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

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 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

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 disappears.

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

technology.