1. Unit 5 Power Devices
and Display Devices
Department of Electronics and Communication
Institute of Road and Transport Technology, Erode, INDIA
3. LED – Light Emitting Diode
A light-emitting diode (LED)
is a semiconductor light
source that emits light
when current flows
Sample 2 –LED Lamp
Sample 3 –LED Lamp
4. Figure shows the longer lead is
Anode and shorter lead is cathode. It
emits light only in the Forward bias
5. Working Principle
Electrons in the semiconductor
recombine with holes, releasing energy
in the form of photons. The color of the
light (corresponding to the energy of
the photons) is determined by the
energy required for electrons to cross
the band gap of the semiconductor.
White light is obtained by using
multiple semiconductors or a layer of
light-emitting phosphor on the
LED Symbol and
Its Driver Circuit
LED Application- LED
1.Low Power Consumption
find LEDs in Cars, Bikes, Street
Lights, Home Lighting, Office
Lighting, Mobile Phones,
Televisions and many more.
7. 1. Through-hole LED – single
2. Surface Mount LED lights
3. Bi-color LED
4. RGB LED
5. High Power LED
6. Infra Red LED
● Types of LED
8. LCD – Liquid Crystal Display
● Liquid crystal (LC) is an organic substance that has both
a liquid form and a crystal molecular structure. In
this liquid, the rod-shaped molecules are normally in a
parallel array, and an electric field can be used to control
the molecules. Most LCDs today use a type of liquid
crystal called Twisted Nematic (TN).
11. ● The basic display using liquid crystals is composed of six main
components: a polarizing filter, a glass plate that has a transparent
electrode pattern, the liquid crystal material, a clear common electrode
on glass, a polarizer whose axis is crossed compared to the first
polarizer, and either a reflective surface or a light source. Without the
liquid crystal between the polarizers, the crossed polarizers would block
out the light, making the screen appear dark.
● The main principle behind liquid crystal molecules is that when an
electric current is applied to them, they tend to untwist. This causes a
change in the light angle passing through them. This causes a change in
the angle of the top polarizing filter with respect to it. So little light is
allowed to pass through that particular area of LCD. Thus that area
becomes darker comparing to others.
12. ● The LCD glass has transparent electrical conductors plated onto each side
of the glass in contact with the liquid crystal fluid and they are used as
electrodes. These electrodes are made of Indium-Tin Oxide (ITO). When an
appropriate drive signal is applied to the cell electrodes, an electric field is
set up across the cell. The liquid crystal molecules will rotate in the direction
of the electric field. The incoming linearly polarized light passes through the
cell unaffected and is absorbed by the rear analyzer. The observer sees a
black character on a sliver gray background. When the electric field is turned
off, the molecules relax back to their 90 degree twist structure. This is
referred to as a positive image, reflective viewing mode.
13. 1.The liquid crystal displays
(LCDs) are used in aircraft
2.It is used as a display screen in
3.For displaying images used in
4.The television is main
applications of LCD.
5.Mostly the computer monitor is
made up of LCDs.
15. Photo Transistor
Phototransistors are either tri-
terminal (emitter, base and
collector) or bi-terminal (emitter
and collector) semiconductor
devices which have a light-
sensitive base region as shown in
sample Figures. Although all
transistors exhibit light-sensitive
nature, these are specially
designed and optimized for photo
16. These are made of diffusion or ion-implantation and have much larger collector
and base regions in comparison with the ordinary transistors. These devices
can be either homojunction structured or heterojunction structured as shown in
17. Working Principle of Photo transistor
Phototransistors work in a similar way to photoresistors commonly known as
LDR (light dependent resistor) but are able to produce both current and
voltage while photoresistors are only capable of producing current due to
change in resistance. Phototransistors are transistors with the base terminal
exposed. Instead of sending current into the base, the photons from striking
light activate the transistor. This is because a phototransistor is made of a
bipolar semiconductor and focuses the energy that is passed through it.
These are activated by light particles and are used in virtually all electronic
devices that depend on light in some way. All silicon photosensors
(phototransistors) respond to the entire visible radiation range as well as to
18. Advantages of Phototransistor
● Phototransistors produce higher current than photodiodes.
● Phototransistors are relatively inexpensive, simple, and small enough to fit
several of them onto a single integrated computer chip.
● Phototransistors are very fast and are capable of providing nearly
● Phototransistors produce a voltage, that photo-resistors cannot do so.
19. Disadvantages of Phototransistor
● Phototransistors that are made of silicon are not capable of handling voltages
over 1,000 Volts.
● Phototransistors are also more vulnerable to surges and spikes of electricity as
well as electromagnetic energy.
● Phototransistors also do not allow electrons to move as freely as other devices
do, such as electron tubes.
20. Applications of Phototransistor
● Punch-card readers.
● Security systems
● Encoders – measure speed and direction
● IR detectors photo
● electric controls
● Computer logic circuitry.
● Lighting control (highways etc)
● Level indication
● Counting systems
22. Opto-Coupler or Opto-
An opto-isolator (also called an
optocoupler, photocoupler, or optical
isolator) is an electronic component
that transfers electrical signals
between two isolated circuits by using
light. Opto-isolators prevent high
voltages from affecting the system
receiving the signal.
23. Working Principle
The basic design of an optocoupler,
also known as an Opto-isolator,
consists of an LED that produces
infra-red light and a semiconductor
photo-sensitive device that is used to
detect the emitted infra-red beam.
Both the LED and photo-sensitive
device are enclosed in a light-tight
body or package with metal legs for
the electrical connections as shown.
24. Assume a photo-transistor device as shown.
Current from the source signal passes
through the input LED which emits an infra-
red light whose intensity is proportional to
the electrical signal.
This emitted light falls upon the base of the
photo-transistor, causing it to switch-ON
and conduct in a similar way to a normal
The base connection of the photo-transistor
can be left open (unconnected) for
maximum sensitivity to the LEDs infra-red
light energy or connected to ground via a
suitable external high value resistor.
25. Applications of Opto coupler
microprocessor input/output switching
DC and AC power control
signal isolation and power supply regulation
Note: The electrical signal being transmitted can be either analogue
(linear) or digital (pulses).
26. Solar Cell
A solar cell, or photovoltaic 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. Individual
solar cell devices can be combined to
form modules, otherwise known as solar
Sample 1 Sample 2
Sample 3 Sample 4
27. A solar cell is basically a
junction diode, although its
construction it is little bit
different from conventional
p-n junction diodes. A very
thin layer of p-type
semiconductor is grown on a
relatively thicker n-type
Then apply a few finer electrodes on the top of the p-type semiconductor layer.
These electrodes do not obstruct light to reach the thin p-type layer. Just below the
p-type layer there is a p-n junction. A current collecting electrode at the bottom of
the n-type layer. The entire assembly encapsulated by thin glass to protect the solar
cell from any mechanical shock.
28. Working Principle
When light reaches the p-n junction, the light photons can easily enter in the junction,
through very thin p-type layer. The light energy, in the form of photons, supplies
sufficient energy to the junction to create a number of electron-hole pairs. The incident
light breaks the thermal equilibrium condition of the junction. The free electrons in the
depletion region can quickly come to the n-type side of the junction.
Similarly, the holes in the depletion can quickly come to the p-type side of the junction.
Once, the newly created free electrons come to the n-type side, cannot further cross
the junction because of barrier potential of the junction.
Similarly, the newly created holes once come to the p-type side cannot further cross
the junction became of same barrier potential of the junction. As the concentration of
electrons becomes higher in one side, i.e. n-type side of the junction and
concentration of holes becomes more in another side, i.e. the p-type side of the
junction, the p-n junction will behave like a small battery cell. A voltage is set up which
is known as photo voltage.
29. Materials Used in Solar Cell
The materials which are used for this purpose must have
band gap close to 1.5ev. Commonly used materials are-
3.CdTe (Cadmium Telluride)
4.CuInSe2 (Copper Indium Selienide)
Advantages of Solar Cell
1.No pollution associated with it.
2.It must last for a long time.
3.No maintenance cost.
Disadvantages of Solar Cell
1.It has high cost of installation.
2.It has low efficiency.
3.During cloudy day, the energy cannot be produced and
also at night we will not get solar energy.
Uses of Solar Generation Systems
1.It may be used to charge batteries.
2.Used in light meters.
3.It is used to power calculators and wrist
4.It can be used in spacecraft to provide electrical
30. CCD – Charge Coupled Device
a semiconductor device that is used especially as an optical
sensor and that stores charge and transfers it sequentially to
an amplifier and detector
CCD- Argus 2.1 Mega
CCD- Sony 10.1
Mega pixel camers
31. A charged coupled device (CCD) allows the transportation of electrically charged analog
signals through different capacitors set up in a series. This device is controlled by a clock
signal that oscillates between high and low states. The entire system acts as a shift register that
has its inputs and outputs linked together in series, which allows a charged coupled device to
be used as a way of delaying analog signals.
The way that a charged coupled device works in capturing images is by focusing an image that
is projected from a lens onto a photoactive capacitor array. This results in each capacitor
accumulating an electric charge that is proportional to the light intensity of the image. This
captures a two-dimensional image that is transferred to a charge amplifier, which in turn
coverts it into voltage. This image is then stored digitally in a memory module and can be
A basic charged coupled device is efficient in capturing luminance, but it has difficulty
rendering color. To solve this problem, modern digital cameras use a device called a Bayer
mask over the CCD. It links four pixels into blocks and filters different luminance levels as
different colors. These pixels are colored, with one being red, one blue and two green, because
the human eye can identify green more readily than other colors.
A DIAC is a diode that conducts electrical current only after its breakover
voltage (VBO) has been reached. DIAC stands for “Diode for Alternating
Current ”. A DIAC is a device which has two electrodes, and it is a member of
the thyristor family. DIACs are used in the triggering of thyristors.
39. Construction Details
•Note that neither terminal is referred to
as the cathode.
•Instead, there is an anode 1 (or
electrode 1 or MT1) and an anode 2 (or
electrode 2 or MT2).
•When anode 1 is positive with respect to
anode 2, the semiconductor layers
of particular interest are p1n2p2 and n3.
•For anode 2 positive with respect to
anode 1, the applicable layers are
p2n2p1 and n1.
40. DIAC V-I Characteristics
•As the voltage is increased from zero in
either direction, a small amount of leakage
current occurs, as shown in characteristic
•When VBO is reached in either direction, the
diac fires (ie, starts to conduct).
•There is then a negative-differential-
resistance region, similar to that of UJT.
•After the breakover voltage is reached, the
diac conducts current easily and has very
little internal resistance.
•The diac looks like a semiconductor diode.
It is not coded for polarity because it acts the
same in both directions.
The diacs, because of their symmetrical bidirectional switching
characteristics, are widely used as triggering devices in
triac phase control circuits employed for lamp dimmer, heat control,
universal motor speed control
A silicon controlled
rectifier or semiconductor
controlled rectifier is a four-
layer solid-state current-controlling
SCRs are unidirectional devices (i.e.
can conduct current only in one
direction) as opposed to TRIACs,
which are bidirectional
SCR Symbol &
43. Working Principle
There are three modes of
operation for an SCR depending
upon the biasing given to it:
● Forward blocking mode (off
● Forward conduction mode
● Reverse blocking mode (off
44. Forward blocking mode
In this mode of operation, the anode (+) is given a positive voltage while the cathode (−) is
given a negative voltage, keeping the gate at zero (0) potential i.e. disconnected. In this case
junction J1 and J3 are forward-biased, while J2 is reverse-biased, allowing only a small
leakage current from the anode to the cathode. Below breakover voltage J2 offers very high
resistance to the current and the SCR is said to be in the off state.
Forward conduction mode
An SCR can be brought from blocking mode to conduction mode in two ways: Either by
increasing the voltage between anode and cathode beyond the breakover voltage, or by
applying a positive pulse at the gate. Once the SCR starts conducting, no more gate voltage
is required to maintain it in the ON state.
There are two ways to turn it off:
1. Reduce the current through it below a minimum value called the holding current, or
2. With the gate turned off, short-circuit the anode and cathode momentarily with a push-
button switch or transistor across the junction.
45. Reverse blocking mode
When a negative voltage is applied to the anode and a positive voltage to the cathode, the
SCR is in reverse blocking mode, making J1 and J3 reverse biased and J2 forward biased.
The device behaves as two reverse-biased diodes connected in series. A small leakage
current flows. This is the reverse blocking mode. If the reverse voltage is increased, then at
critical breakdown level, called the reverse breakdown voltage (VBR), an avalanche occurs at
J1 and J3 and the reverse current increases rapidly.
47. ● SCR Applications
high-voltage AC power control
applications, such as
3. motor control.
4.rectification of high-power AC
in high-voltage dc power transmission
5.control of welding machines
48. ● TRIAC
TRIAC (triode for alternating current) is a
three terminal electronic component that
conducts current in either direction when
triggered. Its formal name is bidirectional
triode thyristor or bilateral triode thyristor.
A thyristor is analogous to a relay in that a
small voltage induced current can control
a much larger voltage and current. The
illustration on the right shows the circuit
symbol for a TRIAC where "A1" is Anode 1,
"A2" is Anode 2, and "G" is Gate. Anode 1
and Anode 2 are normally termed Main
Terminal 1 (MT1) and Main Terminal 2
52. Figure : Triggering modes. Quadrants, 1 (top right),
2 (top left), 3 (bottom left), 4 (bottom right)
With the gate open, MT2 is made positive
with respect to MT1 for a forward biased
traic. Hence traic operates in forward
blocking mode until the voltage across the
triac is less than the forward breakover
voltage. Similarly for a reverse biased triac,
MT2 is made negative with respect to MT1
with gate open.
Until the voltage across the triac is less
than the reverse breakover voltage, device
operates in a reverse blocking mode. A
traic can be made conductive by either
positive or negative voltage at the gate
It can operate and switch both
half cycles of an AC waveform.
In DC applications, SCRs are
required to be connected with a
parallel diode to protect against
reverse voltage. But the triac
may work without a diode, a
safe breakdown is possible in
very small switching
Triacs are less reliable than
triacs are used as AC power
triggering devices for SCRs,
three position static switch,
Triac as a switch and phase control
54. ● Power BJTPower BJT is a three terminal device
with very large current and power
handling capacity and offer high
voltage resistance in off state .The
construction of a power BJT is slightly
different than that of a normal logic
transistor.It has an extra lightly doped
(n-) region called as collector drift
region in addition to (base contact,
emitter contact and collector contact
with N,P and N region depending upon
the configuration of BJT).
What are Power Transistors?
“POWER TRANSISTORS ARE
SOLID STATE COMPONENTS
ACTING AS HIGH SPEED
SWITCHES THAT OPERATE
ONLY IN CUT-OFF AND
56. Power BJT Structure:
● The construction of the
Power Transistor is different
from the signal transistor as
shown in the figure.
The n- layer is added in the
power BJT which is known
as drift region.
57. A Power BJT has a four layer structure of alternating P and N type doping as shown
in above NPN transistor.
It has three terminals labeled as Collector, Base, Emitter.
In most of Power Electronic applications, the Power Transistor works in Common
Emitter configuration. ie, Base is the input terminal, the Collector is the output
terminal and the Emitter is common between input and output
In power switches NPN transistors are most widely used than PNP transistors.
The characteristics of the device is determined by the doping level in each of the
layers and the thickness of the layers.
The thickness of the dirft region determines the breakdown voltage of the Power
58. ● Power BJT VI Characteristics
The VI characteristics of the Power BJT is
different from signal level transistor.
The major differences are Quasi saturation
region & secondary breakdown region.
The Quasi saturation region is available
only in Power transistor characteristic not
in signal transistors. It is because of the
lightly doped collector drift region present
in Power BJT.
The primary breakdown is similar to the
signal transistor’s avalanche breakdown.
Operation of device at primary and
secondary breakdown regions should be
avoided as it will lead to the catastrophic
failure of the device.
59. ● APPLICATIONS OF
1.SMPS(Switch mode power supply)
commonly used in computers.
2.Final audio amplifier in stereo
4.DC to AC inverters.
5.Relay and display drivers.
6.AC motor speed controllers.
7.Power control circuits.
60. ● Power MOSFETA power MOSFET is a specific type of
effect transistor (MOSFET) designed
to handle significant power levels.
Compared to the other power
semiconductor devices, such as an
insulated-gate bipolar transistor (IGBT)
or a thyristor, its main advantages are
high switching speed and good
efficiency at low voltages.
62. The figure depicts DMOS structure.
Following are the properties of DMOS device:
The DMOS device uses a double diffusion process.
The p-region and the n+ source regions are diffused through common window. This
is defined by edge of the gate.
The p-region is being diffused deeper compare to n+ source.
The surface channel length is defined as the lateral diffusion distance between the
p-substrate and the n+ source.
The breakdown voltage and on-resistance are two important parameters of DMOS
Due to high voltage and high frequency characteristics it is similar to BJT.
The very high breakdown voltage is achieved due to lightly doped drift region
between Drain and channel regions.
The n-drift region thickness should be as thin as possible in order to achieve lower
64. Following are the properties of VMOS device:
It consists of shaped groove.
Due to source at top and drain at bottom, the current flows vertically rather
V shaped gate makes cross-sectional area of source to drain path larger.
Hence lower ON resistance of the device can be achieved which allows
much higher power.
The gate consists of metallised area over the V groove which controls
current flow in P-region.
VMOS structure is more complex compare to traditional FET device. This
makes it more expensive.
65. ● Advantages
Due to its low gate drive power,
fast switching speed,
easy advanced paralleling
ease of application, and
ease of repair.
66. ● Applications
wide range of applications, such
low-voltage motor controllers,
automobile control electronics,
67. Thank u & Best of luck
Department of Electronics and Communication
Institute of Road and Transport Technology, Erode, INDIA