How Can a Diode Produce Light? |
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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. 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 technology. For more information on LEDs and other semiconductor
devices, check out the links in the next section. |