With the coming of energy-saving era, high power light-emitting diodes (LEDs) are promising to
replace other technologies such as incandescent and fluorescent bulbs due to improved luminescent
efficiencies and extended lifetime.
Introduction
Light emitting diodes, commonly called LEDs. They do dozens of different Functions and are
found in all kinds of devices. Among other things, they form the numbers on digital clocks, transmit
information from remote controls, light up watches and tell you when your appliances are turned on.
Collected together, they can form images on a jumbo television screen or illuminate a traffic light.
Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike
ordinary incandescent bulbs, they don't have a filament that will burn out, and they don't get
especially hot. They are illuminated solely by the movement of electrons in a semiconductor material,
and they last just as long as a standard transistor.
What is a Diode?
A diode is the simplest sort of semiconductor device. Broadly speaking, a semiconductor is a
material with a varying ability to conduct electrical current. Most semiconductors are made of a
poor conductor that has had impurities (atoms of another material) added to it. The process of adding
impurities is called doping.
A diode comprises a section of N-type material bonded to a section of P-type material, with
electrodes on each end. This arrangement conducts electricity in only one direction. When no voltage
is applied to the diode, electrons from the N-type material fill holes from the P-type material along
the junction between the layers, forming a depletion zone. In a depletion zone, the semiconductor
material is returned to its original insulating state -- all of the holes are filled, so there are
no free electrons or empty spaces for electrons, and charge can't flow.
At the junction, free electrons from the N-type
material fill holes from the P-type material. This
creates an insulating layer in the middle of the
diode called the depletion zone.
To get rid of the depletion zone, you have to get electrons moving from the N-type area to the
P-type area and holes moving in the reverse direction. To do this, you connect the N-type side of
the diode to the negative end of a circuit and the P-type side to the positive end. The free
electrons in the N-type material are repelled by the negative electrode and drawn to the positive
electrode. The holes in the P-type material move the other way. When the voltage difference between
the electrodes is high enough, the electrons in the depletion zone are boosted out of their holes
and begin moving freely again. The depletion zone disappears, and charge moves across the diode.
When the positive end of the circuit is hooked up to the N-type layer and the negative
end is hooked up to the P-type layer, free electrons collect on one end of the diode and
holes collect on the other. The depletion zone gets bigger.
The interaction between electrons and holes in this setup has an interesting side effect -- it generates light!.
How Can a Diode Produce Light?
Light is a form of energy that can be released by an atom. It is made up of many small particle-like
packets that have energy and momentum but no mass. These particles, called photons, are the most basic
units of light.
For an electron to jump from a lower orbital to a higher orbital, something has to boost its energy
level. Conversely, an electron releases energy when it drops from a higher orbital to a lower one.
This energy is released in the form of a photon. A greater energy drop releases a higher-energy photon,
which is characterized by a higher frequency. As we saw in the last section, free electrons moving
across a diode can fall into empty holes from the P-type layer. This involves a drop from the
conduction band to a lower orbital, so the electrons release energy in the form of photons.
This happens in any diode, but you can only see the photons when the diode is composed of certain
material. The atoms in a standard silicon diode, for example, are arranged in such a way that the
electron drops a relatively short distance. As a result, the photon's frequency is so low that it
is invisible to the human eye -- it is in the infrared portion of the light spectrum. This isn't
necessarily a bad thing, of course: Infrared LEDs are ideal for remote controls, among other things.
Visible light-emitting diodes (VLEDs), such as the ones that light up numbers in a digital clock,
are made of materials characterized by a wider gap between the conduction band and the lower orbitals.
The size of the gap determines the frequency of the photon -- in other words, it determines the color
of the light.
LED Advantages
While all diodes release light, most don't do it very effectively. In an ordinary diode, the
semiconductor material itself ends up absorbing a lot of the light energy. LEDs are specially
constructed to release a large number of photons outward. Additionally, they are housed in a plastic
bulb that concentrates the light in a particular direction. As you can see in the diagram, most of
the light from the diode bounces off the sides of the bulb, traveling on through the rounded end.
LEDs have several advantages over conventional incandescent lamps. For one thing, they don't
have a filament that will burn out, so they last much longer. Additionally, their small plastic
bulb makes them a lot more durable. They also fit more easily into modern electronic circuits.
But the main advantage is efficiency. In conventional incandescent bulbs, the light-production
process involves generating a lot of heat (the filament must be warmed). This is completely wasted
energy, unless you're using the lamp as a heater, because a huge portion of the available electricity
isn't going toward producing visible light. LEDs generate very little heat, relatively speaking.
A much higher percentage of the electrical power is going directly to generating light, which cuts
down on the electricity demands considerably.
Up until recently, LEDs were too expensive to use for most lighting applications because they're
built around advanced semiconductor material. The price of semiconductor devices has plummeted over
the past decade, however, making LEDs a more cost-effective lighting option for a wide range of
situations. While they may be more expensive than incandescent lights up front, their lower cost
in the long run can make them a better buy. In the future, they will play an even bigger role in
the world of technology