emitting diodes, commonly called LEDs, are real unsung
heroes in the electronics world. They do dozens of different
jobs 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.
In this edition of HowStuffWorks,
we'll examine the simple principles behind these ubiquitous
blinkers, illuminating some cool principles of electricity and
light in the process.
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.
In the case of LEDs, the conductor material is typically
aluminum-gallium-arsenide (AlGaAs). In pure
aluminum-gallium-arsenide, all of the atoms bond perfectly to
their neighbors, leaving no free electrons
(negatively-charged particles) to conduct electric current. In
doped material, additional atoms change the balance, either
adding free electrons or creating holes where electrons
can go. Either of these additions make the material more
A semiconductor with extra electrons is called N-type
material, since it has extra negatively-charged
particles. In N-type material, free electrons move from a
negatively-charged area to a positively charged area.
A semiconductor with extra holes is called P-type
material, since it effectively has extra
positively-charged particles. Electrons can jump from
hole to hole, moving from a negatively-charged area to a
positively-charged area. As a result, the holes themselves
appear to move from a positively-charged area to a
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
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 negative end of the circuit is
hooked up to the N-type layer and the positive end is
hooked up to P-type layer, electrons and holes start
moving and the depletion zone
If you try to run current the other way, with the P-type
side connected to the negative end of the circuit and the
N-type side connected to the positive end, current will not
flow. The negative electrons in the N-type material are
attracted to the positive electrode. The positive holes in the
P-type material are attracted to the negative electrode. No
current flows across the junction because the holes and the
electrons are each moving in the wrong direction. The
depletion zone increases. (See How
Semiconductors Work for more information on the entire
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
The interaction between electrons and holes in this setup
has an interesting side effect -- it generates light!
In the next section, we'll find out exactly why this is.
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.
Photons are released as a result of moving electrons. In an
atom, electrons move in orbitals around the
nucleus. Electrons in different orbitals have different
amounts of energy. Generally speaking, electrons with greater
energy move in orbitals farther away from the nucleus.
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.
(Check out How Light
Works for a full explanation.)
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.
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
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.
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.
For more information on LEDs and other semiconductor
devices, check out the links in the next section.