Mymensingh Engineering College: An Assignment On Optoelectronics EEE - 809

Mymensingh Engineering College
(University of Dhaka)
An Assignment on Optoelectronics
EEE - 809
Submitted by, Submitted to,
K. M. Ahsan-uz-Zaman Md. Mehedi Hasan
Roll : 735 Lecturer,

7 Batch
th
Dept. of EEE,
Dept. of EEE Mymensingh Engineering College
Submission Date: Signature:

Topic of the Assignment: 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.
2. Working principle of PN Junction LED?

The simplicity in the fabrication (in p-n junction diode form) and driving
circuitry of LED makes it very attractive as solid state light source in a
wide variety of technical as well as commercial applications. The LED
converts input electrical energy into spontaneous optical radiation through
the injection luminescence or electroluminescence process. During its
normal operation, the electrons and holes are injected as minority carriers
into the p- and n-sides of forward bias p-n junction diode respectively, as
shown in Figure below,
Figure: Carrier injection in forward bias p-n junction diode leading towards
spontaneous emission.
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
3. Advantage, Disadvantage & Application of LED?
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.
5. What is spontaneous emission and simulated emission?

Spontaneous emission: Electron drops from an excited state to a lower
state emitting a photon. It is the process by which a quantum system such
as an atom, molecule, nanocrystal or nucleus in an excited state undergoes
a transition to a state with a lower energy and emits quanta of energy.
Light from an atom is a fundamental process that plays an essential role
in many phenomena in nature and forms the basis of many applications,
such as fluorescent tubes, older television screens (cathode ray tubes),
plasma display panels, lasers, and LED.
Stimulated emission: When the photon of the same frequency

interacts with electron in excited state which drops to lower state. It is the
process by which an atomic electron interacting with an electromagnetic
wave of a certain frequency may drop to a lower energy level, transferring
its energy to that field. A new photon created in this manner has the same
phase, frequency, polarization, and direction of travel as the photons of
the incident wave. This is in contrast to spontaneous emission which
occurs without regard to the ambient electromagnetic field.
6. Comparison between LED & LASER?
LED LASER
1. It emits the light by spontaneous 1. Here, the light is by stimulated
emission emission
2. The emitted light is incoherent, i.e., 2. It possesses a coherent beam with
the photons are in the random phase identical phase relation of emitted
among themselves. photons.
3. The emitted light power is relatively 3. The output power is high (few MW to
low. GW).
4. It requires smaller applied bias and 4. It requires relatively higher driving
operates under relatively lower current power and higher injected current
densities. density is required.
5. It possesses a simple structural design 5. Its fabrication requires more steps
and does not require optical cavity and and special care in comparison to an
mirror facets like laser, as it operates LED.
under relatively lower power.
6. It does not suffer catastrophic 6. It has relatively higher degradation
degradation and is more reliable in uses. chances during long period operation
7. Construction and working principle of planner surface

of LED?
Among various LEDs, the most simple and widely used one is planar
surface type as shown in Fig.6. A p-n+ junction with very thin p-layer (top
emitting side) as compared to heavily doped wide n+ layer, is grown on
low resistive and lattice matched n+ substrate. The external bias is applied
through the ohmic contact layers present at the top and bottom while the
emitted light comes from the thin p-layer as indicated.
Fig. Structural view of planer surface LED.
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.
9. Loses in LED and how to minimize loses?
LEDs have three distinct optical loss mechanisms like,
i. Loss of certain portion of emitted photons due to re-absorption
by the LED material itself to recreate the electron–hole pair.
ii. Loss of generated photon during its vertical incident at the
semiconductor-air interface.
iii. Photon loss due to its total internal reflection at incidence upon
LED top planer surface with angles greater than the critical
angles.
Loss Minimization:
To minimize the loss of photon due to reabsorption process (first
category), the top p- layer of LED is made thin that prevents the traveling
of the emitted photons to the long distance of its exposed planer layer. The
second and third types of loss mechanism can be minimized by using
dome-shaped LED.

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Mymensingh Engineering College

Submitted by, Submitted to,

K. M. Ahsan-uz-Zaman Md. Mehedi Hasan

Roll : 735 Lecturer,

Submission Date: Signature:

2. Working principle of PN Junction LED?

3. Advantage, Disadvantage & Application of LED?

5. What is spontaneous emission and simulated emission?

Stimulated emission: When the photon of the same frequency

7. Construction and working principle of planner surface

8. Injection, Quantum and extraction efficiency?

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