What is a Light Emitting Diode?

The lighting emitting diode is a p-n junction diode. It is a specially doped diode and
made up of a special type of semiconductors. When the light emits in the forward
biased, then it is called a light-emitting diode.

he LED symbol is like a diode symbol except for two small arrows that specify the
emission of light, thus it is called LED (light-emitting diode). The LED includes two
terminals namely anode (+) and the cathode (-). The LED symbol is shown below.

LED Symbol

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.
How does the Light Emitting Diode Work?

The light-emitting diode simply, we know as a diode. When the diode is forward
biased, then the electrons & holes are moving fast across the junction and they are
combined constantly, removing one another out. Soon after the electrons are moving
from the n-type to the p-type silicon, it combines with the holes, then it disappears.
Hence it makes the complete atom & more stable and it gives the little burst of
energy in the form of a tiny packet or photon of light.

Working of Light Emitting Diode

The above diagram shows how the light-emitting diode works and the step by step
process of the diagram.

 From the above diagram, we can observe that the N-type silicon is in red
color including the electrons which are indicated by the black circles.
 The P-type silicon is in the blue color and it contains holes, they are
indicated by the white circles.
 The power supply across the p-n junction makes the diode forward biased
and pushing the electrons from n-type to p-type. Pushing the holes in the
opposite direction.
 Electron and holes at the junction are combined.
 The photons are given off as the electrons and holes are recombined.

 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.
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

Working Principle of LED

 The working principle of the Light-emitting diode is based on the quantum theory.
The quantum theory says that when the electron comes down from the higher
energy level to the lower energy level then, the energy emits from the photon. The
photon energy is equal to the energy gap between these two energy levels. If the
PN-junction diode is in the forward biased, then the current flows through the
diode.

Working Principle of
LED
 The flow of current in the semiconductors is caused by the flow of holes in the
opposite direction of current and the flow of electrons in the direction of the
current. Hence there will be recombination due to the flow of these charge
carriers.

 The recombination indicates that the electrons in the conduction band jump
down to the valence band. When the electrons jump from one band to another
 band the electrons will emit the electromagnetic energy in the form of photons
and the photon energy is equal to the forbidden energy gap.

 The infrared radiation is said to be as heat because the silicon and the
germanium semiconductors are not direct gap semiconductors rather these are
indirect gap semiconductors. But in the direct gap semiconductors, the maximum
energy level of the valence band and minimum energy level of the conduction
band does not occur at the same moment of electrons.

 Therefore, during the recombination of electrons and holes are migration of

electrons from the conduction band to the valence band the momentum of the
electron band will be changed.

I-V Characteristics of LED

There are different types of light-emitting diodes are available in the market and
there are different LED characteristics which include the color light, or wavelength
radiation, light intensity. The important characteristic of the LED is color. In the
starting use of LED, there is the only red color. As the use of LED is increased with
the help of the semiconductor process and doing the research on the new metals for
LED, the different colors were formed.

I-V Characteristics of LED

The following graph shows the approximate curves between the forward voltage and
the current. Each curve in the graph indicates a different color. The table shows a
summary of the LED characteristics.
Advantages and Disadvantages of LED’s
The advantages of light-emitting diode include the following.
 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
The disadvantages of light-emitting diode include the following.
 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

What is the Difference between a Diode and a LED?

The main difference between a diode and a LED includes the following.

Diode
LED
The semiconductor device like a diode The LED is one type of diode, used to
conducts simply in one direction. generate light.
The LED is designed with the gallium
phosphide & gallium arsenide whose
The designing of the diode can be done withelectrons can generate light while
a semiconductor material & the flow of transmitting the energy.
electrons in this material can give their
 
energy the heat form.
The diode changes the AC into the DC The LED changes the voltage into light
It has a high reverse breakdown voltage It has a low-reverse breakdown voltage.
The on-state voltage of the diode is 0.7v for The on-state voltage of LED
silicon whereas, for germanium, it is 0.3v approximately ranges from 1.2 to 2.0 V.
The diode is used in voltage rectifiers,
clipping & clamping circuits, voltage
multipliers.
 
The applications of LED are traffic
  signals, automotive headlamps, in
medical devices, camera flashes, etc.
Liquid Crystal Display:
 Liquid Crystal Display (LCD) is a flat display screen used in electronic devices
such as laptop, computer, TV, cell phones and portable video games. As the
name says liquid crystal is a material which flows like a liquid and shows some
properties of solid. These LCD are very thin displays, and it consumes less power
than LEDs.

 Molecular arrangement of Liquid Crystal:

 As the name says the molecular structure of liquid crystal is in between solid
crystal and liquid isotropic. In Liquid crystal display (LCD) nematic type of liquid
crystal molecular arrangement is used in which molecules are oriented in some
degree of alignment. For example, when we increase the temperature the ice
cube melts, and liquid crystal is like the state in between ice cube and water.
Construction:

 Construction of LCD consists of two polarized glass pieces. Two electrodes are
used, one is positive and the other one is negative. External potential is applied to
LCD through this electrodes and it is made up of indium-tin-oxide. Liquid crystal
layer of about 10µm- 20µm is placed between two glass sheets. The light is
passed or blocked by changing the polarization.

Working of Liquid Crystal Display

The basic working principle of LCD is blocking of light. It does not produce light on
its own. So external light source is used. When the external light passes from one
polarizer to the next polarizer, external supply is given to the liquid crystal ,the
polarized light aligns itself so that the image is produced in the screen.

 
 Working of LCD

The indium oxide conducting surface is a transparent layer which is placed on both
the sides of the sealed thick layer of liquid crystal . When no external bias is applied
the molecular arrangement is not disturbed. 

Working of LCD
When the external bias is applied the molecular arrangement is disturbed and it and
that area looks dark and the other area looks clear.

LCD Display 

In the segment arrangement, the conducting segment looks dark and the other
segment looks clear. To display number 2 , the segments A,B,G,E,D are energized.

 Positive and Negative LCDs:

In positive LCD display the segments are dark and the background is white and the
polarizers are placed perpendicular to each other. In the negative LCD display the
segments are white in the dark background and the polarizers are aligned to each
other.
Advantages:

 It is thin and compact

 Low power consumption
 Less heat is emitted during operation
 Low cost
Disadvantages:

 Speed of operation is low

 Lifespan is less
 Restricted viewing angles

Applications:

 Used in digital wrist watch

 Display images in digital cameras
 Used in numerical counters
 Display screen in calculators
 Mainly used in television
 Used in mobile screens
 Used in video players
 Used in image sensing circuits
Photoconductor – Working & Its Application
 0

 The material which can conduct the electricity within them are called
conductors, and the material whose conductivity increases as soon as the
light falls on it, that material is called a photoconductor.

 The increased electrical conductivity due to absorption of any type of light like
infrared light, visible light, gamma rays, or ultraviolet light by the conductor
material is called photoconductivity.

 The photoconductivity is mainly observed in semiconductors. And the symbol

of the photoconductive cell is shown below.

Photoconductive cell symbol

 When a load resistor and bias voltage are used in series with a semiconductor,
it is possible to monitor the voltage drop across the load resistor as the
current in the circuit changes due to variations in the material’s electrical
conductivity.

 Below is a diagram of the photoconductor’s creation. The base of the

photoconductor is shaped like a disc, and a long strip of light-sensitive
material is laid across it in a zigzag pattern. On each side of the strip, the
connecting terminals are fastened to the conducting substance. To safeguard
the light-sensitive material, a clear plastic cover is placed over a large strip of
it that is located between the two conductors.

Construction of photoconductive cell

 To create photoconductive cells, two materials called CdSe (cadmium
 selenide) and CdS (cadmium sulfide) are used. These two materials react to
variations in light intensity very slowly. Therefore, the response time for CdSe
is about 10 ms, whereas the response time for CDS may be 100 ms.

 A fundamental internal semiconductor structure must be taken into account

to fully comprehend this mechanism. When compared to conductors, the
number of charge carriers like electrons in the conduction band is
substantially lower in the energy-band diagram of semiconductors. But the
valence band also contains charge carriers, such as holes.

 There are two types of semiconductors intrinsic and extrinsic. The pure
semiconductor is called an intrinsic semiconductor and when impurities are
added to it to increase its conductivity then it is called an extrinsic
semiconductor.

 Multiple mechanisms by which photons cause electrons to be expelled from

the valence band and injected into the conduction band result in the
photoconductive effect. Intrinsic photoconductivity is the phenomenon in
which the quantity of conduction electrons and holes rises simultaneously. P-
type extrinsic photoconductivity is the phenomenon whereby more holes are
created when electrons from a filled band are injected into empty impurity
levels. The phenomenon is referred to as n-type extrinsic photoconductivity if
electrons are expelled from impurity levels and injected into the conduction
band. It is also feasible to excite intrinsic and extrinsic photoconductivity
simultaneously.

Applications of Photoconductor
  Photoconductive materials are used in the manufacture of photoelectric
devices.

 A simple application of photoconductive cells is for relay control. When the

light on the photoconductive cell exceeds a particular value, the current
through the circuit increases and the relay operates. When light is less, the
current is not sufficient enough to energize the relay. The circuit for it is
shown below.

 Making streetlights turn on and off automatically according to the level of daylight.

Photoconductive Cell Construction and Working:

Photoconductive Cell Construction and Working – Light striking the surface of a
material can provide sufficient energy to cause electrons within the material to break
away from their atoms. Thus, free electrons and holes (charge carriers) are created
within the material, and consequently its resistance is reduced. This is known as the
Photoconductive effect.
The Photoconductive Cell Construction and Working is illustrated in Fig, and the
graphic symbol is shown in Fig. Light-sensitive material is arranged in the form of a
long strip zigzagged across a disc-shaped base. The connecting terminals are fitted
to the conducting material on each side of the strip; they are not at the ends of the
strip. Thus, the light sensitive material is actually a short, wide strip between the two
conductors. For added protection, a transparent plastic cover is usually included.

Cadmium sulphide (CdS) and cadmium selenide (CdSe) are the two materials
normally used in photoconductive cell manufacture. Both respond rather slowly to
changes in light intensity. For cadmium selenide, the response time (tres) is around
10 ms, while for cadmium sulphide it may be as long as 100 ms. Temperature
sensitivity is another important difference between the two materials There is a large
change in the resistance of a cadmium selenide cell with changes in ambient
temperature, but the resistance of cadmium sulphide remains relatively stable. As
with all other devices, care must be taken to ensure that the power dissipation is not
excessive. The spectral response of a cadmium sulphide cell is similar to that of the
human eye; it responds to visible light. For a cadmium selenide cell, the spectral
response is at the longer wavelength end of the visible spectrum and extends into
the infrared region.

Characteristics and Parameters:

Typical illumination characteristic for a photoconductive cell are shown in Fig. 20-11.
It is seen that, when the cell is not illuminated its resistance can be greater than 100
kΩ. This is known as the dark resistance of the cell. When the cell is illuminated, its
resistance might fall to a few hundred ohms. Note that the scales on the illumination
characteristic are logarithmic.

A typical photoconductive cell specification is shown in Fig. As well as maximum

voltage and power dissipation, the cell dark resistance and the resistance at a 10-lx
illumination is listed. Note the wide range of cell resistance at a 10 lx. The light
wavelength that gives peak response (λP) is also given on the specification. Cell
sensitivity is sometimes used, and this is simply the cell current for a given voltage
and given level of illumination.
Applications:

Figure shows a photoconductive cell used for relay control. When the cell is
illuminated, its resistance is low and the relay current is at its maximum. Thus, the
relay is energized. When the cell is dark, its high resistance keeps the current down
to a level too low to energize the relay. Resistance R1 is included to limit the relay
current to the desired level when the cell resistance is low.

What is Photovoltaic or Solar Cell? – Construction,

Working and Advantages

An electrical device which converts light energy into electrical energy through the
photovoltaic effect is known as photovoltaic cell or PV cell or solar cell. A
photovoltaic cell is basically a specially designed p-n junction diode.

Construction and Working of Photovoltaic Cell

The construction of a photovoltaic cell is shown in the following figure.

A photovoltaic cell consists of a base metal plate and it is made of either steel or
aluminum over which a metallic selenium layer is situated which is light sensitive
and acts as the positive terminal.
An electrically conducting layer of cadmium oxide is applied by sputtering over the
selenium layer. This cadmium oxide layer is sufficiently thin in order to allow light to
reach the selenium and as it is electrically conducting, hence acts as the negative
terminal. A strip of metal sprayed on the edge of the top surface which forms the
negative contact.
The transparent varnish layer is used to protect the front surface of the photovoltaic
cell.
When light falls on the surface of selenium layer through the layer of cadmium oxide,
the selenium compound releases the electrons that are sufficient to maintain the
flow of current through the external circuit connected between the positive and
negative terminals.

Advantages of Using Photovoltaic Cells

The advantages of using photovoltaic cells are listed below −

 Photovoltaic cells do not cause pollution while producing electricity.
 The operating cost of photovoltaic cells is low as source of energy is
natural light.
 The maintenance cost of PV cells is also minimum as they need little
maintenance.
  Photovoltaic cells have long lifespan. They are highly reliable.
 PV cells are the best renewable energy sources.

Disadvantages of Photovoltaic Cell

Following are some of the disadvantages of using photovoltaic cells −

 The operation of photovoltaic cells depends upon the light energy of
the Sun, thus their operation depends upon the weather.
 Storage of electricity produced by the photovoltaic cells is expensive
and complicated.
 They require more space for installation.

Applications of Photovoltaic Cell

The applications of photovoltaic cells include the following −

 Remote lighting systems
 Emergency power
 For Satellites power supplies
 In photometric measurements
 As portable power supplies such as solar car, etc.

Optocouplers
Definition: An optocoupler or optoelectronic coupler is an electronic
component that basically acts as an interface between the two separate
circuits with different voltage levels. Optocouplers are common component
by which electrical isolation can be supplied between the input and output
source. It is a 6 pin device and can have any number of photodetectors.

Here, a beam of light emitted by a light source exists as an only contact

between input and output. Due to this, we can have an insulation resistance of
megaohms between the two circuits. In high voltage applications where the
voltage difference between the two circuits differs by several thousand volts,
such isolation is favourable. The use of all such electronic isolators lies in all
that conditions where the signal is to pass between two isolated circuits.

Till now we have talked about an isolated circuit but one should know the
meaning of it before going into any further aspect.

What is an isolated circuit?

Isolated circuits are the circuits which do not have a common conductor in

between them and proper isolation is maintained.

As we are already aware of the fact that the information signal highly contains
noise and additional distortions in it which can be beyond the tolerance limit
of the logic circuit at the output end during transmission. Optical couplers can
be used to work on both ac and dc high voltages.

Construction of an Optocoupler
An optocoupler mainly consists of an infrared LED and a photosensitive
device that detects the emitted infrared beam. The semiconductor
photosensitive device can be a photodiode, phototransistor, a Darlington pair,
SCR or TRIAC.

Let’s have a look at the basic diagram of an Optocoupler:

The infrared LED and the device that are light sensitive is packed in a single
package. The LED is kept on the input side and the light-sensitive material is
placed on the output side. A resistance is connected at the beginning of the
circuit which is used to limit the current and the other resistance is connected
between the supply voltage and the collector terminal.

Before proceeding further let’s see the pin description of an optocoupler:

  Pin 1: Anode

 Pin 2: Cathode

 Pin 3: Ground

 Pin 4: Emitter

 Pin 5: Collector

 Pin 6: Base
The base terminal of the phototransistor is externally available. A single
phototransistor is used at the output stage of a simple isolating optocoupler.

Working Of an Optocoupler
An Optocoupler is a combination of LED and a Photodiode packed in a single
package. As we can see in the below-shown circuit diagram, when a high
voltage appears across the input side of the Optocoupler, a current start to
flow through the LED.

Due to this current LED will emit light. This emitted light when falls on
a phototransistor cause a current to flow through the same.
The current flowing through the phototransistor is directly proportional to
the supplied input voltage. An input resistance placed at the beginning of the
circuit will decrease the amount of current flowing through the LED if its value
is increased. As the LED glows due to this current, hence, when current will be
low so as the light intensity of LED.

As we have already discussed earlier the intensity of emitted light by the LED
will be equal to the corresponding current flowing through the phototransistor.
This means that the low-intensity light emitted by the LED will cause a low-
level current to flow through the phototransistor. Thus a changing voltage is
generated across the collector-emitter terminal of the transistor.

In this way, an incoming signal from the input circuit is coupled to the output
circuit.

Types of Optocoupler

The various types of the optocoupler are shown in the diagram given below:
For the use in DC circuits, photo-transistor and photo-Darlington devices are
majorly used. In the case of AC, powered circuits photo-SCR and photo-TRIAC
are used.

There exist some other forms of source-sensor configurations like LED-LASER,

LED-photodiode, reflective optocoupler, slotted optocoupler etc.

Advantages

1. Optocouplers allow easy interfacing with logic circuits.

2. Electrical isolation provides circuit protection.

3. It allows wideband signal transmission.

4. It is small in size and lightweight device.

Disadvantages

1. The operational speed of Optocouplers is low.

 2. In case of a very high-power signal, the possibility of signal
coupling may arise.
Applications

1. It is used in high power inverters.

2. It is used in high power choppers.

3. In AC to DC converters optocouplers are widely used.