Optoelectronics Basics: Advantages of Optoelectronic Devices
Optoelectronics Basics: Advantages of Optoelectronic Devices
Optoelectronics Basics: Advantages of Optoelectronic Devices
Blackbody Radiation:
Reflection: When a ray of light approaches a smooth polished surface and the light
ray bounces back, it is called the reflection of light.
Refraction: A change of direction that light undergoes when it enters a medium with
a different density from the one through which it has been traveling — for example,
when, after moving through air, it passes through a prism.
Total internal reflection: Total internal reflection refers to the complete reflection
of a ray of light within a given medium from the surrounding surface. Here, the ray
of light continues to be reflected within the medium (glass, water etc.) without being
refracted off.
Interference: When two light waves from different coherent sources meet together,
then the distribution of energy due to one wave is disturbed by the other. This
modification in the distribution of light energy due to super- position of two light
waves is called "Interference of light".
LED
1. What is LED?
A light emitting diode (LED) is essentially a PN junction opto-
semiconductor that emits a monochromatic (single color) light when
operated in a forward biased direction. LEDs convert electrical energy
into light energy.
The injected excess minority carriers are then try to diffuse away quickly
from the junction and subsequently recombine with majority carriers
either by radiative and nonradiative ways. The diode should be designed
in such a way that it can support the radiative recombination as strong as
possible. Under such case, the emitted spontaneous radiation photon
energy be given by,
hν = Eg
Advantages:
i. Energy efficient (produce more light per watt)
ii. Long lifetime (60,000 Hours or more)
iii. Rugged(made-up of solid material, no breakage like filament)
iv. No warm-up period(achieve full bright light in nanoseconds)
v. Not effected by cold temperature(used in sub zero weather)
vi. Directional(direct the light where you want)
vii. Environment Friendly(contains no mercury)
viii. Controllable(brightness and color can be controlled)
ix. Can sustain over frequent on-off cycle
Disadvantages:
i. Very expensive than other lighting technologies
ii. Requires accurate voltage & constant current flow
iii. Can shift color due to age & temperature
iv. Cannot be used in high temperature (Lead to device failure)
Applications:
i. Vehicle indicator lights and brake lights.
ii. Currently Audi & BMW integrate high power LEDs.
iii. Mobile phone flash lights. (Surface Mount Diode)
iv. LED screens for advertising & information.
v. Due to low power consumption, small size & long life
vi. LEDs are used in many electrical equipment. (indicator)
vii. Now a days airports, hotels, subways, shopping centers and some
homes feature LEDs.
viii. LED based traffic signal has been successful & is alsogrowing
rapidly.
4. History of LED?
Since the discovery of the light emitting diode (LED) in the early 1900s,
it is one the oldest and simplest optoelectronic devices which have found
tremendous scientific and industrial applications in display systems,
optical communication networks, sensors, logic devices, tail light in
automobiles and many more. LEDs along with lasers are basically
electroluminescence type of devices, where light emission supported by
generation of excess carriers by electric field or current injection into the
devices. From the mid-1950s, the entire effort on designing efficient
LEDs rests on alloy materials of III-V (like GaAs, GaP, GaN, AlGaAs,
GaAsP, GaInP, GaInP, AlGaInP, etc.) and II-VI (like ZnS, ZnTe etc.)
semiconductors. These materials, being direct bandgap by nature, support
primarily radiative light emission, and hence ensure higher efficiency.
Moreover, the light emission from these binary, ternary or quaternary
semiconducting materials covers a wide range of light starting from
infrared (IR) to visible (white light or single color light such as green blue,
red, yellow, etc.) as well as ultra violet (UV) region. Nowadays, with the
advent of modern semiconductor growth techniques like molecular beam
epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), etc.
along with bandgap engineering using compound semiconductors, it is
possible to fabricate various solid-state light sources including LEDs and
lasers which are active in visible, UV and IR region of the spectrum.
The top or the active layer (p-region) is made thinner than the heavily
doped n + layer for higher efficiency of LED. By using p-n structure
(rather than simple p-n junction), the depletion region can be pushed into
p-region of the junction (i.e., nearer to the top layer) where major carrier
recombination takes place. Thus, the emitted photon has the minimum
chance to be reabsorbed by the device material. In addition, by using very
high quality (e.g., defect-free except doping atoms) material, the trap
assisted recombination current is made nearly negligible.
8. Injection, Quantum and extraction efficiency?
Injection efficiency: It is the ratio of the number of electrons injected into
the LED to the number of electrons supplied by the power sources. It is
denoted by ηinj.
Quantum efficiency: It is the proportion of all electron-hole
recombinations in the active region that are radiative, producing photons.
It is denoted by ηint.
Extraction efficiency: Once the photons are produced within the
semiconductor device, they have to escape from the crystal in order to
produce a light-emitting effect. Extraction efficiency is the proportion of
photons generated in the active region that escape from the device. It is
denoted by ηext.
❖ LASER types
No. Laser Type Example
1. Solid State laser Ruby Laser, Nd:YAG Laser
2. Gas laser He-Ne Laser, CO2 Laser, Argon – ion laser
3. Liquid Laser SeOCl2 Laser, Europium Chelate Laser
4. Dye laser Rhodamine 6G laser, Coumarin dye laser
5. Semiconductor Laser GaAs laser, GaAsP laser
Direct Conversion
In this method, due to electrical energy applied
in direct band gap semiconductor like Ga As,
recombination of electrons and holes takes
place. During the recombination process, the
electrical energy is directly is converted into
light energy.
In this method, a combination of two gases are used. The excited states of
A and B nearly matches in energy. In the first step atoms of gas A are
excited to their higher energy state. Now the excited A atoms at higher
energy state collide with B atoms in the lower state. So, B atoms gain
energy and they are excited to a higher state. Hence, A atoms lose energy
and return to lower state.
❖ Flow chart of LASER action.
It is specifically fabricated p-n junction diode. This diode emits laser light
when it is forward biased.
Working Principle:
The active medium of a semiconductor diode laser is made of a p-n
junction diode made from the single crystal of GaAs. This crystal is cut in
the form of a platter having thickness of 0.5μm. The plate consists of two
parts having an n-type and p-type conductivity. The electrical voltage is
applied to the crystal through the electrode fixed on the upper surface.
Finally, the light beam comes out from a polished surface that acts like an
optical resonator.
When the PN junction is forward biased with large applied voltage, the
electrons and holes are injected into junction region. The region around
the junction contains a large amount of electrons in the conduction band
and a large amount of holes in the valence band. When the population
density reaches higher level, a condition of population inversion is
achieved. The electrons and holes recombine with each other and this
recombination’s produce radiation in the form of light.
When the forward – biased voltage is increased, more and more light
photons are emitted and the light production instantly becomes stronger.
These photons will trigger a chain of stimulated recombination resulting
in the release of photons in phase. The photons moving at the plane of the
junction travels back and forth by reflection between two sides placed
parallel and opposite to each other and grow in strength. After gaining
enough strength, it gives out the laser beam of wavelength 8400oA.
Advantages:
Disadvantages:
Application:
Construction:
layer of p–type GaAs (3rd layer) will act as the active region. This layer
is sandwiched between two layers having wider band gap of p-type
GaAlAs (2nd layer) and n-type GaAlAs (4th layer).
The end faces of the junctions of 3rd and 4th layer is well polished and
parallel to each other. They act as an optical resonator.
Working Principle:
Disadvantages:
1. It is very difficult to grow different layers of PN junction.
2. The cost is very high.
Application:
1. This type of laser is mostly used in optical applications
2. It is widely used in computers, especially on CD-ROMs.
2. Requirements of photodetectors.
3. Mechanism of photodetector.
Optical characteristics
6. Noise in photodiodes.
7. PIN photodiodes.
8. Avalanche photodiode (APD).
The avalanche photodiode possesses a similar structure to that of the PN
or PIN photodiode. The main difference of the avalanche photodiode to
other forms of photodiode is that it operates under a high reverse bias
condition. As a photon enters the depletion region and creates a hole
electron pair, these charge carriers will be pulled by the very high electric
field away from one another. Their velocity will increase to such an extent
that when they collide with the lattice, they will create further hole
electron pairs and the process will repeat.
The avalanche action enables the gain of the diode to be increased many
times, providing a very much greater level of sensitivity.
9. Comparison of PIN & APD.
Solar Cell
❖ What is Solar cell? Advantages of it.
❖ Photovoltaic effect.
❖ Single Solar cell.
Similarly, the newly created holes once come to the p-type side cannot
further cross the junction because 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.
If we connect a small load across the junction, there will be a tiny current
flowing through it.