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Light Emitting Diodes 1
Recent advances in solid state lighting techniques allow LEDs to generate light at levels that is similar to
traditional lighting systems. Now high power LEDs are available which have the potential to replace the
fluorescent lamps. Lattice LED is the new generation LED used in display boards and TV screens
Light Emitting Diode is now a part of our life. As a simple indicator to TV monitor, LED has its significant
role in modern electronics.
Some LED Physics
The LED has a semiconductor chip placed in its centre. The semiconductor consists of two regions
namely a P region that has positive charge carriers and an N region with negative charge carriers. There
are three layers in the chip. An active photon generating material is sandwiched between the P and N
type materials so that photons will be generated when the electrons and holes combines. That is when a
potential difference is applied between the P and N materials through the LED terminals, holes from the
P layer and electrons from the N layer move towards the active material where they combine to
produce the light though the phenomenon of Electroluminescence.
How LED gives color?
The color of photon emission depends on the chemical doped in the active material. Chemicals like
Gallium, Arsenic and Phosphor are used to give color to the light emission. Some of the chemicals used
in LEDs are
1. Infrared LED- Aluminium gallium arsenide
2. Red LED – Aluminium gallium arsenide, Gallium arsenide phosphide, Gallium phosphide
3. Green LED – Aluminium gallium phosphide, Gallium nitride
4. Yellow LED – Aluminium gallium phosphide, Gallium arsenide phosphide, Gallium phosphide
5. Orange LED – Aluminium gallium indium phosphide
6. Blue LED – Indium gallium nitride, Silicon carbide, Sapphire, Zinc selenide
7. White LED – Gallium nitride based indium gallium nitride
8. Ultraviolet LED – Indium gallium nitride, Aluminium gallium nitride.
Energy level of an LED
The amount of energy in the light generated from an LED depends on the voltage applied to the LED
and the electric charge of the electrons present in the LED. The electric charge of an electron is q=
-1.6x10-19 joule. The forward voltage of LED range from 1.8 to 3.6 volts in different types. Therefore
the energy of emission can be calculated using the formula E=qV. q is the electric charge of electron
and V is the forward voltage drop of LED.
Wattage of an LED
LED is a current dependent device. Minimum 20 mA current is required to get sufficient brightness.
If excess current is flowing through the LED, its semiconductor heats up and gradually deteriorate.
This leads to poor performance and finally LED will be destroyed. Wattage of the LED is the forward
voltage multiplied by the forward current. In high current LEDs, forward current can go up to 350
mA. In these devices the wattage depends on the forward voltage drop ranging from 1.8 volts to 4
volts. Therefore an average of 1 watt is found in high current LEDs.
Light output
Light from the LED depends on the material used and the amount of current flowing through it.
Luminous intensity of LED depends on the current flowing through it. Typical LED requires minimum
20 mA current to give sufficient brightness. Some semiconductor material is affected by
temperature. High power Red and Orange LEDs suffer light loss at high temperature. The light
output and viewing angle of LED depends on the semiconductor as well as the epoxy casing of LED.
LED with high luminous intensity will have narrow viewing angle because light rays will concentrate
into a beam. Diffused LED will scatter light so that intensity will be less.
How many years an LED live?
LEDs have more life when compared to ordinary bulb. The life of LED depends on the physical
parameters and operating current. When the light from the LED reduces to half, it indicates its
degradation. High quality LEDs can operate more than 1 lakh hours but ordinary LEDs will not live
more than 50,000 hours.
The Trick of White LED
There is no semiconductor material that emits pure white light. White light is a feeling created by
the mixing of the primary colors - Red, Green and Blue. The white LED has an Indium gallium nitride
blue chip coated with phosphor. This phosphor can emit white light when energized with blue or
ultraviolet photons. The blue light emitting chip in the LED is coated with a converter material.
When this converter material is activated by blue light, it emits yellow light. The mixture of blue and
yellow light is perceived by the eye as white light. The light from the white LED has both peaks in the
blue and yellow region but human eye will identify the peaks as white light. That is why, some white
LEDs looks more blue and some more yellow. This is due to the difference in the peaks. White LED
requires minimum 3 volts for its operation and its forward current can go up to 40 mA.
More technologies
LED technology is advancing and the latest potential devices include High power white LEDs, Lattice
LEDs, LED monitors etc. High power LEDs can give more than 1000 lumens and the device is brighter
than a 50 watts halogen lamp. These LEDs are widely used to make flood lights, room lights and also
in display purpose. Lattice LED is the new generation LED using the photon lattice technology. Large
area chip is constructed using the lattice method. These ultra high power LED lattice can operate at
an input power as high as 100 watts. Lattice LED are used in sign boards, display devices , LED TV
etc.
D.Mohankumar