductors
When a tetravalent atom such as Si or Ge is
doped with a pentavalent atom, it occupies
the position of an atom in the crystal lattice
of the Si atom.
The four of the electrons of the pentavalent
atom bond with the four neighbouring
silicon atoms, and the fifth one remains
weakly bound to the parent atom.
As a result, the ionization energy required
to set the fifth electron free is very low, and
the electrons become free to move in the
lattice of the semiconductor. Such
semiconductors are termed as n-type
semiconductors. https://www.youtube.com/c/padhmakumarpk

N-Type Semi Conductor

p-type Semiconductors
When a tetravalent atom such as Si or Ge is
doped with a trivalent impurity such as Al, B,
In, etc., the dopant atom has one less electron
than the surrounding atoms of Si or Ge. Thus,
the fourth atom of the tetravalent atom is
free, and a hole or vacancy is generated in the
trivalent atom.
In such materials, the holes are the charge
carriers, and such semiconductors are termed
p-type semiconductors

P-Type Semi Conductor

Intrinsic Semiconductor Vs Extrinsic Semiconductor
Basis of Comparison Intrinsic Semiconductor Extrinsic Semiconductor
Impurity present in the
material
Intrinsic semiconductors are pure forms of
semiconductors, hence they do not have a
significant amount of impurity.
Extrinsic semiconductors are made by adding some
impurity to the pure form of semiconductors.
Electrical conductivity They exhibit poor electrical conductivity. Electrical conductivity in the case of extrinsic
semiconductors is significantly high as compared to
intrinsic semiconductors
The density of charge
carriers
In intrinsic semiconductors, the number of
free electrons in the conduction band is equal
to the number of holes in the valence band.
The number of electrons and holes are not equal in
extrinsic semiconductors and depends on the type of
extrinsic semiconductor.
Dependency of electrical
conductivity
The electrical conductivity of intrinsic
semiconductors depends only on the
temperature.
The electrical conductivity of extrinsic semiconductors
depends on the temperature as well as the amount of
doped impurity.
Position of Fermi level In intrinsic semiconductors, the Fermi energy
levels lie in the middle of the valence and
conduction band.
In extrinsic semiconductors, the Fermi level shifts
towards the valence or conduction band.
Examples Examples include the crystalline forms of
pure silicon or germanium.
Examples include Silicon (Si) and germanium (Ge)
crystals with impurity atoms of As, Sb, P, etc., or In, B,
Al, etc.

Majority carriers and Minority carriers
• An extrinsic semiconductor has a large number of charge carriers, for example
an n-type semiconductor has a large number of electrons as well as large
number of holes. Since the number of electrons, are much larger than the
number of holes in a N-type semiconductor, electrons are majority carriers
and holes are minority carriers.
• Similarly in a p-type semiconductor the number of holes are much larger than
the number of electrons. Therefore in a p-type semiconductor, holes are
majority carriers and electrons are minority carriers.

N-Type Semiconductor Vs P-Type Semiconductor
n-type semiconductor p-type semiconductor
1. Electrons are majority charge carriers and
holes are minority charge carriers.
1. Holes are majority charge carriers and
electrons are minority charge carriers.
2. The majority of charge carriers move from
low potential to high potential.
2. The majority of charge carriers move from
high potential to low potential.
3. An donor is a doping atom that can produce
an n-type semiconductor whenever introduced
to a semiconductor.
3. An acceptor is a contaminating atom that
could produce a p-type semiconductor
whenever introduced to a semiconductor.
4. Pentavalent impurities are added. 4. Trivalent impurities are added.
5. It has donor energy levels that are very
close to the conduction band.
5. It has acceptor energy levels that are very
close to the valence band.

PN Junction – definition
• A P-N junction is an interface or a boundary between two semiconductor
material types, namely the p-type and the n-type, inside a semiconductor.
• The P-N junction is created by the method of doping. The p-side or the
positive side of the semiconductor has an excess of holes, and the n-side or the
negative side has an excess of electrons.
• Both P and N sides together in a semi-conductor obtained by doping is called
a PN junction (PN junction diode)

Formation of PN Junction

Formation of PN Junction
• Let us consider a thin p-type silicon semiconductor sheet. If we add a small
amount of pentavalent impurity to this, a part of the p-type Si will get converted
to n-type silicon.
• This sheet will now contain both the p-type region and the n-type region and a
junction between these two regions.
• There is a difference in the concentration of holes and electrons at the two sides
of a junction. The holes from the p-side diffuse to the n-side, and the electrons
from the n-side diffuse to the p-side. These give rise to a diffusion current
across the junction.

Depletion region
• When an electron diffuses from the n-side to the p-side, an ionised donor is left behind
on the n-side, which is immobile. As the process goes on, a layer of positive charge is
developed on the n-side of the junction.
• Similarly, when a hole goes from the p-side to the n-side, an ionized acceptor is left
behind on the p-side, resulting in the formation of a layer of negative charges in the p-
side of the junction. This region of positive charge and negative charge on either side of
the junction is termed as the depletion region.
• Depletion region or depletion layer is a region in a P-N junction diode where no mobile
charge carriers are present. Depletion layer acts like a barrier that opposes the flow of
electrons from n-side and holes from p-side.
• The thickness of the depletion region is merely around one-tenth of a micrometre.

Drift and Diffusion currents in a semiconductor
Drift and diffusion are responsible for generating current in semiconductors and
the overall current density is the sum of the drift and diffusion currents.

Diffusion Current
The movement of charge carriers from higher concentration to lower concentration
generates diffusion current. This occurs when a semiconductor is doped non-uniformly
then there is a non-uniform distribution of carriers or a concentration gradient.
Nature’s way of attaining equilibrium in this case is through diffusion of particles
(carriers) and this gives rise to a diffusion current.
This process does not require an external electric field and is primarily dependent on
the repulsive forces between carriers of the same charge that are highly concentrated in
an area.
The diffusion current is proportional to the concentration gradient or how nonuniformly
the carriers were initially distributed.

• Also, an electric field develops, directed from the p-side to the n-side of
the junction. This is because of the positive space-charge region on the n-
side and the negative space-charge region on the p-side of the junction.
• This electric field is responsible for the movement of electrons from the
p-side to the n-side and holes from the n-side to the p-side. This motion of
charged carriers due to the electric field is called drift. Hence, a drift
current starts which is opposite in direction to the diffusion current.

Drift Current
Drift current arises from the movement of carriers in response to an applied
electric field. Positive carriers (holes) move in the same direction as the
electric field while negative carriers (electrons) move in the opposite direction.
The net motion of charged particles generates a drift current that is in the same
direction as the applied electric field.
The drift velocity increases with
increasing electric field.
The drift current follows Ohm’s law
and is mainly influenced by the
external field and charge carrier
concentration.

Drift current Vs Diffusion current
Drift current Diffusion current
1. It is due to the movement of carriers in
response to an implemented electric field.
1. The motion of charge carriers from higher concentration to lower
concentration produces diffusion current.
2. Positive carriers or holes flow in the same
direction as the electric field, while negative
carriers or electrons flow in the reverse
direction.
2. When a semiconductor is doped non-uniformly, there is a non-
uniform concentration of carriers or a concentration gradient.
3. The net movement of charged particles
creates a drift current in the identical direction
as the implemented electric field.
3. Nature’s way of achieving equilibrium, in this case, is through the
diffusion of carriers, and this provides rise to a diffusion current. This
process does not need an external electric field and depends primarily
on the repulsive forces between carriers of the same charge highly
concentrated in an area.
4. The drift velocity rises with an increasing
electric field and provides the mobility of the
transmitters.
4. The repulsive forces will drive carriers' diffusion, leading to a
variation in concentrations and eventually a uniform arrangement.
5. The drift current obeys Ohm’s law and is
mainly affected by the external field and
charge carrier concentration.
5. The primary carrier concentrations also define the diffusion
current's direction—the current progress to the direction where there
is initially a higher density of electrons or a feebler concentration of
holes.

PN Junction Diode

Biasing Conditions for p-n Junction Diode
In a p-n junction diode, there are two operational regions:
• p-type
• n-type
The voltage applied determines one of three biasing conditions for p-n junction diodes:
• There is no external voltage provided to the p-n junction diode while it is at zero bias.
• Forward bias: The p-type is linked to the positive terminal of the voltage potential,
while the n-type is connected to the negative terminal.
• Reverse bias: The p-type is linked to the negative terminal of the voltage potential,
while the n-type is connected to the positive terminal.

Biasing of PN Junction – Types (FB & RB)
Reverse Bias
Forward Bias

Forward Bias
• When the p-type is connected to the battery’s
positive terminal and the n-type to the
negative terminal, then the P-N junction is
said to be forward-biased.
• When the P-N junction is forward biased, the
built-in electric field at the P-N junction and
the applied electric field are in opposite
directions.
• When both the electric fields add up, the
resultant electric field has a magnitude lesser
than the built-in electric field. This results in
a less resistive and thinner depletion region.
• The depletion region’s resistance becomes
negligible when the applied voltage is large.
In silicon, at the voltage of 0.6 V, the
resistance of the depletion region becomes
completely negligible, and the current flows
across it unimpeded.

Reverse Bias
• When the p-type is connected to the
battery’s negative terminal and the n-type
is connected to the positive side, the P-N
junction is reverse biased.
• In this case, the built-in electric field and
the applied electric field are in the same
direction.
• When the two fields are added, the
resultant electric field is in the same
direction as the built-in electric field,
creating a more resistive, thicker
depletion region.
• The depletion region becomes more
resistive and thicker if the applied voltage
becomes larger.

V-I Characteristics of P-N Junction Diode

Knee Voltage or Cut-in voltage
• The voltage at which the forward diode current increases rapidly is known as
Knee voltage or cut in voltage . Knee voltage for germanium is 0.3V & for
silicon is 0.7V.

Reverse Break down Voltage
• Breakdown voltage of a diode is the reverse-bias voltage at which current
increases suddenly across it.
• If reverse bias is increased, the current through PN junction will also increase
resulting in formation of a voltage called breakdown voltage.
We know that the reverse current through the junction diode is due to flow of minority carriers
(i.e., flow of electrons from p to n side and holes from n to p-side of p-n junction diode).
As the reverse bias voltage across the junction is increased, the electric field at the junction
becomes significant.
When the reverse bias voltage becomes equal to zener voltage (i.e. V=Vz), then the electric
field strength across the junction becomes quite high. This electric field across the junction is
sufficient to pull valence electrons from the host atoms on the p-side and accelerate them
towards n-side. The movement of these electrons across the junction accounts for high current
which is observed at the break down reverse voltage.

Resistances of a Diode
• Resistance is the opposition offered to the flow of current through the device.
Hence, diode resistance can be defined as the effective opposition offered by
the diode to the flow of current through it.
• Ideally, a diode is expected to offer zero resistance when forward biased and
infinite resistance when reverse biased.
• However, no device can ever be ideal. Thus, practically, every diode is seen to
offer a small resistance when forward biased, and a considerable resistance
when reverse biased.
• One can characterize the given diode regarding its forward and reverse
resistances.

Diode resistances – classifications

Forward Resistance
• Even after forward biasing, the diode will not conduct until it reaches a
minimum threshold voltage level. After the applied voltage exceeds this
threshold level, the diode starts to conduct. We refer the resistance, offered by
the diode under this condition as the forward resistance of the diode.
• That is, the forward resistance is nothing but the resistance offered by the
diode when the diode is working in its forward biased condition.
• Forward resistance is classified into two types viz., static or dynamic
depending on whether the current flowing through the device is DC (Direct
Current) or AC (Alternating Current), respectively.

Static or DC Resistance
It is the resistance offered by the diode to the flow of DC through it when we
apply a DC voltage to it. Mathematically the static resistance is expressed as the
ratio of DC voltage applied across the diode terminals to the DC current flowing
through it

Dynamic Resistance
It is the resistance offered by the diode to the
flow of AC through it when we connect it in a
circuit which has an AC voltage source as an
active circuit element.
The dynamic resistance is given as the ratio of
change in voltage applied across the diode to the
resulting change in the current flowing through
it.
Dynamic resistance is the reciprocal of the slope
of the volt-ampere characteristics.

Reverse Resistance
When we connect the diode in reverse biased condition, there will be a small
current flowing through it which is called the reverse leakage current. Due to this
current flow, the diode exhibits reverse resistance characteristic
𝑅𝑟 =
𝑉
𝑟
𝐼𝑟
Where, Vr and Ir are the reverse voltage and the reverse current respectively.

Breakdowns in PN Junctions
The p-n diode can react to negative potential in
two different ways
(a) Avalanche breakdown
(b) Zener breakdown.

Avalanche Breakdown:
• This type of breakdown occurs in the presence
of a high electric field.
• When we apply a high electric field in a
reverse biased condition, the electrons start
gaining high kinetic energy. These electrons
start breaking other covalent bonds and start
creating more hole-electron pairs.
• These pairs start crossing the depletion region
and contribute to a high reverse biased
current. The breaking of bond is an
irreversible process, and the p-n junction is
completely destroyed after an avalanche
breakdown. https://www.youtube.com/c/padhmakumarpk

Zener Breakdown:
Zener Breakdown is a controlled way of
creating breakdown in p-n junction diodes.
The p-n junction has to be heavily doped so
that the electrons in the valence bond of p-type
region can jump easily to the conduction band
of n-type region.
This temporary breakdown occurs due to the
high electric field. As it does not contribute to
a chain reaction, the effects of Zener
breakdown is temporary.

Peak inverse voltage specification, PIV
The maximum value of the reverse voltage that a PN junction or diode can
withstand without damaging itself is known as its Peak Inverse
Voltage. This rating of Peak Inverse Voltage (PIV) is given and described in
the datasheet provided by the manufacturer.
However, if the voltage coming across the junction at reverse biased
condition increases beyond this specified value, the junction will get
damaged.

Forward Voltage Drop
• Forward Voltage Drop is the voltage drop across the diode when it is
forward conduction.
• For silicon diodes Vf = 0.6 V.
• For germanium diodes Vf = 0.3 V.

Forward current
• Forward current refers to the current that flows through a diode when it is
forward-biased
• The amount of forward current that can flow through a diode is determined by
its construction and the amount of forward voltage applied to it.

Maximum Power Rating of a Diode
• The Maximum Power Rating is defined as the maximum power that a PN
junction or diode can dissipate without damaging the device itself.
• The power dissipated at the junction is equal to the product of junction
current and the voltage across the junction.
• If the power developed across the junction is more than the maximum power
rating, the junction will be overheated and it may be destroyed.

Reverse saturation current
The migration of minority carriers from the neutral
regions to the depletion zone results in a portion of the
reverse current in a semiconductor diode known as the
reverse saturation current.
In other words it is the part of the reverse current in a
semiconductor diode which is caused by the diffusion of
minority carriers from the neutral regions to the
depletion region.

Specifications of a diode
• Maximum Forward Current (If): The maximum current that can flow through the diode in the
forward direction without causing damage.
• Maximum Reverse Voltage (Vr): The maximum voltage that can be applied across the diode in the
reverse direction without causing breakdown.
• Forward Voltage Drop (Vf): The voltage drop across the diode when it is forward-biased and
conducting current.
• Reverse Leakage Current (Ir): The small amount of current that flows through the diode when it is
reverse-biased.
• Response Time (Reverse Recovery Time): The time it takes for the diode to switch from
conducting in the forward direction to blocking in the reverse direction or vice versa.
• Power Dissipation (Pd): The maximum power that the diode can safely handle without overheating.
• Junction Temperature (Tj): The maximum temperature that the diode can withstand without being
damaged.
• Capacitance (Cj): The junction capacitance of the diode, which affects its behavior at high
frequencies.
• Reverse Breakdown Voltage (Vbr): If the reverse voltage exceeds this value, the diode will break
down and allow significant current to flow in the reverse direction.
• Reverse Recovery Time (trr): The time taken by the diode to recover from the reverse bias
condition and return to a low-impedance state after it has been forward biased.

Applications of Diodes
Some Common Applications of Diodes are:
• Rectifiers
• Voltage Multipliers
• Clipper Circuit
• Clamping Circuit
• Protection Circuit
• In Logic Gates
• Flyback Circuits
• Light Emission
• Light Detection
• AM Envelope Detector
• Frequency Mixer

Module – II
Bipolar Junction Transistor (BJT)
and
Transistor amplifier

Module – II Module Outcomes

Module – II Contents

Bipolar Junction Transistor
• W. Shockley, J. Barden, and W. Brattain invented the transistor in 1947.
• The term ‘transistor’ is derived from the words ‘transfer’ and ‘resistor.’
• These words describe the operation of a BJT which is the transfer of an input
signal from a low resistance circuit to a high resistance circuit.

Bipolar Junction Transistor (BJT) Structure & Symbols
Symbols

Bipolar Junction Transistor (BJT)
• A bipolar junction transistor is a three-terminal semiconductor device that
consists of two p-n junctions which are able to amplify or magnify a signal.
• It is a current controlled device.
• The three terminals of the BJT are the base, the collector, and the emitter.
• A signal of a small amplitude applied to the base is available in the amplified
form at the collector of the transistor. This is the amplification provided by the
BJT.
• An external source of DC power supply is required for the amplification
process.

Construction of Bipolar Junction Transistor
• BJT is a semiconductor device that is constructed with 3 doped semiconductor
Regions i.e. Base, Collector & Emitter separated by 2 p-n Junctions.
• Bipolar transistors are manufactured in two types, PNP and NPN, and are
available as separate components, usually in large quantities.
• The prime use or function of this type of transistor is to amplify current. This
makes them useful as switches or amplifiers.
• They have a wide application in electronic devices like mobile phones,
televisions, radio transmitters, and industrial control.

Types of Bipolar Junction Transistors
There are two types of bipolar junction transistors:
• PNP bipolar junction transistor
• NPN bipolar junction transistor

Unbiased transistor
• When no external supply is connected to a transistor then the transistor is in
unbiased condition.
• When external supply is connected to a transistor then the transistor is in
biased condition.
(An unbiased transistor means a transistor with no external voltage
(biasing)is applied. Obviously, there will be no current flowing
from any of the transistor leads.)

Unbiased transistor – two diode equivalent circuit

Unbiased transistor
• Since transistor is like two pn junction diodes connected back to back,
there are depletion regions at both the junctions, emitter junction and collector
junction, as shown in the Fig
• During diffusion process, depletion region penetrates more deeply into the
lightly doped side in order to include an equal number of impurity atoms in the
each side of the junction.
• As shown in the Fig. depletion region at emitter junction penetrates less ie the heavily
doped emitter and extends more in the base region.
• Similarly, depletion region at collector junction penetrates less in the heavily doped
collector and extends more in the base region.
• As collector is slightly less doped than the emitter, the depletion layer Width at the
collector junction is more than the depletion layer width at the emitter junction

Transistor Biasing
Biasing is the process of providing DC voltage which helps in the functioning of
the circuit.
The proper flow of zero signal collector current and the maintenance of proper
collector emitter voltage during the passage of signal is known as Transistor
Biasing. The circuit which provides transistor biasing is called Biasing Circuit.
(A transistor is based in order to make the emitter base junction forward biased and collector
base junction reverse biased, so that it maintains in active region, to work as an amplifier.)

Junction Voltages 𝑽𝑩𝑬, 𝑽𝑩𝑪, 𝑽𝑪𝑬

Transistor Regions of Operation
• The DC supply is provided for the operation of a transistor. This DC supply is
given to the two PN junctions of a transistor which influences the actions of
majority carriers in these emitter and collector junctions.
• The junctions are forward biased and reverse biased based on our requirement.
• Forward biased is the condition where a positive voltage is applied to the p-
type and negative voltage is applied to the n-type material.
• Reverse biased is the condition where a positive voltage is applied to the n-type
and negative voltage is applied to the p-type material.
• The supply of suitable external dc voltage is called as biasing. Either forward or
reverse biasing is done to the emitter and collector junctions of the transistor.
• These biasing methods make the transistor circuit to work in four kinds of
regions such as Active region, Saturation region, Cutoff region and Inverse
active region *

Biasing and Region of Operations
EMITTER
JUNCTION
COLLECTOR
JUNCTION
REGION OF
OPERATION
Forward biased Forward biased Saturation region
Forward biased Reverse biased Active region
Reverse biased Forward biased Inverse active region
Reverse biased Reverse biased Cutoff region

Need for DC biasing
• If a signal of very small voltage is given to the input of BJT, it cannot be
amplified. Because, for a BJT, to amplify a signal, two conditions have to be
met.
• The input voltage should exceed cut-in voltage for the transistor to be ON.
• The BJT should be in the active region, to be operated as an amplifier.
• If appropriate DC voltages and currents are given through BJT by
external sources, so that BJT operates in active region and
superimpose the AC signals to be amplified, then this problem can
be avoided.
• The given DC voltage and currents are so chosen that the transistor
remains in active region for entire input AC cycle. Hence DC biasing
is needed. https://www.youtube.com/c/padhmakumarpk

Active Region
This region lies between saturation and cutoff. The transistor operates in active
region when the emitter junction is forward biased and collector junction is
reverse biased. In the active state, collector current is β times the base current,
i.e., 𝐼𝑐 = 𝛽𝐼𝐵
Where 𝐼𝑐= Collector current
𝐼𝐵=Base current
𝛽 = current amplification factor
This is the region in which transistors have many applications. This is also called as linear
region. A transistor while in this region, acts better as an Amplifier.

Saturation region
This is the region in which transistor tends to behave as a closed
switch. The transistor has the effect of its collector and Emitter being
shorted. The collector and Emitter currents are maximum in this mode
of operation.
The transistor operates in saturation region when both the emitter and
collector junctions are forward biased. As it is understood that, in the
saturation region the transistor tends to behave as a closed switch, we can
say that, 𝐼𝐶 = 𝐼𝐸
𝐼𝐸= Emitter current

Cut-off region
This is the region in which transistor tends to behave as an open switch. The
transistor has the effect of its collector and base being opened. The collector,
emitter and base currents are all zero in this mode of operation.
The transistor operates in cutoff region when both the
emitter and collector junctions are reverse biased. As in
cutoff region, the collector current, emitter current and base
currents are nil, we can write as 𝐼𝑐= 𝐼𝐵= 𝐼𝐸=0
𝐼𝐵= Base current
𝐼𝐸= Emitter current

NPN Transistor Working
• The operation of the NPN transistor is based on the fact that when a small current
flows through the base-emitter junction, it can control a much larger current flowing
between the collector and emitter.
• The transistor consists of three layers of semiconductor material. The middle layer is
the base, and the other two are the emitter and collector. The base-emitter and base-
collector junctions are formed by doping the middle layer with different impurities.
The base-emitter junction is doped with a high concentration of p-type impurities,
and the base-collector junction is doped with a high concentration of n-type
impurities.
• This creates a depletion region around the base-collector junction and an
accumulation region around the base-emitter junction. When a small current
flows through the base-emitter junction, it creates a large electric field in the
depletion region which modulates the flow of current between the collector and
emitter. This allows the transistor to be used as an amplifier or switch.

Current flow in a transistor
• In an NPN transistor, the base controls the amount of current passing through it
while allowing current flow from the emitter to the collector.
(The goal of the NPN transistor is to allow electrons to move from the emitter to
the collector (so conventional current flows from collector to emitter). The base,
which regulates the number of electrons the emitter "emits," receives electrons from
the emitter. The collector "collects" the majority of the electrons released and
passes them on to the circuit's subsequent component.)
• PNP transistors, on the other hand, are made to flow current from collector to
emitter.
(A PNP operates in a similar but reverse manner. The base still regulates the current
flow, but it now goes from emitter to collector in the reverse direction. The emitter
emits "holes" (a hypothetical absence of electrons), which are gathered by the
collector, rather than electrons.)

Current flow in NPN and PNP transistors

Currents in a transistor
There are three different types of currents are flows in a transistor. Which are
• The current flow through emitter terminal is called Emitter current and it is
denoted by IE
• The current flow through the collector terminal is called collector current, It is
symbolized by IC
• Like that IB is referred to base current which flows through base terminal.
• Also a few parts of charge carriers from emitter recombine with holes in base
region and remaining charge carriers flow through the collector. Therefore,
almost all of IE crossed to the collector and small portion of IE flows into the base
terminal. The current that leaves collector base junction αdc * IE and the value of
αdc is typically 0.96 to 0.99.

Current relations in NPN
• In NPN transistor, junction J2 (collector-base)
will be reverse biased, therefore there is a reverse
leakage current ICBO (collector-base leakage
current) flows towards the collector terminal. In
PNP transistor the current flows due to the holes
moving from n-types base to p-type collector.
• Now the collector current is sum of the portion of
IE which flows across the collector-base junction
and the reverse saturation current. hence
Since ICBO << IC
Therefore, αdc is approximately equal to the ratio of
collector current and emitter current. αdc is also referred
to as the common base current gain.

Transistor Configurations
A Transistor has 3 terminals, the emitter, the base and the collector. Using these 3
terminals the transistor can be connected in a circuit with one terminal common to both
input and output in a 3 different possible configurations.
The three types of configurations are
• Common Base,
• Common Emitter and
• Common Collector configurations.
In every configuration, the emitter junction is forward biased and the
collector junction is reverse biased.

Common Base (CB) Configuration
• The name itself implies that the
Base terminal is taken as common
terminal for both input and output
of the transistor.
• let us consider NPN transistor in
CB configuration. When the
emitter voltage is applied, as it is
forward biased, the electrons from
the negative terminal repel the
emitter electrons and current flows
through the emitter and base to the
collector to contribute collector
current. The collector
voltage VCB is kept constant
throughout this.
In the CB configuration, the input current
is the emitter current IE and the output
current is the collector current IC.

Current Amplification Factor α
The ratio of change in collector current ΔIC to the change in emitter current
ΔIE, when collector voltage VCB is kept constant, is called as Current
amplification factor.
It is denoted by 𝛼 =
ΔIC
ΔIE

Common Emitter (CE) Configuration
The name itself implies that
the Emitter terminal is taken as
common terminal for both input
and output of the transistor.
Just as in CB configuration, the
emitter junction is forward
biased and the collector junction
is reverse biased. The flow of
electrons is controlled in the
same manner. The input current
is the base current IB and the
output current is the collector
current IC here.

Base Current Amplification factor β
• The ratio of change in collector current Δ𝐼𝐶 to the change in base current Δ𝐼𝐵. is
known as Base Current Amplification Factor. It is denoted by β.
• It is denoted by β =
ΔIC
ΔIB

Relation between β and α
Current amplification ratio in CB mode,𝛼 =
ΔIC
ΔIE
Base current amplification ratio,β =
ΔIC
ΔIB
𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶
Δ𝐼𝐸 = Δ𝐼𝐵 + Δ𝐼𝐶
Δ𝐼𝐵 = Δ𝐼𝐸 − Δ𝐼𝐶
β =
ΔIC
ΔIB
=
ΔIC
Δ𝐼𝐸
−
Δ𝐼𝐶
Dividing by ΔIE, β =
ΔIC
ΔIB
=
Δ𝐼𝐶
Δ𝐼𝐸
Δ𝐼𝐸
Δ𝐼𝐸
−
Δ𝐼𝐶
Δ𝐼𝐸
=
α
1−α
β=
α
1−α
From the above equation, it is evident that, as α approaches 1, β reaches infinity. Hence, the
current gain in Common Emitter connection is very high. This is the reason CE configuration
is mostly used in all transistor applications.

The equation for collector current
In the Common Emitter configuration, IB is the input current and IC is the output current.
ICBO = Reverse Leakage Current between Collector and Base while Emitter is Open. (IE=0)
ICEO = Reverse Leakage Current between Collector and Emitter while Base is Open. (IB=0)

Effect of temperature in leakage current
• ICBO is the part of ICEO .
• I(CEO) means the current from collector to emitter when base is not connected.
• I(CBO) means current from collector to base when emitter is not connected
• ICEO roughly doubles every 100 C rise in temperature ; This will add to Ic and
hence increase the power dissipation. Increase in power dissipation will
increase the temperature. This will increase the ICEO and the cycle continues,
eventually the transistor will be damaged. This phenomenon is known as
thermal runaway.

Common Collector (CC) configuration
The name itself implies that
the Collector terminal is taken as
common terminal for both input and
output of the transistor.
Just as in CB and CE configurations,
the emitter junction is forward biased
and the collector junction is reverse
biased. The flow of electrons is
controlled in the same manner. The
input current is the base
current IB and the output current is
the emitter current IE here.

Current Amplification Factor 𝛾
The ratio of change in emitter current ∆𝐼𝐸to the change in base current ∆𝐼𝐵 is
known as Current Amplification factor in common collector (CC)
configuration.
𝛾 =
∆𝐼𝐸
∆𝐼𝐵

Transistor Characteristics
• Transistor Characteristics are the plots which represent the relationships
between the current and the voltages of a transistor in a particular
configuration.
• The characteristic-curves of a transistor can be of the following types
• Input Characteristics: These describe the changes in input current
with the variation in the values of input voltage keeping the output
voltage constant.
• Output Characteristics: This is a plot of output current versus output
voltage with constant input current.
• Current Transfer Characteristics: This characteristic curve shows the
variation of output current in accordance with the input current,
keeping output voltage constant.

CB configuration characteristics
In common base configuration circuit is shown in figure. Here base is grounded
and it is used as the common terminal for both input and output.
It is also called as grounded base configuration. Emitter is used as a input terminal where as
collector is the output terminal.

Input characteristics (of CB Configuration)
It is defined as the characteristic curve drawn
between input voltage to input current whereas
output voltage is constant.
To determine input characteristics, the collector
base voltage VCB is kept constant at zero and
emitter current IE is increased from zero by
increasing VEB.This is repeated for higher fixed
values of VCB.
A curve is drawn between emitter current and
emitter base voltage at constant collector base
voltage is shown in figure. When VCB is zero EB
junctions is forward biased. So it behaves as a
diode so that emitter current increases rapidly.

Output Characteristics (of CB Configuration)
• It is defined as the characteristic curve drawn between
output voltage to output current whereas input current is
constant. To determine output characteristics, the emitter
current IE is kept constant at zero and collector current Ic
is increased from zero by increasing VCB. This is repeated
for higher fixed values of IE.
• From the characteristic it is seen that for a constant value
of IE, Ic is independent of VCB and the curves are parallel
to the axis of VCB.As the emitter base junction is forward
biased the majority carriers that is electrons from the
emitter region are injected into the base region.
In CB configuration a variation of the base-collector voltage results in a variation of the
quasi- neutral width in the base. The gradient of the minority-carrier density in the base
therefore changes, yielding an increased collector current as the collector-base current is
increased. This effect is referred to as the Early effect.

CE configuration characteristics
In common emitter configuration circuit is shown in figure. Here emitter is
grounded and it is used as the common terminal for both input and output. It is
also called as grounded emitter configuration. Base is used as a input terminal
whereas collector is the output terminal.

• It is defined as the characteristic curve drawn
between input voltages to input current
whereas output voltage is constant.
• To determine input characteristics, the
collector base voltage VCB is kept constant at
zero and base current IB is increased from
zero by increasing VBE.This is repeated for
higher fixed values of VCE.
• A curve is drawn between base current and
base emitter voltage at constant collector base
voltage is shown in figure. Here the base
width decreases. So curve moves right as
VCE increases.
Input characteristics (of CE Configuration)

• It is defined as the characteristic curve drawn
between output voltage to output current
whereas input current is constant.
• To determine output characteristics, the base
current IB i s kept constant at zero and
collector current Ic is increased from zero by
increasing VCE.This is repeated for higher
fixed values of IB.
• From the characteristic it is seen that for a
constant value of IB, Ic is independent of
VCB and the curves are parallel to the axis of
VCE.
Output Characteristics (of CE Configuration)
The output characteristic has 3 basic regions:
Active region –defined by the biasing arrangements.
Cutoff region – region where the collector current is 0A
Saturation region- region of the characteristics to the left of VCB = 0V.

Regions of operation

Comparison of CB, CE and CC configurations
Characteristic
Common
Base
Common
Emitter
Common
Collector
Input Impedance Low Medium High
Output Impedance Very High High Low
Phase Shift 0o 180o 0o
Voltage Gain High Medium Low
Current Gain Low Medium High
Power Gain Low Very High Medium

Key features of common-emitter (CE) configuration
There are several key features of the common-emitter configuration that make it so
popular:
• The gain of a common emitter amplifier is determined by the transistor’s beta (β).
The higher the beta, the higher the gain.
• Common emitters have high input impedance and low output impedance. This
makes them ideal for use as voltage amplifiers.
• Common emitters are typically used in amplification stages where a large voltage
gain is required.
• The CE configuration provides both High Current and Voltage gain
• CE configuration is best for amplification because of its high power gain
• One of the main disadvantages of common emitter amplifiers is that they are subject
to parasitic effects such as capacitive coupling and feedback. These effects can
reduce the overall performance of the amplifier.
• Another disadvantage is that common emitters tend to be less stable than other
amplifier topologies.

Transistor as an Amplifier
• A transistor acts as an amplifier by raising the
strength of a weak signal. The DC bias voltage
applied to the emitter base junction, makes it remain
in forward biased condition. This forward bias is
maintained regardless of the polarity of the signal.
• The low resistance in input circuit, lets any small
change in input signal to result in an appreciable
change in the output. The emitter current caused by
the input signal contributes the collector current,
which when flows through the load resistor RL,
results in a large voltage drop across it.
• Thus a small input voltage results in a large output
voltage, which shows that the transistor works as an
amplifier.

Transistor action
Transistor action refers to amplifier action. For transistor action,
• Emitter must be heavily doped than collector
• Width of collector must be higher
• Emitter base junction must be forward biased
• Base collector junction must be reverse biased
• Base region must be thin to minimize recombination of carrier injected from emitter

Module – III
Working principle of
JFET, MOSFET & UJT

Module – III Module Outcomes

Module – III Contents

JFET - Junction Field Effect Transistor
JFET or Junction Field Effect Transistor is one of the simplest types of field-effect
transistor. Contrary to the Bipolar Junction Transistor, JFETs are voltage-
controlled devices.
In JFET, the current flow is due to the majority of charge carriers. However, in
BJTs, the current flow is due to both minority and majority charge carriers.
Since only the majority of charge carriers are responsible for the current flow,
JFETs are unidirectional.
The first working model of junction field-effect transistors was made in 1953.

Symbols and Structurs of JFET

JFET properties
• Junction field effect transistor (JFET): Junction field effect transistor is a three
terminal semiconductor device consisting of a gate, a drain, and a source. These can
operate on the principle of an electric field, which controls the flow of electrons from
source to drain.
• Gate: This terminal plays a significant role in controlling the flow of current through
this semiconductor device.
• Source and drain: The source is the terminal where the current enters, and the drain
is the terminal where the current exits. The flow of electrons can be controlled by the
gate.
• High impedance: The impedance of the JFET is high, which can be suitable for
applications like amplifiers where the loading of the input signal of the amplifier is
desired.
• Biasing: This is the process of applying an external signal to the gate terminal in JFET
to get a specific operating point.

Construction of JFET
N-Channel JFET
P-Channel JFET

Features of a JFET
• Temperature sensitivity: The JFETs are sensitive to temperature variations,
which allows them to adapt their properties with changing temperatures. In
certain applications, proper thermal management may be necessary.
• Low Noise: JFETs are known for their low noise, which is valuable in
applications such as amplifier and sensors where signal integrity is essential.
• High input impedance: The high input impedance of JFETs remains
relatively stable over a range of temperatures and voltages, making them
suitable for precision applications.
• Variants: JFETs, which allow circuit design flexibility and enable applications
that require both sourcing and sinking current, are available in two types of
channels: N channel and P Channel.

JFET Working (N-Channel)
• In this JFET on end of the n-channel is connected through a ohmic contact
to the drain terminal and another end is connected to the source
terminal. The two p type material is connected to the gate terminal. When
the drain to source terminal is forward biased with the external supply the
conduction begins. Due to the biasing a two pn junction becomes reverse
biased near the drain and source terminal.
• The region of the source is powered with a lower value of the potential than
the drain due to this the region of depletion is more at the drain terminal
compared to the source terminal. As the potential is increased, the width of
the region of depletion at the junctions tends to increase.
• This is the major reason due to which the flow of the majority concentration
of carriers can be evident from the terminal drain to the lower potential
applied terminal called source. These majority charge carriers flow is noted
in a linear manner. As the value of the differences in potential increases
between the terminals of drain and the source, the flow of the currents tends
to be in continuous.
• But the at a particular voltage at the drain and the source, the transistor
reaches the condition of voltage known as the pinch-off voltage. Here
current flow reaches the saturation level at the transistor. In this case, the
transistor is referred to as the resistor that is controlled by the voltage.

Characteristics of JFET
• Output characteristics
• Transfer characteristics

Output characteristic V-I curves of a typical junction FET

Current equation of JFET
The drain current ID flowing through the channel is zero when applied voltage VGS is equal to
pinch-off voltage VP. In normal operation of JFET the applied gate voltage VGS is in between 0
and VP, In this case the drain current ID flowing through the channel can be calculated as
follows.
Where
• ID = Drain current
• IDSS = maximum saturation current
• VGS = gate to source voltage
• VP = pinched-off voltage

Regions of operation of FETs
(2) Pinch-OFF Region
When VGS increases in negative direction, depletion region’s width increases and drain the
source resistance will reach its maximum, and this region of operation is called pinch-off.
Theoretically there should be no evident flow of the current at the pinch off condition.
(1) Ohmic Region
The region at which the current value of the drain behaves
linearly to the applied voltage as input at the drain and the
source terminal is referred to as the Ohmic Region of
field-effect transistor. In this case, the FET acts like a
resistor that is controlled by the voltage.

(3) Saturation Region
In this region, the behavior of the transistor turns as the
conductor. The current at the drain gets controlled by the
amount of the voltage applied at the terminals of the source
and gate. Where the voltage value at the drain and the
source don’t get affected but at this region, the maximum
value of the current flows from the terminals of drain
towards the source. This region in FET is also referred to
as Active Region. Hence the FET is considered to be fully
ON at this region.
(4) Breakdown Region
As the value of the voltage applied between the terminals of the drain and the source is
considered to be high due to this the current value at the drain tends to increase drastically.
Hence the channel at the drain and the terminal source gets affected resulting in the breakdown
condition. The device should not be operated under this condition. These voltage values are
generally specified in the data sheet.

Transfer Characteristics of JFET
The transfer characteristics of a JFET plotted between the drain current (Id) and
drain source voltage (Vds). It can be determined by keeping the Vds constant,
and drain current can be observed by changing the gate source voltage. So we
can observe that when the gate source voltage Vgs is increased, the drain current
Id decreases.
When the drain source voltage is constant, it can be observed that the value of
the drain current varies inversely with respect to the gate source voltage. The
above transfer characteristics curve of JFET is described below; it can be
observed that the value of drain current varies inversely with respect to gate-
source voltage (vgs) when the drain-source voltage is constant.

Advantages of Junction Field Effect Transistor (JFET)
• Stability: It offers good stability in various operating conditions.
• Low power consumption: it consumes little power, which makes it energy
efficient.
• High impedance: JFETs have a high input impedance; these high input
impedances can be well suited for amplifier circuits.
• Simplicity: JFETs are relatively simple to use and do not require the complex
biasing arrangements often found in other transistors.
• No Gate Current: JFETs have no gate current flow, which simplifies circuit
design in applications where current flow must be avoided.

Disadvantages of Junction Field Effect Transistor (JFET)
• Unipolar Device: JFETs are unipolar devices, because the current flow can be
controls through the movement of only one type of charge carrier (either
electrons or holes).
• Gate-Source Leakage: JFETs can exhibit gate-source leakage currents,
which is required in strict leakage current applications.
• Limited Availability: The availability is less, finding specific JFETs with
specific characteristics can be challenging
• Low gain: it has low gain as compared to other types of transistors and cannot
be used in high-gain applications.
• Cost: It is expensive, which can impact the overall cost of electronic devices.

Applications of Junction Field Effect Transistor
• Low-Noise Amplifiers: JFETs are ideal for low-noise amplifier applications in high-
frequency signal processing, and audio circuits.
• High-Impedance Preamplifiers: JFETs are used in preamplifiers for high-
impedance sensors, such as piezoelectric accelerometers and certain types of
microphones, to maintain signal integrity.
• Switching Circuits: JFETs can be used as electronic switches in low-power and
high-frequency applications where fast switching is required, such as in RF
switching circuits.
• Sample and Hold circuits: In sample-and-hold circuits, JFETs can sample the input
signal and hold its value until the next sampling period.
• Voltage Regulators: JFETs can also be used in voltage regulators to maintain stable
output.
• Oscillators: JFETs are important in making oscillators that create repeating
waveforms. They can also control the frequency, to generate stable waveforms.

Comparison of BJT vs JFET
Aspect Bipolar Junction Transistor (BJT) Junction Field-Effect Transistor (JFET)
Operation Principle
BJT relies on the movement of both electrons
and holes.
JFET operates based on the movement of majority charge carriers
(either electrons or holes) within a single type of semiconductor
material.
Types NPN and PNP configurations. N-channel and P-channel configurations.
Control Controlled by current (base current in BJT). Controlled by voltage (gate-source voltage in JFET).
Input Impedance Lower input impedance compared to JFET.
Higher input impedance, making it more suitable for high-impedance
applications.
Voltage Gain Generally lower voltage gain compared to JFET. Generally higher voltage gain.
Noise Higher noise levels compared to JFET.
Lower noise levels, making JFETs suitable for high-fidelity
applications.
Power Consumption Higher power consumption. Lower power consumption.
Speed Faster switching speeds. Relatively slower switching speeds.
Temperature Sensitivity Sensitive to temperature variations. Less sensitive to temperature variations.
Applications
Commonly used in audio amplifiers, digital
circuits.
Often used in high-input impedance applications, such as in
amplifiers and analog switches.

Metal Oxide Semiconductor Field-effect Transistor (MOSFET)
• A Metal Oxide Semiconductor Field-effect Transistor (MOSFET, MOS-FET, or MOS
FET) is a field-effect transistor (FET with an insulated gate) where the voltage determines
the conductivity of the device.
• It is used for switching or amplifying signals. The ability to change conductivity with the
amount of applied voltage can be used for amplifying or switching electronic signals.
• MOSFETs are now even more common than BJTs (bipolar junction transistors) in digital and
analog circuits.

MOSFET Structure

MOSFET Structure
• It is a four-terminal device with Source (S), Drain (D), Gate (G), and body
(B) terminals. The body (B) is frequently connected to the source terminal,
reducing the terminals to three.
• It works by varying the width of a channel along which charge carriers flow
(electrons or holes).
• The charge carriers enter the channel at the source and exit via the drain.
• The width of the channel is controlled by the voltage on an electrode called
Gate which is located between the source and the drain. It is insulated from the
channel near an extremely thin layer of metal oxide.

MOSFET Types
• P-Channel Depletion MOSFET
• P-Channel Enhancement MOSFET
• N-Channel Depletion MOSFET
• N-Channel Enhancement MOSFET

MOSFET Symbols

P-Channel MOSFET
• The drain and source are heavily doped p+ region
and the substrate is in n-type. The current flows due
to the flow of positively charged holes, and that’s
why known as p-channel MOSFET.
• When we apply negative gate voltage, the electrons
present beneath the oxide layer experience repulsive
force and are pushed downward into the substrate,
the depletion region is populated by the bound
positive charges which are associated with the
donor atoms.
• The negative gate voltage also attracts holes from
the P+ source and drain region into the channel
region.

N-Channel MOSFET
The drain and source are heavily doped N+
region and the substrate is p-type. The current
flows due to the flow of negatively charged
electrons and that’s why known as n-channel
MOSFET.
When we apply the positive gate voltage, the
holes present beneath the oxide layer experience
repulsive force, and the holes are pushed
downwards into the bound negative charges
which are associated with the acceptor atoms.
The positive gate voltage also attracts electrons
from the N+ source and drain region into the
channel thus an electron-rich channel is formed.

Construction of MOSFET
(N-Channel Depletion Type)
The circuit of a MOSFET is usually constructed as follows:
• Its base is formed of a P-type semiconductor.
• Its two types of bases are highly doped with an N-type of impurity (which is shown as
n+ in the diagram below). And is connected as a channel
• The terminals, source, and drain originate from the heavily doped region of the base.
• The substrate layer is coated with a layer of insulating silicon dioxide.
• On top of silicon dioxide, a thin insulated metal plate is kept (which acts as a capacitor
in the MOSFET) as gate.
• From the thin metallic plate, the gate terminal is bought out.
• A DC circuit is formed when a voltage source is connected between these two doped
N-type regions.

N-Channel Depletion MOSFET Working and
Characteristics
Since the channel is already present in
depletion type, unlike the enhancement type
MOSFET, you also will get drain current at
zero gate voltage.

N Channel Depletion MOSFET – Working
• In our first case, let us connect the Gate terminal to the
ground and apply a positive voltage at the drain and
source.
• On applying positive VDS, the electrons in the N
channel will move towards the positive drain terminal,
and the drain current will start flowing from drain to
source. On increasing VDS further, keeping VGS 0, a
time will come where ID will become constant, and
that value of drain current is called saturation current.
• We can conclude from this discussion that when VGS
= 0 and VDS > 0, current ID flows from drain to
source, and on increasing VDS further, ID = IS = IDSS
Apply VGS = 0

Apply VGS < 0
When VGS is negative, holes from the P-type substrate
will attract towards the negative gate terminal and
recombine with electrons in the N channel, forming
electron-hole pairs.
On increasing negative potential at the gate, more
electron-hole combinations will occur, decreasing the
number of free electrons in the N channel.
As a result, ID decreases. A time will come when the
drain current will become zero. The negative gate voltage
at which the drain current is zero is called pinch-off
voltage or VP.
We can conclude from this discussion at pinch-off, VGS =
VP, VDS > 0, and ID = 0
“This mode of operation is called Depletion mode”

On applying VGS > 0
When VGS is postitive, the minority carriers in
the p-type substrate, i.e. electrons, will get
attracted towards the gate terminal, thereby
increasing the concentration of electrons in the
N-channel.
As a result, the drain current will increase and
exceed the saturation current.
We can conclude from this discussion,
when VGS > 0 and VDS > 0, then ID > IDSS.
“This mode is called Enhancement mode”

Transfer characteristics & Drain Characteristics of N-Channel DE-MOSFET

Advantages of MOSFET :
• The operational speed of MOSFET is higher than that of JFET.
• Input impedance is much higher as compared to JFET.
• It can be easily used in case of high current applications.
• These devices provide an easy manufacturing process.
Disadvantages of MOSFET :
• It is a delicate device and is easily destroyable.
• Excessive application of gate to source voltage VGS may destroy the
thin SiO2 layer.

Applications of MOSFET
• It is used in inverter.
• It is used in digital circuits.
• It is used as a high-frequency amplifier.
• It is used in brushless DC motor drives.
• It is used in electronic DC relays.
• It is used in SMPS.
• It is used as a switch.
• In amplifiers.

Unijunction Transistor (UJT)
• The Unijunction Transistor or UJT for short, is a solid state three
terminal device that can be used in gate pulse, timing circuits and trigger
generator applications to switch and control either thyristors and triac’s for
AC power control type applications.
• Like diodes, unijunction transistors are constructed from separate P-type
and N-type semiconductor materials forming a single (hence its name
Uni-Junction) PN-junction within the main conducting N-type channel of
the device.

Structure of UJT
• The UJT consists of an n-type silicon semiconductor bar
with an electrical contact on each end. The terminals of
these connections are called Base terminals (B1 and B2).
• Near to base B2, a pn-junction is formed between a p-type
emitter and the n-type silicon bar. The terminal of this
junction is called emitter terminal (E).
• Since the device has three terminals and one pn-junction,
for this region this is called as a Unijunction Transistor
(UJT).
The device has only pn-junction so it forms a diode. Because the two base leads
are taken from one section of the diode, hence the device is also called
as Double-Based Diode.
The emitter is heavily doped while the n-region is lightly doped. Thus, the
resistance between base terminals is very high when emitter terminal is open.

UJT’s Symbol, Structure & Equivalent Circuit

Operation of UJT - With Emitter Open
• When the voltage VBB is applied with emitter open.
• A potential gradient is established along the n-type silicon
bar. As the emitter is located close to the base B2, thus a
major part of VBB appears between the emitter and base
B1.
• The voltage V1 between emitter and B1, establishes a
reverse bias on the pn-junction and the emitter current is
cut off, but a small leakage current flows from B2 to
emitter due to minority charge carriers.
• Thus, the device is said to be in OFF state.

With Emitter at Positive Potential
• When a positive voltage is applied at the emitter terminal, the pn-
junction will remain reverse biased till the input voltage is less than
V1.
• A soon as the input voltage at emitter exceeds V1, the pn-junction
becomes forward biased. Under this condition, holes are supplied
from p-type region into the n-type bar.
• These holes are repelled by positive B2 terminal and attracted
towards the B1 terminal. This increase in the number of holes in the
emitter to B1 region results in the decrease of resistance of this
section of the bar.
• Because of this, the internal voltage drop from emitter to B1 region
is reduced, thus the emitter current (IE) increases. As more holes
are supplied, a condition of saturation is reached.
• At the point of saturation, the emitter current is limited by the
emitter power supply. Now, the device is conducting, hence said to
be in ON state.

Internal structure of UJT, On biasing with VBB
If emitter voltage VE > VD + VA, then diode will conduct towards the N-type
bar, causing a reduction in its resistance, and UJT is said to be ‘fired’.

UJT features
• The resistance of silicon bar is called as the inter-base resistance (has a value from 4 kΩ to 10 kΩ).
• The resistance RB1 is the resistance of the bar between emitter and B1 region. The value of this is variable and
depends upon the bias voltage across the pn-junction.
• The resistance RB2 is the resistance of the bar between emitter and B2 region.
• The emitter pn-junction is represented by a diode.
• With no voltage applied to the UJT, the value of inter-base resistance is given by
𝑅𝐵𝐵 = 𝑅𝐵1 + 𝑅𝐵2
The intrinsic stand-off ration (ƞ) of UJT is given by 𝜂 =
𝑅𝐵1
𝑅𝐵𝐵
=
𝑅𝐵1
𝑅𝐵1+𝑅𝐵2
The value of ƞ generally lies between 0.51 and 0.82.
The voltage across RB1 is ,𝑉1 = 𝑉𝐵𝐵 ∗
𝑅𝐵1
𝑅𝐵𝐵
= 𝜂𝑉𝐵𝐵
The Peak Point Voltage (VP) of the UJT 𝑉𝑃 = 𝜂𝑉𝐵𝐵 + 𝑉𝐷

VI Characteristics of UJT
The curve between emitter voltage (VE) and emitter current (IE) of UJT, at a given value of
VBB is known as emitter characteristics of UJT.

Applications of UJT
• Relaxation Oscillator:
• Thyristor Firing Circuit:
• Phase Control:

Advantages
• UJTs have a negative resistance region that enables specific applications
such as oscillators.
• They possess a high input impedance, making them useful in various circuit
configurations.
• Due to their simplicity and ruggedness, UJTs are reliable and cost-effective
devices.
Disadvantages
• UJTs are not ideal for high-frequency applications due to the charge storage
effect.
• They have limited gain, which restricts their use in amplification circuits.
• There is a relatively high variation in the parameters of UJTs, which may
cause inconsistency in their performance.

Module – IV
Linear and non linear
wave shaping circuits

Module – IV Module Outcomes

Module – IV Contents

Rectifier
A rectifier is an electronic device that converts an alternating current (AC) into a
direct current (DC) by using one or more P-N junction diodes.
This process is known as rectification.
AC input DC output

Classification of rectifiers
• Half wave rectifier
• Full wave rectifier
• Centre tapped full wave rectifier
• Bridge rectifier

Half wave rectifier
• Half-wave rectifiers transform AC voltage to DC voltage using a single diode.
A half wave rectifier is defined as a type of rectifier that allows only one-half
cycle of an AC voltage waveform to pass while blocking the other half cycle.

Working of Half wave rectifier
• During the positive half cycle, diode gets forward biased and current
pass through the load and voltage drops across the load resistor
• During negative half cycle, diode gets reverse biased and no current
pass through the load and no voltage drop across the load resistor

Waveforms
Input
Output

Average output voltage in Half wave rectifier

Ripple
• A ripple is defined as the fluctuating AC component in the rectified DC
output.
• The rectified DC output could be either DC current or DC voltage.
• When the fluctuating AC component is present in DC current it is known
as the current ripple while the fluctuating AC component in DC voltage is
known as the voltage ripple.

Ripple Factor
Ripple factor measures the ripple content of the waveform
The ripple factor is defined as
The ratio of the RMS value of an alternating current component in the
rectified output to the average value of rectified output.
The ripple factor is denoted as γ. It is a dimensionless quantity and always
has a value less than unity.

Ripple Factor of Half Wave Rectifier
• We know that the ripple factor is equal to the ratio of RMS value and the
average value of the rectifier output, which is given as:
• From the formula of ripple factor, we know that
• Rearranging the above equation, we get the ripple factor of half wave rectifier
as:

Peak Inverse Voltage (PIV)
• Peak Inverse Voltage (PIV) or Peak Reverse Voltage (PRV) refer to the
maximum voltage a diode or other device can withstand in the reverse-
biased direction before breakdown. Also may be called Reverse
Breakdown Voltage.
• the voltage that appears across the terminals of the semiconductor diode
once it is reverse biased
When the maximum value of the sinusoidal AC supply
is ‘Vm’, the highest voltage across the diode is
reversed bias can also equivalent to ‘Vm’. So, the PIV
of the semiconductor diode in HWR (half-wave
rectifier) is equivalent to the maximum value of the
voltage supply.
That is ‘Vm’

Transformer Utilization Factor (TUF)
Transformer Utilization Factor (TUF) is defined as the ratio of DC power
output of a rectifier to the effective Transformer VA rating used in the same
rectifier. Effective VA Rating of transformer is the average of primary and
secondary VA rating of transformer.

Transformer Utilization Factor (TUF) of Half Wave Rectifier
DC Power Output, Pdc = Average Current x Average Voltage
= (Im/π) (Vm/π)
= (Im Vm)/π2
VA rating of Transformer = (Vm/√2)(Im/2)
= (Vm Im) /(2x√2)
TUF = DC Power Output / VA Rating of Transformer
= [(Im Vm)/π2] / [(Vm Im) /(2x√2)]
= [(2x√2)/π2]
= 0.2865

Form Factor
• The form factor is the ratio of RMS value to the DC value. For a half-
wave rectifier, the form factor is 1.57.
Rectifier Efficiency
• Rectifier efficiency is the ratio of output DC power to the input AC
power. For a half-wave rectifier, rectifier efficiency is 40.6%.

Characteristics of Half Wave Rectifier
• No of diodes = 1 No
• Ripple factor = 1.21
• DC output voltage =
𝑉𝑚
𝜋
• Form factor = 1.57
• Rectifier efficiency = 40.6%
• TUF (Transformer utilization factor) = 28.6%
• Output frequency = fm (same as input frequency)
• Efficiency = 40.6%

Centre tap full wave rectifier

Working of Centre tap full wave rectifier
• During +ve half cycle, current starts
from A, then flows through D1, RL
and transformer’s center point.
• During -ve half cycle, current starts
from B, then flows through D2, RL
and transformer’s center point.
• During both cycles, the direction of
current through the load resistor RL
is same direction

Bridge rectifier

Bridge Rectifier working
• During +ve half cycle, D3 and D1 are
in forward bias condition, D4 and D2
are in reverse bias condition.
• Current starts from A flows through
D3, RL, D1 and flows towards B.
• During -ve half cycle, D4 and D2 are in
forward bias condition, D3 and D1 are
in reverse bias condition.
• Current starts from B flows through
D2, RL, D4 and flows towards A.
• In Both half cycles, the direction of
current through the Load resistor RL is
same. https://www.youtube.com/c/padhmakumarpk

Characteristics of Full Wave Rectifiers
• No of diodes = 2 No or 4 Nos
• Ripple factor = 0.482
• DC output voltage = 2
𝑉𝑚
𝜋
• Form factor = 1.1
• Rectifier efficiency = 40.6%
• TUF (Transformer utilization factor) = 81.2% (Bridge) , 63.6 % (Center tap)
• Output frequency = 2fm (Double the input frequency)
• Efficiency = 81.2%

Compare Half wave,Full wave & Bridge rectifier

Filters
• Rectifier is a device that converts AC into DC by using one diodes. But the
output of rectifiers is pulsating DC (means contains both AC component and
DC component).Hence, to remove all the AC components we use filters.
• Filter is a circuit which removes the AC component from a rectifier output.

Need for filters in DC power supply
• The ripple in the signal denotes the presence of some AC component. This ac
component has to be removed, in order to get pure dc output. So, we need a
circuit that smoothens the rectified output into a pure dc signal.
• A filter circuit is one which removes the ac component present in the rectified
output and allows the dc component to reach the load.
• The following figure shows the functionality of a filter circuit.
A filter circuit is constructed using two main
components, inductor and capacitor
•An inductor allows dc and blocks ac.
•A capacitor allows ac and blocks dc.

Series Inductor Filter
As an inductor allows dc and blocks ac, a filter called Series Inductor Filter can
be constructed by connecting the inductor in series, between the rectifier and the
load. The figure below shows the circuit of a series inductor filter.
The rectified output when passed through this filter, the inductor blocks the ac
components that are present in the signal, in order to provide a pure dc. This is a
simple primary filter.

Shunt Capacitor Filter
As a capacitor allows ac through it and blocks dc, a filter called Shunt Capacitor
Filter can be constructed using a capacitor, connected in shunt, as shown in the
following figure.
The rectified output when passed through this filter, the ac components present in the signal
are grounded through the capacitor which allows ac components. The remaining dc
components present in the signal are collected at the output.

L-C Filter
A filter circuit can be constructed using both inductor and capacitor in order to
obtain a better output where the efficiencies of both inductor and capacitor can be
used. The figure below shows the circuit diagram of a LC filter.
The rectified output when given to this circuit, the inductor allows dc components to pass
through it, blocking the ac components in the signal. Now, from that signal, few more ac
components if any present are grounded so that we get a pure dc output. This filter is also
called as a Choke Input Filter as the input signal first enters the inductor. The output of this
filter is a better one than the previous ones.

𝝅- Filter
• This is another type of filter circuit which is very commonly used. It has
capacitor at its input and hence it is also called as a Capacitor Input Filter.
Here, two capacitors and one inductor are connected in the form of π shaped
network. A capacitor in parallel, then an inductor in series, followed by another
capacitor in parallel makes this circuit.
• If needed, several identical sections can also be added to this, according to the
requirement. The figure below shows a circuit for 𝜋 filter.

Working of a Pi filter
In this circuit, we have a capacitor in parallel, then an inductor in series,
followed by another capacitor in parallel.
• Capacitor C1 − This filter capacitor offers high reactance to dc and low
reactance to ac signal. After grounding the ac components present in the signal,
the signal passes to the inductor for further filtration.
• Inductor L − This inductor offers low reactance to dc components, while
blocking the ac components if any got managed to pass, through the capacitor
C1.
• Capacitor C2 − Now the signal is further smoothened using this capacitor so
that it allows any ac component present in the signal, which the inductor has
failed to block. https://www.youtube.com/c/padhmakumarpk

Comparison of filters

Bridge rectifier with filters

Wave shaping Circuits

Classification of Wave shaping Circuits

Linear Wave shaping circuits
• A Signal can also be called as a Wave. Every wave has a certain shape when
it is represented in a graph. This shape can be of different types such as
sinusoidal, square, triangular, etc. which vary with respect to time period
• Linear elements such as resistors, capacitors and inductors are employed to
shape a signal in linear wave shaping circuits.
Examples
• RC Differentiator
• RC Integrator
• RLC circuits
• Passive filters

RC Differentiator
• The passive RC differentiator is a series connected RC network that produces
an output signal which corresponds to the mathematical process of
differentiation.
• A passive RC differentiator is nothing more than a capacitance in series with a
resistance, that is a frequency dependent device which has reactance in series
with a fixed resistance. The output voltage depends on the circuits RC time
constant and input frequency.

RC Integrator as High Pass Filter
• At low input frequencies the reactance, XC of the capacitor is high
blocking any DC voltage or slowly varying input signals. While at
high input frequencies the capacitors reactance is low allowing
rapidly varying pulses to pass directly from the input to the output.
• This is because the ratio of the capacitive reactance (XC) to resistance
(R) is different for different frequencies and the lower the frequency
the less output. So for a given time constant, as the frequency of the
input pulses increases, the output pulses more and more resemble the
input pulses in shape. Thus the RC integrator act as High Pass Filter
Where 𝑋𝑐 =
1
2𝜋𝑓𝐶
𝑉𝑜𝑢𝑡 = 𝑖 𝑥 𝑅
= 𝑅 𝑥
𝑑𝑞
𝑑𝑡
= 𝑅 ⤫
𝑑(𝐶.𝑉𝐶)
𝑑𝑡
= 𝑅𝐶 ⤫
𝑑 𝑉𝐶
𝑑𝑡
𝑉𝑜𝑢𝑡 = 𝑅𝐶
𝑑 𝑉𝑖𝑛
𝑑𝑡
𝑅 ≪
1
𝜔𝐶
𝑓𝑜𝑟 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑖𝑒𝑠 𝜔 ≪
1
𝑅𝐶
, 𝑉𝑖𝑛 ≅ 𝑉𝑜𝑢𝑡
The charge q on the capacitor C at any instant is

RC Differentiator – Waveforms

Time constant of RC differentiator
The rate at which the capacitor charges (or
discharges) is directly proportional to the
amount of resistance and capacitance giving
the time constant of the circuit.
Thus the time constant of a RC differentiator
circuit is the time interval that equals the
product of R and C.
t=RC

Waveforms with different RC value
• A differentiator circuit is used to produce
trigger or spiked typed pulses for timing
circuit applications.
• TV Circuits
• Trigger circuits

RC Integrator
The RC integrator is a series connected RC network that produces an output
signal which corresponds to the mathematical process of integration.
For a passive RC integrator circuit, the input is connected to a resistance while
the output voltage is taken from across a capacitor being the exact opposite to
the RC Differentiator Circuit. The capacitor charges up when the input is high
and discharges when the input is low.

Basic Electronics for diploma students as per technical education Kerala Syllabus

Basic Electronics for diploma students as per technical education Kerala Syllabus

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