readsA 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 cal 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 makes the material more conductive.
A semiconductor with extra electrons is cal 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 cal 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 negatively-charged area.
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 fil, 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 cal the
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 repel 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
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
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.
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! In the next section, we'll find out exactly why this is.
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, cal
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. 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.
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