The document discusses special purpose diodes including LEDs, photo diodes, and laser diodes. It begins with an introduction to optoelectronics and what diodes are. LEDs emit light when electrons fall to a lower energy level. Photo diodes convert light into an electric current. Laser diodes produce coherent light when electrons recombine across the junction. The document then covers the construction, working principles, applications and diagrams of these three types of special purpose diodes.

1. Special Purpose Diodes:
1. LEDs
2. Photo Diodes
3. Laser Diodes
A PRESENTATION BY:
ABHIJEET GUPTA : 140110111001
DARSHAK PADSALA: 140110111008
HIREN PATEL : 140110111035
PRERAK TRIVEDI : 140110111045
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2. Contents
Optoelectronics
LEDs Photo Diodes Laser Diodes
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3. What Diodes Actually Are?
As the name suggests di-ode means a device having two junctions or
electrodes.
It is usually made up of semiconductors but some are made up of
metallic materials too.
Diodes can be used as rectifiers, signal limiters, voltage regulators,
switches, signal modulators, signal mixers, signal demodulators, and
oscillators.
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4. What Is Opto-Electronics?
Before moving on the special purpose diodes, let’s understand the
term OPTOELCETRONICS.
OPTOELECTRONICS is the technology which combines both optics
and electronics.
This field includes many devices based on the action of PN Junction
diode.
Few Optoelectronic devices are:
LEDs, Photodiode, Optocouplers and Laser Diode.
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5. Light Emitting Diodes work on the principle of the emission of
specific-frequency radiant energy whenever electrons fall from a
higher energy level to a lower energy level.
LEDs rule the world of lighting because of their low energy
consumption, smaller size, faster switching and long life.
Just like an ordinary diode LEDs also have anode and cathode which
are properly biased.
A General Info About LEDs
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6. What Are Different Parts Of LED?
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7. What Is The Basic Circuit Of An LED?
 The basic circuit diagram shows a source connected to a
resistor and an LED and the outward arrows symbolize the
radiated light.
 In a forward-biased LED, free electrons cross the PN
junction and fall into holes. As these electrons fall from a
higher to a lower energy level, they radiate energy in the
form of photons.
 In ordinary diodes, this energy is radiated in the form of
heat. But in an LED, the energy is radiated as light. This
effect is referred to as electroluminescence.
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8. How are the colors obtained?
The colour of the light, which corresponds to the wavelength energy
of the photons, is primarily determined by the energy band gap of
the semiconductor materials that are used.
By using elements like gallium, arsenic, and phosphorus, a
manufacturer can produce LEDs that radiate red, green, yellow, blue,
orange, white or infrared (invisible) light.
LEDs with various colours have various applications like red, blue
etc. are used for decoration and indication whereas infrared are used
for security purposes.
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9. ELECTRONIC DEVICES AND CIRCUITS, GCET

10. LED Voltage and Current
Here, the resistor shown is the usual current-limiting
resistor that prevents the current from exceeding the
maximum current rating of the diode.
Since the resistor has a node voltage of VS on the left
and a node voltage of VD on the right, the voltage
across the resistor is the difference between the two
voltages.
With Ohm’s law, the series current is:
𝐼𝑆 =
𝑉𝑆 − 𝑉𝐷
𝑅 𝑆
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11. What Photo Diodes Are?
 A photodiode is a semiconductor device that converts light into
current.
Current is generated when photons are absorbed in a photodiode.
It contains optical filters, built in lenses and special semiconductor
material that is capable of emitting electrons or are photo sensitive.
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12. What Is The Construction Of Photo Diode?
In the photodiode fabrication process a thick intrinsic layer is
inserted between the p-type and n-type layers. The middle layer may
be either completely intrinsic or very lightly doped to make it and n-
layer. In some instances it may be grown as an epitaxial layer onto
the substrate, or alternatively it may be contained within the
substrate itself.
One of the main requirements of the photodiode is to ensure that
the maximum amount of light reaches the intrinsic layer. One of the
most efficient ways of achieving this is to place the electrical
contacts at the side of the device as shown.
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13. What Is The Construction Diagram Of
Photo Diode?
 This enables the maximum amount of light to reach the active
area. It is found that as the substrate is heavily doped, there is
very little loss of light due to the fact that this is not the active
area.
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14. On What Principle Does Photo Diode
Work?
 A photodiode is a p–n junction or PIN structure. When a photon of
sufficient energy strikes the diode, it creates an electron-hole pair.
This mechanism is also known as the inner photoelectric effect.
If the absorption occurs in the junction's depletion region, or one
diffusion length away from it, these carriers are swept from the
junction by the built-in electric field of the depletion region.
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15. Thus holes move toward the anode, and electrons toward the
cathode, and a photocurrent is produced.
The total current through the photodiode is the sum of the dark
current (current that is generated in the absence of light) and the
photocurrent, so the dark current must be minimized to maximize
the sensitivity of the device.
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16. How Does The Photo Diode Work?
When photons of energy greater than 1.1 eV hit the diode, electron-
hole pairs are created. The intensity of photon absorption depends
on the energy of photons – the lower the energy of photons, the
deeper the absorption is. This process is known as the inner
photoelectric effect.
If the absorption occurs in the depletion region of the p-n junction,
these hole pairs are swept from the junction - due to the built-in
electric field of the depletion region. As a result, the holes move
toward the anode and the electrons move toward the cathode,
thereby producing photocurrent.
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17. Graph
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18. Applications Of Photo Diodes
Cameras
Medical devices
Safety equipment
Optical communication devices
Position sensors
Bar code scanners
Automotive devices
Surveying instruments
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19. How Laser Diodes Look Like?
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20. What is Laser Diode?
A laser diode, also known as an injection laser or diode laser, is a
semiconductor device that produces coherent radiation (in which
the waves are all at the same frequency and phase) in the visible or
infrared (IR) spectrum when current passes through it.
The laser diode is the most common type of laser produced with a
wide range of uses that include, but are not limited to, fibre optical
communication , barcode readers , laser pointers , laser printing,
laser scanning and increasingly directional lighting sources .
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21. How does it work?
When current is passed through semiconductors both negatively
charged electron from N type semiconductor and positively charged
hole from P-type semiconductor begin to flow towards PN junction.
Holes exist at lower energy level than free electrons. So, electron
must lose small amount of energy to combine with hole.
It combines with hole by releasing single photon of light.
Top and bottom surfaces of PN junction is coated with mirrored
material to trap photon of light.
Photon bounces around inside the PN junction on mirrored top and
bottom surfaces.
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22. Photon encourages other holes and electron to combine and they
also release their own photon of light.
Photon that are released by other electron and holes are of same
polarity, same phase and of same direction of the original photon.
The process continue and more photon is produced. Photons will
keep increasing in numbers until whole PN junction is filled with
laser light.
Some of the laser light exists the rear of laser diode and hit photo
diode
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23. The photo diode uses this information to regulate voltage to the
laser diode.
Vast majority of the light exist through the front of the laser diode
This diffracted light must be made into typical straight laser beam
This is done by passing the light through collimating lens
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24. ELECTRONIC DEVICES AND CIRCUITS, GCET

25. Applications of Laser Diode
In telecommunication as easily modulated and easily coupled light
sources for fibre optics communication.
In various measuring instruments, such as rangefinders. Another
common use is in barcode readers.
Visible lasers, typically red but later also green, are common as laser
pointers.
Both low and high-power diodes are used extensively in the printing
industry both as light sources for scanning (input) of images and for
very high-speed and high-resolution printing plate (output)
manufacturing.
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26. Infrared and red laser diodes are common in CD players, CD-ROMs
and DVD technology.
Violet lasers are used in HD DVD and Blu-ray technology.
Diode lasers have also found many applications in laser absorption
spectrometry (LAS) for high-speed, low-cost assessment or
monitoring of the concentration of various species in gas phase.
 High-power laser diodes are used in industrial applications such as
heat treating, cladding, seam welding and for pumping other lasers,
such as diode-pumped solid-state lasers.
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27. Thank You 
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Doubts?