Explain the properties, characterstics and applications of :
1,Solar cells
2,light emitting diodes
3, schokky cells
4,zenor diodes
1,Solar cells
Solar cell is the basic unit of solar energy generation system where electrical energy is to be
extracted directly from light energy without any intermediate process. The working of a solar
cell solely depends upon its photovoltaic effect, hence a solar cell also known as photovoltaic
cell. A solar cell is basically a semiconductor p-n junction device. It is formed by joining p type (high
concentration of hole or deficiency of electron) and n-type (high concentration of
electron) semiconductor material. at the junction excess electrons from n-type try to diffuse to
p-side and vice-versa. Movement of electrons to the p-side exposes positive ion cores in n side, while
movement of holes to the n-side exposes negative ion cores in the p-side. This
results in an electric field at the junction and forming the depletion region. When sunlight
falls on the solar cell, photons with energy greater than band gap of the semiconductor are
absorbed by the cell and generate electron-hole (e-h) pair. These e-h pairs migrate
respectively to n- and p- side of the pn junction due to electrostatic force of the field across
the junction. In this way a potential difference is established between two sides of the cell.
Typically a solar or photovoltaic cell has negative front contact and positive back contact. A
semiconductor p-n junction is in the middle of these two contacts like a battery. If these two
sides are connected by an external circuit, current will start flowing from positive to negative
terminal of the solar cell. This is basic working principle of a solar cell. For silicon, the band
gap at room temperature is Eg = 1.1 eV and the diffusion potential is UD = 0.5 to 0.7 V
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Applications of solar cells
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At NREL, we see potential for photovoltaics (PV) everywhere. As we pursue advanced materials
and next-generation technologies, we are enabling PV across a range of applications and locations.
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Solar Farms
Many acres of PV panels can provide utility-scale power—from tens of megawatts to more than a
gigawatt of electricity. These large systems, using fixed or sun-tracking panels, feed power into
municipal or regional grids.
 Remote Locations
 It is not always cost-effective, convenient, or even possible to extend power lines to locations
where electricity is needed. PV can be the solution—for rural homes, villages in developing
nations, lighthouses, offshore oil platforms, desalination plants, and remote health clinics.
Stand-Alone Power
 In urban or remote areas, PV can power stand-alone devices, tools, and meters. PV can meet the
need for electricity for parking meters, temporary traffic signs, emergency phones, radio
transmitters, water irrigation pumps, stream-flow gauges, remote guard posts, lighting for
roadways, and more.
Power in Space
From the beginning, PV has been a primary power source for Earth-orbiting satellites. High-efficiency PV
has supplied power for ventures such as the International Space Station and surface rovers on the Moon
and Mars, and it will continue to be an integral part of space and planetary exploration.

Photovoltaic cell characterstics
Solar cells convert power of sunlight into electric power. As an introduction, therefore, Chapter 1 is
devoted to a brief characterization of sunlight and basic electric parameters of solar cells. The power of
sun is given in terms of the solar constant, the power spectrum and power losses in earth atmosphere
expressed by the so-called air mass. The basic characteristics of a solar cell are the short-circuit current
(ISC), the open-circuit voltage (VOC), the fill factor (FF) and the solar energy conversion efficiency (η).
The influence of both the diode saturation current density and of ISC on VOC, FF and η is analyzed for
ideal solar cells. The importance of concentrated sunlight for increasing η is shown. Tolerable series and
parallel resistances are introduced as an evaluation criterion for resistive losses in real solar cells. The
influence of the series resistance (Rs) and parallel resistance (Rp) on ISC, VOC, FF and η is investigated.
The specific role of Rs and Rp is discussed in detail for the dependence of η on ISC. Concepts are
described for measuring the basic characteristics of solar cells and their dependencies on light intensity,
temperature and light spectra. Attention is paid to principle work with various kinds of load resistances,
to the function of a pyranometer, of a sun simulator and to the measurement of the quantum efficiency
of solar cells.
2,Light emitting diode
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The Light-emitting diode is a two-lead semiconductor light source. In 1962, Nick Holonyak has come up
with the idea of a light-emitting diode, and he was working for the general electric company. The LED is
a special type of diode and they have similar electrical characteristics to a PN junction diode. Hence the
LED allows the flow of current in the forward direction and blocks the current in the reverse direction.
The LED occupies a small area which is less than 1 mm2. The applications of LEDs used to make various
electrical and electronic projects. In this article, we will discuss the working principle of the LED and its
applications.

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
The LED symbol is similar to 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 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 Ptype 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.
The Light-emitting diode is a two-lead semiconductor light source. In 1962, Nick Holonyak has come up
with the idea of a light-emitting diode, and he was working for the general electric company. The LED is
a special type of diode and they have similar electrical characteristics to a PN junction diode. Hence the
LED allows the flow of current in the forward direction and blocks the current in the reverse direction.
The LED occupies a small area which is less than 1 mm2

LED characterstics
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
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is increased with the help of the semiconductor process and doing the research on the new metals for
LED, the different colors were formed.

LED applications
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
3, schokky cells
Diode is one of the basic components that are commonly used in electronic circuit designs, it can be
commonly found in rectifiers, clippers, clampers and many other commonly used circuits. It is a twoterminal semiconductor device that allows the current flow in only one direction that is form Anode to
Cathode (+ to -) and blocks the current flow in reverse direction, i.e., Cathode to Anode. The reason
behind it that it has approx. Zero resistance in the forward direction while infinite resistance in reverse
direction. There are many types of Diodes each with its unique property and applications. We have
already learnt about Zener Diodes and its working, in this article we will learn about another interesting
type of diode called Schottky Diode and how it can be used in our circuit designs.
Schottky diode (Named after the German physicist Walter H. Schottky) is another type of semiconductor
diode, but instead of having a P-N junction, Schottky diode has a metal-semiconductor junction and
which reduces capacitance and increases switching speed of Schottky diode, and this makes it different
from other diodes. The Schottky diode also has other names like surface barrier diode, Schottky barrier
diode, hot carrier, or hot-electron diode.
Schottky Diode Symbol
Symbol of the Schottky diode is based on generic diode symbol, but instead of having a straight line it
has an S like structure at the negative end of the diode as shown below. This schematic symbol can
easily be used to distinguish Schottky diode from other diodes when reading a circuit diagram.
Throughout the article we will be comparing the Schottky diode with regular diode for better
understanding.
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Applications of Schottky Diode
Schottky diodes have many applications in the electronics industry because of their unique properties.
Some of the applications are as follows:
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1. Voltage Clamping/Clipping circuits
Clipper circuits and clamper circuits are commonly used in wave shaping applications. Having a low
voltage drop property makes the Schottky diode useful as a clamping diode.
2. Reverse current and discharge protection
As we know, Schottky diode is also called as blocking diode because it blocks the current flow in reverse
direction; it can be used as discharge protection. For example, in Emergency Flash Light, a Schottky
diode is used between a supercapacitor and DC motor to prevent supercapacitor from discharge
through DC motor.
3. Sample-and-hold circuits
Forward biased Schottky diode doesn’t have any minority charge carriers, and due to this, they can
switch more quickly than the typical PN-junction diodes. So Schottky diodes are used in because they
have lower transition time from the sample to the hold step and this results in a more accurate sample
at the output.
4. Power rectifier
Schottky diodes have high current density, and low forward voltage drop means that less power is
wasted than a typical PN junction diode and this makes Schottky diodes more suitable for power
rectifiers.

Characterstics of shottky cells
The V-I (Voltage-Current) characteristics of Schottky diode is shown in the below figure. Along the graph,
the vertical line signifies the current flow and the horizontal line denotes the voltage applied across the
Schottky diode. The V-I characteristics of Schottky diode is almost similar to the P-N junction diode.

Schottky Diode
Nevertheless, the forward voltage drop of Schottky diode is very low when compared to the P-N
junction diode. The forward voltage drop ranges from 0.3 volts to 0.5 volts. The barrier of forward
voltage drop is made of silicon. The forward voltage drop is proportional to the doping concentration of
N type semiconductor. Due to high concentration of current carriers, the V-I characteristic of Schottky
diode is steeper.
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.
4, zenor diode
What is a Zener Diode?
Zener diodes are silicone-based discrete semiconductor devices which allow current to flow
bidirectionally - either reverse or forward. Diodes are comprised of a heavily-doped P-N silicone junction,
which is intended to conduct in the reverse direction once a particular voltage threshold has been met.
Zener diodes have a set reverse breakdown voltage. When this is reached, they start to conduct current
and continue to operate unceasingly in the reverse bias direction without incurring damage. One of the
main benefits of Zener diodes is that a varying range of voltages will still maintain a constant voltage
drop across the diode. As a result, Zener diodes can be used for voltage regulation applications.

Zener Diode Characteristics
Zener diodes operate similarly to conventional diodes when in the forward-bias mode. They have a bias
turn-on voltage of between 0.3 and 0.7V. When connected in the reverse mode, there is a small leakage
current flow in most applications. As the reverse voltage increases to the set breakdown voltage, there
will be current flowing through the diode. When the current increases to a maximum (determined by
the resistors in series), it will then stabilise and remain constant over a wide range of applied voltage.
Irrespective of the current value flowing through the diode, the voltage remains almost constant. This is
also the case with large current changes, providing that the diode current stays between the maximum
current and the breakdown current.
A Zener diode’s strong self-control is highly useful when it comes to regulating and stabilising variations
in load or supply against a voltage source. This makes it a key characteristic as it enables the diode to be
used in a variety of voltage regulator applications.

Zener Diode Uses and Working Applications
Zener diodes are also used in modern applications such as smartphones and Android devices. Many such
uses involve Bluetooth technology. On average, a standard Bluetooth device needs around 3V for
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operation. In this application, a Zener diode would be used to provide the required 3V to the Bluetoothenabled devicce

Zener Diode Applications
Zener diodes are used for a range of applications, including:
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Voltage regulation
Voltage reference
Surge suppression
Switching applications
Clipper circuits
It is possible to use a Zener diode to create a stabilised low-ripple output voltage under variable load
current conditions. When a suitable current limiting resistor is used to pass a minor current from a
voltage source through the diode, sufficient current will be conducted to maintain the required voltage
drop. As the load value is altered, the average voltage output also changes. However, the addition of a
Zener diode can produce an even voltage output.
With that being said, it should also be noted that Zener diodes may occasionally produce It is possible to
use a Zener diode to create a stabilised low-ripple output voltage under variable load current conditions.
When a suitable current limiting resistor is used to pass a minor current from a voltage source through
the diode, sufficient current will be conducted to maintain the required voltage drop. As the load value
is altered, the average voltage output also changes. However, the addition of a Zener diode can produce
an even voltage output.
As Zener diodes can operate in the reverse bias condition, they can be used in voltage regulator circuits
to sustain constant DC voltage output. This constant voltage can be maintained despite any variations in
voltage input or load current changes.
This voltage regulator circuit comprises a current limiting resistor which is connected in series
As Zener diodes can operate in the reverse bias condition, they can be used in voltage regulator circuits
to sustain constant DC voltage output. This constant voltage can be maintained despite any variations in
voltage input or load current changes.
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Idea Lightbulb
How Does a Zener Diode Work?
The principle behind the operation of a Zener diode is determined by the cause of the diode’s
breakdown in the reverse bias condition. There are typically two types – Zener and avalanche.
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Zener Breakdown
A Zener breakdown happens with a reverse bias voltage between 2V and 8V. The intensity of the electric
field is enough to apply force to the valence electrons, separating them from the nuclei – even at this
low voltage. This process forms mobile electron-hole pairs, therefore increasing current flow.
Zener breakdowns typically occur for highly-doped diodes with a large electric field and low breakdown
voltage. More energy is gained by the valence electrons as temperature increases, therefore requiring
less outward voltage. This also means that Zener breakdown voltage reduces alongside temperature.

Avalanche Breakdown
Voltage breakdown also happens in the reverse bias condition, at a minimum of 8V, for light-doped
diodes that have a large breakdown voltage. Electrons flowing through the diode collide with electrons
in the covalent bond, disrupting it. The velocity of the electrons increases as the voltage also increases,
meaning that the covalent bonds can be disrupted more easily. It should also be noted that avalanche
breakdown voltage rises alongside temperature.
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