DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

Presentation
On

ZENER DIODE
by

CH. KRANTHI REKHA,

Assistant Professor,
ECE Dept, VBIT

5/11/2024 VBIT, ECE 1

 Zener Diode
• A heavily doped p-n junction diode that works in reverse bias
conditions is called a Zener Diode.
• They are special semiconductor devices that allow the current
to flow in both forward and backward directions.
• For the Zener diode, the voltage drop across the diode is
always constant irrespective of the applied voltage.
• Thus, Zener diodes are used as a voltage regulator.
• Zener diodes are semiconductor devices that allow current to
flow in both directions but specialize in current flowing in
reverse. Also known as breakdown diodes
 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.
 Breakdown in zener diode
 There are two types of reverse breakdown regions in a
zener 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.
 Avalanche breakdown
• 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. However, avalanche diodes may not be destroyed because
they are carefully designed to operate in avalanche breakdown
region.
• Avalanche breakdown occurs in zener diodes with zener voltage (Vz)
greater than 6V.
 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.
 Zener breakdown
• 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.
 Features of Zener breakdown

• 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.
 Symbol of zener diode
• The symbol of zener diode is shown in below 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.
VI Characteristics of Zener Diode
 Forward Characteristics of Zener Diode
• Forward characteristics of the Zener Diode are similar to the
forward characteristics of any normal diode.
• It is clearly evident from the above diagram in the first quadrant
that the VI forward characteristics are similar to other P-N
junction diodes.
 Reverse Characteristics of Zener Diode
• 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
 Applications of zener diode
• It is normally used as voltage reference
• Zener diodes are used in voltage stabilizers or shunt regulators.
• Zener diodes are used in switching operations
• Zener diodes are used in clipping and clamping circuits.
• Zener diodes are used in various protection circuits
 Zener Diode as a Voltage Regulator

• Zener diode is a silicon semiconductor with a p-n junction that is

specifically designed to work in the reverse biased condition.
• When forward biased, it behaves like a normal signal diode, but
when the reverse voltage is applied to it, the voltage remains
constant for a wide range of currents.
• Due to this feature, it is used as a voltage regulator in d.c.
circuit. The primary objective of the Zener diode as
a voltage regulator is to maintain a constant voltage.
• Let us say if Zener voltage of 5 V is used then, the voltage
becomes constant at 5 V, and it does not change.
 Zener Diode as a Voltage Regulator
• A voltage regulator is a device that regulates the voltage level.
• It essentially steps down the input voltage to the desired level and
keeps it at that same level during the supply. This ensures that even
when a load is applied the voltage doesn’t drop.
• The voltage regulator is used for two main reasons, and they are:
To vary or regulate the output voltage
To keep the output voltage constant at the desired value in spite of
variations in the supply voltage.
• Voltage regulators are used in computers, power generators,
alternators to control the output of the plant.
• There is a series resistor connected to the circuit in order to limit
the current into the diode.
• It is connected to the positive terminal of the d.c. It works in
such a way the reverse-biased can also work in breakdown
conditions.
• We do not use ordinary junction diode because the low power
rating diode can get damaged when we apply reverse bias
above its breakdown voltage.
• When the minimum input voltage and the maximum load
current is applied, the Zener diode current should always be
minimum.
• Since the input voltage and the required output voltage is
known, it is easier to choose a Zener diode with a voltage
approximately equal to the load voltage, i.e. VZ = VL.
• The value of the series resistor is written as RS = (VL −
VZ)IL.
• Current through the diode increases when the voltage
across the diode tends to increase which results in the
voltage drop across the resistor.
• Similarly, the current through the diode decreases when
the voltage across the diode tends to decrease. Here,
the voltage drop across the resistor is very less, and the
output voltage results normally.
5/11/2024 VBIT, ECE 23
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
Presentation
On

TUNNEL DIODE
by

CH. KRANTHI REKHA,

Assistant Professor,
ECE Dept, VBIT

5/11/2024 VBIT, ECE 1

 Tunnel diode definition
• A Tunnel diode is a heavily doped p-n junction diode in which the
electric current decreases as the voltage increases.
• In tunnel diode, electric current is caused by “Tunneling”. The tunnel
diode is used as a very fast switching device in computers. It is also
used in high-frequency oscillators and amplifiers.
• Symbol of tunnel diode
• The circuit symbol of tunnel diode is shown in the below figure. In
tunnel diode, the p-type semiconductor act as an anode and the n-
type semiconductor act as a cathode.
What is a tunnel diode?
• A tunnel diode (also known as a Esaki diode) is a type of semiconductor
diode that has effectively “negative resistance” due to the quantum
mechanical effect called tunneling. Tunnel diodes have a heavily doped pn
junction that is about 10 nm wide. The heavy doping results in a broken
band gap, where conduction band electron states on the N-side are more
or less aligned with valence band hole states on the P-side.
• The application of transistors in a very high in frequency range are
hampered due to the transit time and other effects. Many devices use the
negative conductance property of semiconductors for these high frequency
applications. A tunnel diode is one of the most commonly used negative
conductance devices. It is also known as Esaki diode after L. Esaki for his
work on this effect.
• The concentration of dopants in both p and n region is very high, at
around 1024 – 1025 m-3. The pn junction is also abrupt. For this
reasons, the depletion layer width is very small. In the current voltage
characteristics of tunnel diode, we can find a negative slope region
when a forward bias is applied.
• The name “tunnel diode” is due to the quantum mechanical
tunneling is responsible for the phenomenon that occurs within the
diode. The doping is very high so at absolute zero temperature the
Fermi levels lies within the bias of the semiconductors.
Characteristics of Tunnel Diode
• When reverse bias is applied the Fermi level of the p-side becomes
higher than the Fermi level of n-side. Hence, the tunneling of
electrons from the balance band of p-side to the conduction band of
n-side takes place. With the interments of the reverse bias the tunnel
current also increases.
• When forward bias is applied the Fermi level of n-side becomes
higher that the Fermi level of p-side, thus the tunneling of electrons
from the n-side to p-side takes place. The amount of the tunnel
current is very large than the normal junction current. When the
forward bias is increased, the tunnel current is increased up to certain
limit.
• When the band edge of n-side is the same as the Fermi level in p-side, the tunnel
current is maximum with the further increment in the forward bias the tunnel
current decreases and we get the desired negative conduction region. When the
forward bias is raised further, normal pn junction current is obtained which is
exponentially proportional to the applied voltage. The V-I characteristics of the
tunnel diode is given,
• The negative resistance is used to achieve oscillation
 Working of a tunnel diode
• Step 1: Unbiased tunnel diode
• When no voltage is applied to the tunnel diode, it is said to be an unbiased tunnel
diode. In tunnel diode, the conduction band of the n-type material overlaps with
the valence band of the p-type material because of the heavy doping.
• Because of this overlapping, the conduction band electrons at n-side
and valence band holes at p-side are nearly at the same energy level.
So when the temperature increases, some electrons tunnel from the
conduction band of n-region to the valence band of p-region. In a
similar way, holes tunnel from the valence band of p-region to the
conduction band of n-region.
• However, the net current flow will be zero because an equal number
of charge carriers (free electrons and holes) flow in opposite
directions.
• Step 2: Small voltage applied to the tunnel diode
• When a small voltage is applied to the tunnel diode which is less than the built-in voltage of the
depletion layer, no forward current flows through the junction.
• However, a small number of electrons in the conduction band of the n-region will tunnel to the
empty states of the valence band in p-region. This will create a small forward bias tunnel current.
Thus, tunnel current starts flowing with a small application of voltage.
• Step 3: Applied voltage is slightly increased
• When the voltage applied to the tunnel diode is slightly increased, a
large number of free electrons at n-side and holes at p-side are
generated. Because of the increase in voltage, the overlapping of the
conduction band and valence band is increased
• Step 4: Applied voltage is further increased
• If the applied voltage is further increased, a slight misalign of the conduction
band and valence band takes place.
• Since the conduction band of the n-type material and the valence band of the p-
type material will overlap. The electrons tunnel from the conduction band of n-
region to the valence band of p-region and cause a small current flow. Thus, the
tunneling current starts decreasing.
• Step 5: Applied voltage is largely increased
• If the applied voltage is largely increased, the tunneling current drops to zero. At
this point, the conduction band and valence band no longer overlap and the
tunnel diode operates in the same manner as a normal p-n junction diode.
• If this applied voltage is greater than the built-in potential of the
depletion layer, the regular forward current starts flowing through the
tunnel diode.
• The portion of the curve in which current decreases as the voltage
increases is the negative resistance region of the tunnel diode. The
negative resistance region is the most important and most widely
used characteristic of the tunnel diode.
• A tunnel diode operating in the negative resistance region can be
used as an amplifier or an oscillator.
 Advantages of tunnel diodes
• Long life
• High-speed operation
• Low noise
• Low power consumption
 Disadvantages of tunnel diodes
• Tunnel diodes cannot be fabricated in large numbers
• Being a two terminal device, the input and output are not isolated
from one another.
 Applications of tunnel diodes
• Tunnel diodes are used as logic memory storage devices.
• Tunnel diodes are used in relaxation oscillator circuits.
• Tunnel diode is used as an ultra high-speed switch.
• Tunnel diodes are used in FM receivers.
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
Presentation
On

Varactor Diode
by

CH. KRANTHI REKHA,

Assistant Professor,
ECE Dept, VBIT

1
UNIT V
• Special Purpose Devices:
Completed Topics: Zener Diode -
Characteristics, Zener diode as Voltage
Regulator, Principle of Operation Tunnel diode

Today’s Topics: - Principle of Operation - UJT,

Varactor Diode,

Next classes: -SCR, Photo diode, Solar cell,

LED, Schottky diode.
 What is a Varactor Diode?
• A varactor diode, also known as a Varicap or volt-cap, is a type of PN
junction diode primarily utilized in the reverse-biased mode.
• It is a device whose capacitance varies with the variation in the applied
reverse bias potential. The term “Varicap” originates from the fusion of
the words “variable” and “capacitor.”
• While standard diodes are used for current conduction, varactor diodes are
specialized for their capacitance characteristics and are primarily operated
in reverse bias to exploit their capacitance properties effectively.
• A varactor diode is a special purpose diode operated in reverse-bias to
form a voltage-controlled capacitor. The width of the depletion region

increases with reverse-bias.

Symbol of Varactor Diode
 Formula of Varactor Diode
CT= ɛA/W
CT :- transition capacitance ɛ :-dielectric constant
A :-capacitor’s plate area W:-depletion layer’s width.
The above relationship shows that transition capacitance is inversely related to
depletion layer width. As a result, if we desire a large capacitance magnitude,
the width should be minimal. And if we use a low reverse voltage, the width
will be minimal. We can simplify it to
CT=CK/(Vb-V)m
CT :-transition capacitance V :-applied voltage
C :-diode capacitance when the device is unbiased
Vb:-barrier voltage at the junction
m :-constant depending upon the material
Q=F/f
Q :-Quality factor of the Varactor Diode F :-maximum operating frequency
f :-the operating frequency
 Construction of Varactor Diode

• The construction of a varactor diode is similar to a regular p-n

junction diode, with some additional characteristics.
• The integral part of Varactor diodes is typically made
from semiconductor materials like silicon (Si) or gallium
arsenide (GaAs). Silicon varactors are commonly used because
they have a lower capacitance range.
• A varactor diode typically consists of a p-n junction. P-N
junctions are created by adding impurities to semiconductor
wafer (doping). The p-type region contains holes ( i.e
positively charged carriers) and the n-type region contains
electrons(negatively charged carriers).
• The p-n junction creates a depletion region at the boundary
between the p-type and n-type regions.
• The width of this region depends on the kind of biasing
technique used in the diode.
• When forward bias is applied this depletion region will be
narrow and when reverse bias is applied the depletion region
will be wide blocking most current flow.
• There are metal contacts added to the p-type and n-type
regions to provide electrical connections. These contacts allow
you to apply an external voltage across the diode.
 Working of Varactor diode
• Under forward bias:
 When a diode is subjected to forward bias, the majority
carriers in both the p and n regions are pushed in the direction
of the battery’s applied voltage, leading to a reduction in the
width of the depletion region.
 Eventually, the diode begins to conduct current more readily.
However, a varactor diode is designed with a specific focus on
its capacitance properties and its ability to store charges. This
is why it is typically operated in reverse bias.
• Under reverse bias:
 When reverse biased, the varactor diode’s capacitance varies
with changes in the applied reverse voltage, making it useful in
various applications like tuning circuits, frequency
modulation, and voltage-controlled oscillators.
• Initially, when there’s no electric force applied, there’s a thin empty region
at the junction between two different materials in a diode. But when we
apply a reverse electric force by connecting it to a battery the wrong way,
something interesting happens.
• The majority carriers in both the p-region (which has positive charges
called “holes”) and the n-region (which has negative charges called
“electrons”) start to move away from the junction. This means they get
pushed apart because of the electric force. As a result, there are fewer of
these majority carriers near the junction.
• So, when we increase the reverse electric force by increasing the voltage
from the battery, the empty region, which we call the “depletion region,”
gets bigger. It’s like the gap between two plates of a capacitor. And the
depletion region behaves like an insulating material in between those
capacitor plates.
• The capacitance at the junction is termed as Transition
Capacitance.
• Now, here’s the important part: as we increase the reverse
voltage (the electric force), the depletion region gets wider.
And because of the formula for capacitance, when the width
(W) of the depletion region increases, the capacitance (CT)
decreases.
• So, in simple terms, when we change the voltage applied to the
diode, the capacitance also changes. It gets smaller when we
increase the voltage because the gap between the plates (the
depletion region) gets bigger.
 Characteristic Curve of Varactor Diode
• The graph shows the non-linear relationship between capacitance and voltage
applied to diode. It is known that capacitance and width have an inverse
relationship which means that as the width of the depletion region increases with
the reverse voltage, the capacitance decreases (i.e. varies inversely as shown in the
graph).
• In short, as the reverse voltage increases, the transition capacitance falls rapidly.
This behaviour can be describes as an exponential pattern.
 ApplicationS of Varactor Diode
• Voltage-Controlled Oscillators(VCOs): Varactor Diodes are used in VCOs. VCOs
are used in Phase-locked Loops and Communication Systems. By Varying the
Voltage Across the Varactor Diode, the Capacitance Changes which leads to change
of the Frequency in the Oscillator.
• Frequency Modulation Tunning in Radios: The Varactor Diodes are also used in
the RF Circuits. By Varying the Voltage Across the Varactor Diode we can adjust
the resonance Frequency of the tuned Circuit which we allow the tuning in FM
radios.
• Phase Shifters: Varactor Diodes are also Implemented in Phase array Antennas. By
Varying the Capacitance in the Certain part of the Antenna Array can lead to
Controlling of the phase of the Emitted Signal Which can be used for beam
Steering and Shaping.
• Voltage-Controlled Filters: Varactor Diodes are used in Voltage Controlled
Bandpass or lowpass Filters. The Cutoff Frequency of the Filter can be Adjusted by
Varying the Voltage across the varactor diode.
 Advantages
• Variable Capacitance: The main advantage of varactor diode
is that we can use it as a variable capacitor. Due to this
property it can be used as a part of frequency tuning circuits.
• Frequency Synthesizer: Due to the small size of varactor
diode and variable capacitance, it can be used in high
frequency elements of electronic devices to generate precise
frequencies.
• Phase Shifters: In PLL(phase-locked loop) circuits, varactor
diodes can be used to create voltage-controlled phase shifters,
allowing precise phase control.
 Advantages
• Frequency Multipliers and Dividers: Varactor
diodes are used in frequency multipliers and dividers
to generate or divide frequencies as needed.
• Economically Affordable: Varactor diodes can be
used in tuning circuits at many levels since they are
economical and affordable. They also generate less
noise as compared to other diodes
 Disadvantages
• Mode-specific: The designing of these diodes is done
to operate them in reverse-bias hence they are not
useful in forward bias.
• Non-linear behaviour: The major drawback of this
diode is its non-linear capacitance-voltage relation
which can result in distortion un many appliances.
• Sensitive: The capacitance of this diode is largely
affected by temperature variations thereby making it
temperature-sensitive.
17
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
Presentation
On

SILICON-CONTROLLED RECTIFIER(SCR )
by

CH. KRANTHI REKHA,

Assistant Professor,
ECE Dept, VBIT
UNIT V
Special Purpose Devices:
Completed Topics: Zener Diode -
Characteristics, Zener diode as Voltage
Regulator, Principle of Operation Tunnel
diode, UJT, Varactor Diode

Today’s Topics: - Principle of Operation –

Silicon controlled rectifier.

Next classes: - Photo diode, Solar cell, LED,

Schottky diode.
 Contents
1. SCR
1. Basic symbol and construction and Equivalent circuit
2. Biasing, Operation and its I-V characteristics
3. Gate characteristics and Application
4. Sawtooth wave generator
5. SCR as a half and full wave Rectifier
2. UJT
1. Construction
2. Interbase resistance
3. Intrinsic stand-off ratio , I-V Characteristics and application
 SILICON-CONTROLLED RECTIFIER(SCR)
 It is one of the prominent members of the thyristors family (A
thyristor is a four-layer semiconductor device, consisting of alternating
P-type and N-type materials (PNPN). A thyristor usually has three
electrodes: an anode, a cathode and a gate, also known as a control
electrode). It was introduced in 1956 by Bell Telephone
Laboratories
 It is a three terminal four layer or PNPN device. Basically, it is
a rectifier with a control element.
 it consists of three diodes connected back-to-back with a
gate connection.
 It is widely used as a switching device in power control
applications. It can control loads by switching current OFF
and ON up to many thousand times a second.

 It can switch ON for variable lengths of time, thereby

delivering selected amount of power to the load. Hence, it
possesses the advantages of a rheostat and a switch
 Basic symbol and construction of SCR
• The graphic symbol for the SCR is shown in Fig 1 with the corresponding
connections to the four-layer semiconductor structure.

• In Fig 2 , if forward conduction is to be established, the anode must be positive

with respect to the cathode. This is not, however, a sufficient criterion for turning
the device on. A pulse of sufficient magnitude must also be applied to the gate to
establish a turn-on gate current, represented symbolically by IGT.

+
IA

IGT

-
 SCR two state equivalent circuit
• A more detailed examination of the basic operation of an SCR is
best effected by splitting the four-layer pnpn structure of Fig1.
into two three-layer transistor structures as shown in Fig 2.and
then considering the resultant circuit.
 Biasing
• With the polarity of V as shown in Fig.(a), the junctions J1 and J3 become
forward biased whereas J2 is reverse- biased.

• Hence, no current (except leakage current) can flow through the SCR.
.
• In Fig. (b), polarity of V has been reversed. It is seen that, now, junctions J1 and
J3 become reverse-biased and only J2 is forward-biased. Again, there is no flow
of current through the SCR.
 Operation
• In above Fig. current flow is blocked due to reverse-biased junction J2. However,
when anode voltage is increased, a certain critical value called forward break over
voltage (VBO) is reached, when J2 breaks down and SCR switches suddenly to a
highly conducting state

• Under this condition, SCR offers very little forward resistance (0.01 Ω –1.0 Ω) so that
voltage across it drops to a low value (about 1 V) as shown in Fig. and current is limited
only by the power supply and the load resistance. Current keeps flowing indefinitely
until the circuit is opened briefly.

• With supply connection as in Fig.(b), the current through the SCR is blocked by the
two reverse biased junctions J1 and J3. When V is increased, a stage comes when Zener
breakdown occurs which may destroy the SCR .

• Hence, it is seen that SCR is unidirectional device.

 SCR I-V Characteristics
• The characteristics of an SCR are provided in Fig. for various values of gate current. The
currents and voltages of usual interest are indicated on the characteristic.
• A brief description of each follows
I. Forward breakover voltage mode V(BR) F* : is that voltage
above which the SCR enters the conduction region.

II. Holding current (IH): is that value of current below which

the SCR switches from the conduction state to the forward
blocking region under stated conditions.

III. Forward and reverse blocking regions are the regions

corresponding to the open circuit condition for the
controlled rectifier which block the flow of charge (current)
from anode to cathode.

IV. Reverse breakdown voltage is equivalent to the Zener or

avalanche region of the fundamental two-layer
semiconductor diode.
 Gate characteristics

(1) (2)
The characteristics of Fig1. are an expanded version of the shaded region of Fig. 2.
• In Fig.1, the three gate ratings of greatest interest, PGFM, IGFM, and VGFM
are indicated. Each is included on the characteristics in the same manner
employed for the transistor. Except for portions of the shaded region, any
combination of gate current and voltage that falls within this region will fire
any SCR in the series of components for which these characteristics are
provided. Temperature will determine which sections of the shaded regio1n0
must be avoided..
 ApplicationS
 Main application of an SCR is as a power control device.

 Consequently, it never dissipates any appreciable amount of power even when

controlling substantial amounts of load power. For example, one SCR requires
only 150 mA to control a load current of 2500 A.

 Other common areas of its application include.

I. regulated power supplies,

II. static switches,
III. motor controls,
IV. inverters,
V. relay controls,
VI. battery chargers,
VII. heater controls,
VIII. phase control.
IX. Half and Full wave rectifier
X. Sawtooth wave generator
 Sawtooth Wave Generator
 Figure illustrate the simple sawtooth generator employing a gate turn-off
switch(GTO) and Zener diode.
 When the supply is energized the GTO will turn on, resulting in the short circuit
equivalent from anode to cathode.
 The capacitor will then begin to charge toward the supply voltage, the capacitor
c1 charges the Zener potential, a revisal in gate to cathode voltage will result,
establishing reversal in gate current.
 Eventual the negative gate current will large enough to turn the GTO off
 The proper choice of resistance R3 and c1 will result a sawtooth waveform. Once
the output voltage v0 drops below vz the GTO will turn ON and the process will
repeat.
 SCR as Half Wave Rectifier

 SCRs are very useful as rectifiers whose output current can be controlled by
controlling the gate current.

 The ac supply to be rectified is applied to the primary of the transformer

ensuring that the negative voltage appearing at the secondary of the
transformer is less than reverse breakdown voltage of the SCR.

 The load resistance RL is connected in series with anode. A variable resistance r

is inserted in the gate circuit for control of gate current.
 The worth noting point is that in an ordinary half-wave rectifier
using a P-N diode, conduction current flows during the whole
of the positive cycle whereas in SCR half-wave rectifier the
current can be made to flow during the part or full of the
positive half cycle by adjustment of gate current. Hence SCR
operates as a controlled rectifier and hence the name silicon
controlled rectifier.

 The output voltage from the SCR rectifier is not a purely dc

voltage but also consists of some ac components, called the
ripples, along it. The ripple components are undesirable and
need to be removed or filtered out. This is accomplished by
placing a filter circuit between the rectifier and load, as shown
in figures.
 During the negative half cycles of ac voltage appearing across the
secondary, the SCR does not conduct regardless of the gate voltage,
because anode is negative with respect to cathode and also peak
inverse voltage is less than the reverse breakdown voltage. The SCR
will conduct during the positive half cycles provided appropriate gate
current is made to flow. The gate current can be varied with the help
of variable resistance r inserted in the gate circuit for this purpose.
The greater the gate current, the lesser will be the supply voltage at
which SCR will start conducting.

 Assume that gate current is such that SCR starts conducting at a

positive voltage V, being less than peak value of ac voltage, Vmax.
From fig.b, it is clear that SCR will start conducting, as soon as the
secondary ac voltage becomes V in the positive half cycle, and will
continue conducting till ac voltage becomes zero when it will turn-
off. Again in next positive half cycle, SCR will start conducting when
ac secondary voltage becomes V volts.
 SCR as full Wave Rectifier
 Two SCRs are connected across the center taped secondary, as shown in
figure.

 The gates of both SCRs are supplied from two gate control supply circuits.
One SCR conducts during the positive half cycle and the other during the
negative half cycle and thus unidirectional current flows in the load circuit.

 The main advantage of this circuit over ordinary full-wave rectifier circuit is
that the output voltage can be controlled by adjusting the gate current.
2. UNIJUNUCTION TRANSISTOR(UJT)

 It is a three-terminal silicon diode. As its name indicates, it

has only one P-N junction .

 It differs from an ordinary diode in that it has three leads

and it differs from a FET in that it has no ability to amplify.

 However, it has the ability to control a large ac power with a

small signal. It is a three-terminal silicon diode. As its name
indicates, it has only one P-N junction.

 It differs from an ordinary diode in that it has three lead

sand it differs from a FET in that it has no ability to amplify.

 However, it has the ability to control a large ac power with a

small signal.
2.1 Construction
 A slab of lightly doped (increased resistance characteristic) n-type silicon
material has two base contacts attached to both ends of one surface and
an aluminum rod alloyed to the opposite surface.

 The p-n junction of the device is formed at the boundary of the aluminum
rod and the n-type silicon slab. The single p-n junction accounts for the
terminology unijunction. It was originally called a duo (double) base diode
due to the presence of two base
2.2 INTER-BASE RESISTANCE (RBB)

• It is the resistance between B2and B1 i.e. it is the total resistance of the

silicon bar from one end to the other with emitter terminal open.

• It should also be noted that point

A is such that RB1 > RB2. Usually, RB1 = 60% of RB1

• The resistance RB1 has been shown as a variable resistor because its value
varies
inversely as I E.
2.3 INTRINSIC STAND-OFF RATIO
• As seen from Fig, when a battery of 30 V is applied across B2 B1 , there is a progressive fall
of voltage over RBB provided E is open.

• It is obvious from Fig. that emitter acts as a voltage-divider tap on fixed resistanceRBB.

• With emitter open, I1 = I2, the interbase current is given by Ohm’s Law.
I1= I2=VBB /RBB
• It may be noted that part of VBB is dropped over RB2 and part on RB1. Let us
call the voltage drop across RB1 as VA.
• Using simple voltage divider relationship, The voltage division factor is given a special
symbol (η) and the name of ‘intrinsic standoff ratio’

Therefore, VRB1=VBB
2.3 UJT Static Emitter Characteristics
 The decrease in resistance in the active region is due to the holes injected into the n-type
slab from the aluminum p-type rod when conduction is established.

 The increased hole content in the n-type material will result in an increase in the number
of free electrons in the slab, producing an increase in conductivity (G) and a
corresponding drop in resistance (R ↓ 1/G ↑). Three other important parameters for the
unijunction transistor are IP, VV, and IV. Each is indicated on Fig.
• When VBB is switched on, VA is developed and reverse-biases the junction. If VB
is the barrier voltage of the P-N junction, then total reverse bias voltage is = VRB1 +
VB = ηVBB + VB Value of VB for Si is 0.7 V.

• It is obvious that emitter junction will not become forward-biased unless its
applied voltage VE exceeds (ηVBB+ VB). This value of VE is called peakpoint
voltage VP .

• When VE = VP, emitter (peak current), IP starts to flow through RB1 to ground
(i.e. B1). The UJT is then said to have been fired or turned ON. Beyond the valley
point, UJT is in
saturation and VE increases very little with an increasing IE.

• It is seen that only terminals E and B1 are the active terminals whereas B2 is the
bias terminal i.e. it is meant only for applying external voltage across the UJT.

• Generally, UJT is triggered into conduction by applying a suitable positive pulse

at its emitter.

• It can be brought back to OFF state by applying a negative trigger pulse.

 2.3 APPLICATIONS
• One unique property of UJT is that it can be triggered by (or an output
can be taken from) any one of its three terminals.

• Once triggered, the emitter current IE of the UJT increases

regeneratively till it reaches a limiting value determined by the external
power supply. Because of this particular behavior , UJT is used in a
variety of circuit applications.

• Some of which are :

i. phase control
ii. sine wave generator
iii. switching
iv. Pulse generator
v. sawtooth generator
vi. timing and triggercircuits,
vii.voltage or current regulated supplies
 References

Books
 Electronics device and circuit theory by Robert L. Boylestad
 Solid State Electronic Devices by Ben.G. Streetman,S.K Banerjee
 Principle of electronic material and devices by S.O. Kasap

website

 http://www.circuitstoday.com/scr-as-
half-wave-rectifier
 https://www.electronics-
tutorials.ws/diode/schottky-
diode.html
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
Presentation
On

Unijunction Transistor –
Construction, Working Principle, and Characteristic Features
by

CH. KRANTHI REKHA,

Assistant Professor,
ECE Dept, VBIT

1
UNIT V
• Special Purpose Devices:
Completed Topics: Zener Diode - Characteristics, Zener diode as
Voltage Regulator, Principle of Operation Tunnel diode

Today’s Topics: - Principle of Operation - UJT, Varactor Diode

Next classes: -SCR, Photo diode, Solar cell, LED, Schottky diode.
 Definition
• A Unijunction Transistor (UJT) is a three-terminal
semiconductor device. The main characteristics of UJT is
when it is triggered, the emitter current increases re-
generatively until it is limited by emitter power supply.
• Due to this characteristic feature, it is used in applications
like switching pulse generator, saw-tooth wave generator etc.
 Construction of UJT
• The UJT consists of an n-type silicon semiconductor bar with an electrical 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, 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.
Symbol
 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.
 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.
 Important points from the characteristics

• At first, in the cut off region, when the emitter voltage increases from zero,
due to the minority charge carriers, a small current flows from terminal
B2 to emitter. This is called as leakage current.
• Above the definite value of VE, the emitter current (IE) starts to flow and
increases until the peak (VP and IP) is reached at point P.
• After point P, an increase in VE causes a sudden increase in IE with a
corresponding decrease in VE. This is the Negative Resistance Region of
the curve as with the increase in IE, VE decreases.
• The negative resistance region of the curve ends at the valley-point (V),
having valley-point voltage VV and current IV. After the valley-point the
device is driven to saturation.
Equivalent Circuit of UJT
• 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 ratio (ƞ) of UJT is given by

𝑉1 𝑅𝐵1
Ƞ= =
𝑉𝐵𝐵 𝑅𝐵1 +𝑅𝐵2
• The voltage across RB1 is
𝑅𝐵1
𝑉1 = 𝑉 𝐵𝐵 = ƞ 𝑉 𝐵𝐵
𝑅𝐵1 +𝑅𝐵2
• The value of ƞ generally lies between 0.51 and 0.82.
• The Peak Point Voltage (VP) of the UJT
𝑉𝑃 = ƞ 𝑉𝐵𝐵 + 𝑉𝐷
 Advantages of UJT
• Low cost
• Excellent characteristics
• Low power absorbing device under normal operating
conditions
 Applications of UJT
• Oscillators
• Trigger Circuits
• Saw tooth generator
• Bi-stable networks
• Pulse and voltage sensing circuits
• UJT relaxation oscillators
• Over voltage detectors
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
Presentation
On

Photodiode & Light Emitting Diode

by

CH. KRANTHI REKHA,

Assistant Professor,
ECE Dept, VBIT
 Definition of Photodiode
• A photodiode is a PN-junction diode that
consumes light energy to produce an electric
current.
• They are also called a photo-detector, a light
detector, and a photo-sensor.
• Photodiodes are designed to work in reverse
bias condition.
• Typical photodiode materials are Silicon,
Germanium and Indium gallium arsenide.
 Symbol of Photodiode
• The following image shows the symbol of the
photodiode:
 Photodiode Working
• A photodiode is subjected to photons in the form of light which affects the
generation of electron-hole pairs.
• If the energy of the falling photons (hv) is greater than the energy gap (Eg)
of the semiconductor material, electron-hole pairs are created near the
depletion region of the diode.
• The electron-hole pairs created are separated from each other before
recombining due to the electric field of the junction. The direction of the
electric field in the diode forces the electrons to move towards the n-side
and consequently the holes move towards the p-side.
• As a result of the increase in the number of electrons on the n-side and
holes on the p-side, a rise in the electromotive force is observed. Now when
an external load is connected to the system, a current flow is observed
through it.
 Working of a Photodiode
• The more the electromotive force created, the greater the current flow. The
magnitude of the electromotive force created depends directly upon the
intensity of the incident light. This effect of the proportional change in
photocurrent with the change in light intensity can be easily observed by
applying a reverse bias.
• Since photodiodes generate current flow directly depending upon the light
intensity received, they can be used as photo detectors to detect optical
signals. Built-in lenses and optical filters may be used to enhance the power
and productivity of a photodiode.
 Working of a Photodiode
• Generally, when a light is made to illuminate the PN junction,
covalent bonds are ionized.
• This generates hole and electron pairs. Photocurrents are
produced due to generation of electron-hole pairs. Electron
hole pairs are formed when photons of energy more than
1.1eV hits the diode.
• When the photon enters the depletion region of diode, it hits
the atom with high energy. This results in release of electron
from atom structure. After the electron release, free electrons
and hole are produced.
 Connecting a Photodiode in an
External Circuit
• A Photodiode operates in a circuit in reverse bias.
Anode is connected to circuit ground and cathode to
positive supply voltage of the circuit. When
illuminated by light, current flows from cathode to
anode.
• When photodiodes are used with external circuits, they are
connected to a power source in the circuit.
• The amount of current produced by a photodiode will be very
small.
• This value of current will not be enough to drive an electronic
device.
• So when they are connected to an external power source, it
delivers more current to the circuit.
• So, battery is used as a power source. The battery source helps
to increase the current value, which helps the external devices
to have a better performance
 V-I Characteristics of Photodiode

• Photodiode operates in reverse bias condition.

• Reverse voltages are plotted along X axis in volts and reverse
current are plotted along Y-axis in microampere.
• Reverse current does not depend on reverse voltage. When
there is no light illumination, reverse current will be almost
zero.
• The minimum amount of current present is called as Dark
Current. Once when the light illumination increases, reverse
current also increases linearly.
Applications of Photodiode
• Photodiodes are used in simple day-to-day applications. The reason for
their prominent use is their linear response of photodiode to light
illumination.
• Photodiodes with the help of optocouplers provide electric isolation. When
two isolated circuits are illuminated by light, optocouplers are used to
couple the circuit optically. Optocouplers are faster compared to
conventional devices.
• Photodiodes are used in safety electronics such as fire and smoke detectors.
• Photodiodes are used in numerous medical applications. They are used in
instruments that analyze samples, detectors for computed tomography and
also used in blood gas monitors.
Applications of Photodiode
• Photodiodes are used in solar cell panels.
• Photodiodes are used in logic circuits.
• Photodiodes are used in the detection circuits.
• Photodiodes are used in character recognition circuits.
• Photodiodes are used for the exact measurement of the
intensity of light in science and industry.
• Photodiodes are faster and more complex than normal PN
junction diodes and hence are frequently used for lighting
regulation and optical communication
 Light Emitting Diode
• A light-emitting diode (LED) is a semiconductor device that
emits light when an electric current flows through it.
• When current passes through an LED, the electrons recombine
with holes emitting light in the process.
• LEDs allow the current to flow in the forward direction
and blocks the current in the reverse direction.
• Light-emitting diodes are heavily doped p-n junctions.
• Based on the semiconductor material used and the amount of
doping, an LED will emit coloured light at a particular spectral
wavelength when forward biased.
• As shown in the figure, an LED is encapsulated with
a transparent cover so that emitted light can come out.
 LED Symbol

• The LED symbol is the standard symbol for a diode,

with the addition of two small arrows denoting the
emission of light.
 Construction of LED
• The construction of LED is very simple because it is designed
through the deposition of three semiconductor material layers
over a substrate.
• These three layers are arranged one by one where the top
region is a P-type region, the middle region is active and
finally, the bottom region is N-type.
• The three regions of semiconductor material can be observed
in the construction. In the construction, the P-type region
includes the holes; the N-type region includes elections
whereas the active region includes both holes and electrons.
• When the voltage is not applied to the LED, then
there is no flow of electrons and holes so they are
stable.
• Once the voltage is applied then the LED will
forward biased, so the electrons in the N-region and
holes from P-region will move to the active region.
• This region is also known as the depletion region.
Because the charge carriers like holes include a
positive charge whereas electrons have a negative
charge so the light can be generated through the
recombination of polarity charges.
 Working of an LED
• When the diode is forward biased, the minority
electrons are sent from p → n while the minority
holes are sent from n → p.
• At the junction boundary, the concentration of
minority carriers increases. The excess minority
carriers at the junction recombine with the majority
charges carriers.
• The energy is released in the form of photons on
recombination.
• In standard diodes, the energy is released in the form
of heat. But in light-emitting diodes, the energy is
released in the form of photons. We call this
phenomenon electroluminescence.
• Electroluminescence is an optical phenomenon, and
electrical phenomenon where a material emits light in
response to an electric current passed through it.
• As the forward voltage increases, the intensity of the
light increases and reaches a maximum.
What determines the color of an
LED?
• The colour of an LED is determined by the material
used in the semiconducting element.
• The two primary materials used in LEDs are
aluminium gallium indium phosphide alloys and
indium gallium nitride alloys.
• Aluminium alloys are used to obtain red, orange and
yellow light, and indium alloys are used to get green,
blue and white light. Slight changes in the
composition of these alloys change the colour of the
emitted light.
 Types of Light Emitting
Diodes
• There are different types of light-emitting diodes present and some of them
are mentioned below.
• Gallium Arsenide (GaAs) – infra-red
• Gallium Arsenide Phosphide (GaAsP) – red to infra-red, orange
• Aluminium Gallium Arsenide Phosphide (AlGaAsP) – high-brightness red,
orange-red, orange, and yellow
• Gallium Phosphide (GaP) – red, yellow and green
• Aluminium Gallium Phosphide (AlGaP) – green
• Gallium Nitride (GaN) – green, emerald green
• Gallium Indium Nitride (GaInN) – near-ultraviolet, bluish-green and blue
• Silicon Carbide (SiC) – blue as a substrate
• Zinc Selenide (ZnSe) – blue
• Aluminium Gallium Nitride (AlGaN) – ultraviolet
Light Emitting Diode Colours
Light Emitting Diodes I-V
Characteristics
• Before a light emitting diode can “emit” any form of light it
needs a current to flow through it, as it is a current dependant
device with their light output intensity being directly
proportional to the forward current flowing through the LED.
• As the LED is to be connected in a forward bias condition
across a power supply it should be current limited using a
series resistor to protect it from excessive current flow.
• Never connect an LED directly to a battery or power supply as
it will be destroyed almost instantly because too much current
will pass through and burn it out.
• From the table above we can see that each LED has its own
forward voltage drop across the PN junction and this
parameter which is determined by the semiconductor material
used, is the forward voltage drop for a specified amount of
forward conduction current, typically for a forward current of
20mA.
• In most cases LEDs are operated from a low voltage DC
supply, with a series resistor, RS used to limit the forward
current to a safe value from say 5mA for a simple LED
indicator to 30mA or more where a high brightness light
output is needed.
 AdvAntAges of Led’s
• The cost of LED’s is less and they are tiny.
• By using the LED’s electricity is controlled.
• The intensity of the LED differs with the help of the
microcontroller.
• Long Lifetime
• Energy efficient
• No warm-up period
• Rugged
• Doesn’t affect by cold temperatures
• Directional
• Color Rendering is Excellent
• Environmentally friendly
• Controllable
 disAdvAntAges of Led’s
• Price
• Temperature sensitivity
• Temperature dependence
• Light quality
• Electrical polarity
• Voltage sensitivity
• Efficiency droop
• Impact on insects
 Applications of Light Emitting Diode

There are many applications of LED and some of them

are explained below.
LED is used as a bulb in the homes and industries
The light-emitting diodes are used in motorcycles and
cars
These are used in mobile phones to display the
message
At the traffic light signals led’s are used
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
Presentation
On

SCHOTTKY DIODE & Solar

Cell
by

CH. KRANTHI REKHA,

Assistant Professor,
ECE Dept, VBIT
 Definition
• The schottky diode is a type of metal – semiconductor
junction diode, which is also known as hot-carrier diode,
low voltage diode or schottky barrier diode.
• The schottky diode is formed by the junction of a
semiconductor with a metal. Schottky diode offers fast
switching action and has a low forward voltage drop.
• As we are aware that in a PN junction diode, p-type and
n-type are joined together to form a PN junction.
Whereas, in a Schottky diode metals like platinum or
aluminum are used instead of P type semiconductors.
 Symbol
• The symbol for the Schottky barrier diode is based
around the basic diode circuit symbol. The circuit
symbol of the Schottky diode is shown in the figure.
• Schottky Diode Symbol
V-I Characteristics of Schottky Diode
• The V-I characteristics of Schottky diodes are very much similar to the PN
junction diode.
• Current is the dependent variable while voltage is the independent variable
in the Schottky diode.
• The forward voltage drop of the Schottky diode is low between 0.2 to 0.3
volts.
 Characteristics of Schottky
Diode
• The V-I characteristics of Schottky diodes are very
much similar to the PN junction diode. Current is the
dependent variable while voltage is the independent
variable in the Schottky diode.
• Forward Bias (F): When a forward bias voltage is
applied across a Schottky diode (positive voltage on the
anode, negative voltage on the cathode), the V-I
characteristics show that it conducts current very
quickly with a relatively low forward voltage drop. This
rapid conduction is due to the low Schottky barrier at
the metal-semiconductor junction. The forward current
increases exponentially with voltage, similar to a
standard diode.
 Characteristics of Schottky
Diode
• Reverse Bias (R): In the reverse bias condition (positive voltage on
the cathode, negative voltage on the anode), the V-I characteristics
of a Schottky diode display a small reverse current, often referred to
as the leakage current. This reverse current is typically larger than
that of a silicon PN junction diode because of the absence of the
depletion region found in PN junctions. However, it is still relatively
low compared to other diode types.
• Breakdown Voltage: Schottky diodes do not exhibit a well-defined
breakdown voltage like some other diode types. Instead, they have a
reverse breakdown mechanism known as “avalanche breakdown.”
When the reverse voltage exceeds a certain level, the diode can
experience avalanche breakdown, leading to a sudden increase in
reverse current.
• Cut-in Voltage: The cut-in voltage (also called the knee voltage) for
a Schottky diode is typically very close to zero volts, which means it
starts conducting at very low forward bias voltages.
 Schottky Diode Construction
• It is a unilateral junction.
• A metal-semiconductor junction is formed at one end and
another metal semiconductor contact is formed at the other
end. It is an ideal Ohmic bidirectional contact with no potential
existing between the metal and the semiconductor and it is
non-rectifying.
• The built-in potential across the open-circuited Schottky
barrier diode characterizes the Schottky diode.
• Schottky diode is a function of temperature dropping. It
decreases and increasing temperature doping concentration in
N-type semiconductor.
• For manufacturing purposes, the metals of the Schottky barrier
diode like molybdenum, platinum, chromium, tungsten
Aluminium, gold, etc., are used and the semiconductor used is
N-type
 Schottky Diode Construction
• We know that unlike a normal P-N junction diode ,a Schottky diode is
equipped with a metallic contact. On further analysis the construction
of the diode is like:
 Schottky diodes are constructed using semiconductor materials. Silicon
(Si) is a common choice, although other materials can also be used.
 The differentiating feature in Schottky diode is the metal-to-
semiconductor junction. A metal contact (made of metals like Pt, W, Al)
is directly connected to the semiconductor material (usually n-type
silicon).
 The region beneath the metal contact is n-type semiconductor. It has a
higher concentration of electrons (i.e, electrons are the majority charge
carriers).
 Schottky diodes have a depletion region near the junction. Since we do
not have a p-type region, this depletion region is much thinner than a
normal diode.
 The interaction of electrons from the n-type semiconductor with metal
atoms is the major driving factor for formation of this region.
 Working of a Schottky Diode
• The operation relies on the principle that the electrons in different materials
have different potential energy.
• N-type semiconductors have higher potential energy than electrons of
metals.

• When these two are brought into contact, there is a flow of electrons in
both directions across the metal-semiconductor interface.
• A voltage is applied to the Schottky so that the metal is positive when
compared to the semiconductor.
• The voltage opposes the built-in potential and makes the current flow easy.
 Applications of Schottky Diode
• Schottky diodes have been useful for the industry
of electronics that has spotted many applications
in diode rectifiers because of its unique
properties. Here are some major areas where it is
widely used.
• RF mixer and detector diode:
• The Schottky diode consists of its radio frequency
functions owing to its switching speed at the
highest level and top frequency capability. The
Schottky barrier diodes come handy for diode ring
mixers with high performance.
• Power rectifier:
• The Schottky barrier diodes also have functions
with high power as rectifiers. The high density of
current and voltage drop with low forward shows
that the wastage of power is lesser than the
normal PN junction diodes.
• Power OR circuits:
• This diode would be useful for functions where
two different power supplies drive a load like in
battery supply. It is important that the power
coming from supply should not mix with the
others.
• Solar Cell Applications:
• As we know, the solar cells are usually linked
to the batteries that are rechargeable, mostly
batteries with lead-acid since power supply
must be necessary round the clock. Solar cells
would not support the applied charge in
reverse and thus, a diode would be used in a
proportional pattern of the solar cells.
 Features Of Schottky Diode

• Higher efficiency
• Low forward voltage drop
• Low capacitance
• Low profile surface-mount package, ultra-small
• Integrated guard ring for stress protection
• Advantages of Schottky diode
Following are the advantages of Schottky diode:
 The capacitance of the diode is low as the depletion
region of the diode is negligible.
 The reverse recovery time of the diode is very fast, that
is the change from ON to OFF state is fast.
 The current density of the diode is high as the depletion
region is negligible.
 The turn-on voltage of the diode is 0.2 to 0.3 volts,
which is very low.
• Disadvantages of Schottky diode
 The only disadvantage of Schottky diodes is that the
reverse saturation current of the diode is large.
 Difference between Schottky diode and PN
junction diode
Schottky diode PN junction diode

In this diode, the junction is formed In this diode, the junction is formed
between the n-type semiconductor and between the p-type and n-type
the metal plate semiconductors

The forward voltage drop for pn junction

The forward voltage drop is low
diode is more

Reverse recovery loss and reverse Reverse recovery loss and reverse
recovery time are very less recovery time are more

It is a unipolar device It is a bipolar device

The conduction of current happens only The conduction of current happens due
due to the movement of electrons to the movement of electrons and holes
 Solar cell
• Solar cell is an electrical device that converts the energy
of light directly into electricity by the photovoltaic effect,
which is a physical and chemical phenomenon.
• It is a form of photoelectric cell, defined as a device whose
electrical characteristics, such as current, voltage, or
resistance, vary when exposed to light. Solar cells are the
building blocks of photovoltaic modules, otherwise known
as solar panels.
 Overview of Solar Cells
• Solar cells are described as being photovoltaic irrespective of
whether the source is sunlight or an artificial light. They are used as
a photodetector (for example infrared detectors), detecting light or
other electromagnetic radiation near the visible range, or measuring
light intensity.
• Solar cells are often bundled together to make larger units called
solar modules, themselves coupled into even bigger units known as
solar panels.
• Just like the cells in a battery, the cells in a solar panel are designed
to generate electricity; but where a battery’s cells make electricity
from chemicals, a solar panel’s cells generate power by capturing
sunlight instead.
 Solar Cell Structure
• A solar cell is an electronic device which directly
converts sunlight into electricity.
• Light shining on the solar cell produces both a current
and a voltage to generate electric power. This process
requires firstly, a material in which the absorption of
light raises an electron to a higher energy state, and
secondly, the movement of this higher energy electron
from the solar cell into an external circuit.
• The electron then dissipates its energy in the external
circuit and returns to the solar cell. A variety of
materials and processes can potentially satisfy the
requirements for photovoltaic energy conversion, but in
practice nearly all photovoltaic energy conversion uses
semiconductor materials in the form of a p-n junction.
Cross section of a solar cell
 Steps in the operation of a solar
cell
• The basic steps in the operation of a solar cell
are:
The generation of light-generated carriers;
The collection of the light-generated carries to
generate a current;
The generation of a large voltage across the
solar cell; and
The dissipation of power in the load and in
parasitic resistances.
 Working of a Solar cell
• A solar cell is a sandwich of n-type silicon and p-type silicon . It
generates electricity by using sunlight to make electrons hop across
the junction between the different flavors of silicon:
 When sunlight shines on the cell, photons (light particles) bombard
the upper surface.
 The photons (yellow blobs) carry their energy down through the cell.
 The photons give up their energy to electrons (green blobs) in the
lower, p-type layer.
 The electrons use this energy to jump across the barrier into the
upper, n-type layer and escape out into the circuit.
 Flowing around the circuit, the electrons make the lamp light up.
• P-type and n-type silicon are two types of semiconductors used to make
solar cells. Atoms with one fewer electron in their outer energy level than
silicon, like boron or gallium, are added to create p-type silicon.
 An electron vacancy, sometimes known as a “hole,” is generated in boron
because it has one fewer electron than is necessary to make the bonds with
the neighbouring silicon atoms.
 Adding elements, like phosphorus, that have one extra electron in their
outer level than silicon creates the n-type silicon.
 The outer energy level of phosphorus contains five electrons, not four. It
forms bonds with the silicon atoms next to it. However, the bonds do not
involve one electron. It can instead move around freely inside the silicon
framework.
 A solar cell is made of two layers of silicon: one of p-type and one of n-
type. There are more electrons in the n-type layer than positively charged
holes in the p-type layer (which are vacancies due to the lack of valence
electrons).
 The electrons on the n-type layer side of the junction go into the holes on
the other side of the junction (p-type layer) near the junction of the two
layers.
 As a result, the electrons fill the holes in the depletion zone, which is
formed around the junction.
 When all the holes in the depletion zone are filled with electrons, the
p-type side of the depletion zone, where holes were initially present,
now contains negatively charged ions, and the n-type side of the
depletion zone, where electrons were initially present, now contains
positively charged ions.
 Due to the internal electric field produced by these oppositely
charged ions, the n-type layer’s electrons cannot fill the p-type
layer’s holes.
 When sunlight strikes a solar cell, electrons in the silicon are
ejected, causing the silicon to generate “holes”—the vacancies left
behind by the departing electrons.
 Electrons will be moved to the n-type layer and holes to the p-type
layer if this occurs in the electric field. The electrons will cross the
depletion zone from the n-type layer to the p-type layer and then
pass through the external wire back of the n-type layer to go to the
p-type layer if the n-type and p-type layers are connected by a
metallic wire, resulting in an electrical current.
 I-V curve of a solar cell
• The I-V curve of a solar cell is the superposition
of the I-V curve of the solar cell diode in the dark
with the light-generated current.
• The light has the effect of shifting the I-V curve
down into the fourth quadrant where power can
be extracted from the diode. Illuminating a cell
adds to the normal "dark" currents in the diode so
that the diode law becomes:
 Efficiency of solar cells
• The efficiency of a solar cell can be determined by conductivity,
charge carrier separation, reflectance, and thermodynamic
components. The sum of these distinct indicators is the overall
efficiency. The percentage of incident power turned into electricity
determines a solar cell’s power conversion efficiency.
 The system’s peak power, or how much electricity it produces, is a
percentage of the solar energy it receives. A one square metre panel
that produces 200 W of power operates at a 20% efficiency. The
maximum theoretical efficiency of PV cells is around 33%. The
name of this limit is the Shockley-Queisser limit.
 Most cells only convert 10–20% of the energy they receive into
electricity, with laboratory cells having the highest efficiency at
roughly 45% under ideal circumstances.
 Advantages of Solar Cells
• The advantages of solar cells are as follows:
 Lower the cost of your energy bill- The most well-known
benefit of solar cells is that they will make your electricity bills
less expensive.
 Sustainability – Solar energy has the benefit of being a
sustainable substitute for fossil fuels. The sun is certain to exist
for at least a few billion years, whereas fossil fuels have an
expiration date that may be rapidly approaching.
 Lengthy-Term Energy- PV systems frequently have a long
lifespan and are remarkably durable. Additionally, PV panels
frequently come with a guarantee of at least 20 years, making
them a dependable source of electricity for your roof.
 Advantages of Solar Cells
 Selling Energy- It is frequently simpler to sell your home for a greater
price if it contains solar panels. For example, if you wish to invest in
solar cells, several grants and incentives are available in the UK.
 Low Impact on the Environment – Solar energy has a much lower
environmental impact than fossil fuels. Because the technique requires
no fuel combustion, its greenhouse gas emissions are negligible.
Additionally, photovoltaic (PV) solar cells do not use any water to
produce power, whereas concentrating solar thermal plants (CSP) are
relatively inefficient in their water usage, depending on the technology
employed.
 Easy to maintain- Comparing solar PV cells to other renewable energy
sources, they are noted for having lower operating and maintenance
expenses.
 Serene – Due to its lack of noise, solar PV is ideal for residential and
urban use.
 Disadvantages of Solar Cells
• The disadvantages of solar cells are as follows:
 High initial expenditure – Installing solar panels requires a
sizable initial outlay, which not everyone can afford.
Unfortunately, this is a drawback of solar panels, but the future
looks promising as costs are falling.
 Periodic Energy – Since we experience less sun in some parts
of the world, the solar power plant is much more seasonal than
other renewable energy sources. Grid-connected solar arrays
can buy power from the power grid when less energy is
available for collection.
 Solar panels on your residence – Older homes often have
distinct features that might generate shade, making placing
solar panels on them potentially more difficult. A flat roof with
solar cells becomes excessively heavy while drifting snow
may fall below the racks.
 Disadvantages of Solar Cells
 A challenge to disassemble – Solar cells are challenging to
remove and reinstall once placed in another location. Your
system must be dismantled and transported to a different
place, which requires additional money and labour.
 Low energy conversion rate–Solar cells can only convert
roughly 25% of solar energy into electrical power. More
technology advancements are needed to optimize solar
energy better because there is a tremendous possibility to
use it.
 Utilizes a sizable area – The sizable land tracts dedicated
to installing land-mounted PV panels remain in use.
Various Applications of Solar Cells

Solar cells find diverse applications across

various sectors due to their clean energy
production.
In residential settings, solar cells are commonly
used to generate electricity for homes, reducing
dependence on traditional power sources.
In agriculture, solar-powered water pumping
systems help irrigate fields efficiently, especially
in remote areas.
 Various Applications of Solar Cells

 Solar cells power streetlights, offering an

environmentally friendly and cost-effective alternative
to traditional grid-connected lighting.
 Portable electronic devices, such as calculators and
mobile chargers, utilize small solar cells for sustainable
energy supply.
 In spacecraft and satellites, solar cells provide a reliable
source of power for extended missions in outer space.
 Solar-powered vehicles, from cars to bicycles, leverage
solar cells to supplement or entirely replace
conventional fuel sources.