How Light Emitting Diodes (LEDs) Work

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 with 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 alterations make the material more conductive.

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 negatively charged area.

A diode consists of 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 electricity can't flow.

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.

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 process.)

The interaction between electrons and holes in this setup has an interesting side effect – it generates light!

Light is a form of energy that can be released by an atom. It's 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.

As we saw earlier, 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's invisible to the human eye – it's 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 LEDs are used in everything from remote controls to the digital displays on electronics, visible LEDs are popular thanks to their long lifetimes and miniature size. Depending on the materials used in LEDs, they can be built to shine in infrared, ultraviolet, and all the colors of the visible spectrum in between.

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. Most of the light from the diode bounces off the sides of the bulb, traveling on through the rounded end.

For decades, 100-watt incandescent light bulbs have lit up hallways and bedrooms; 60-watt incandescents have shone softer light from reading lamps and closets. But incandescent lights are inefficient, waste lots of energy as heat, and have shorter lifespans than fluorescent lamps. Recently, more-efficient compact fluorescent (CFL) bulbs have become popular alternatives. Where incandescent lights last an average of around 1,200 hours, CFLs can last 10,000 hours [source: EarthEasy]. Unfortunately, CFLs contain toxic mercury that makes them potentially hazardous and a pain to dispose of.

Enter the LED light bulb. LEDs offer the advantages of CFLs – lower power consumption and longer lifetimes – without the downside of toxic mercury [source: Scheer and Moss]. For example, a 60-watt incandescent light bulb draws more than $150 worth of electricity per year and provides about 800 lumens of light; an equivalent compact fluorescent uses less than 15 watts and costs only about $35 of electricity per year. LED bulbs are even better, drawing about 8.5 watts of power, costing about $21 per year, and lasting 25,000 hours or longer [source: EarthEasy]. There are only 8,760 hours in a whole year – imagine how long an LED bulb would last in the average home!

That makes LEDs sound pretty great – and they are – but there are reasons incandescent and compact fluorescent bulbs are still around. The biggest reason is price; LED bulbs cost more than the other options, even though prices for LED bulbs have come down in recent years. However, their longer life spans and dramatically lower power usage help make up for the higher barrier of entry.

In addition to the high cost barrier, LEDs are vulnerable to high temperatures. If LED circuitry gets too hot, more current will pass through the junction mentioned earlier in this article. When the junction gets too hot, it causes the bulb to burn out [source: LEDs Magazine]. And even though LEDs are free of mercury (unlike CFL bulbs), they may contain other dangerous elements such as lead or arsenic [source: Scheer and Moss]. Further, one study found light pollution increased with the use of LED bulbs [source: Brady].

Some people also prefer the look of incandescent light, feeling they're warmer than the yellowish-looking CFL bulbs and the bluish brightness of LEDs. The difference in lighting types can take some adjustment, but LEDs are also available in a variety of hues. LEDs are dimmable (unlike CFLs) and are perfect for encouraging plant growth, since they efficiently put off tons of light without producing heat that could potentially be damaging to plant life.

While CFLs and incandescent bulbs are limited to the color of their enclosure, some LED bulbs can be changed to emit millions of colors. These bulbs are typically more expensive than their white LED bulb counterparts, but they can be programmed to change color with the time of day, giving people more control over their environment.

LEDs have come a long way since the early days of lighting up digital clock faces. In the 2000s, LCD TVs took over the high-definition market and represented a huge step over old standard definition CRT televisions. LCD displays were even a major step above HD rear-projection sets that weighed well over 100 pounds (45.4 kilograms). But LED technology changed the market. While LCD TVs were far thinner and lighter than massive rear-projection sets, they still used cold cathode fluorescent tubes to project a white light onto the pixels that make up the screen. Those add weight and thickness to the TV set. LEDs solved both problems.

Have you ever seen a gigantic flat-screen TV barely an inch thick? It was probably an LED television. Here's where the acronyms get a bit confusing: Those LED TVs are still LCD TVs, because the screens themselves are comprised of liquid crystals. Technically, they're LED-backlit LCD TVs. Instead of fluorescent tubes, LEDs shine light from behind the screen, illuminating the pixels to create an image. Due to the small size and low power consumption of LEDs, LED-backlit TVs are far thinner than regular LCD sets and are also more energy efficient. They can also provide a wider color gamut, producing more vivid pictures.

There are several different types of LED-backlit sets are on the market – and not all are created equal. Many sets use white LED edge lighting to shine light across the display. The only real advantage afforded by these sets is thinness. RGB LED-backlit sets, on the other hand, provide improved color. Some configurations even allow for a technique called local dimming, where LEDs in different parts of the display can be brightened or dimmed independently to create a more dynamic picture [source: Morrison]. And that highlights one more great advantage of LEDs over CFLs: Because the LEDs can actually be instantly toggled on and off, they produce awesome black levels in dark scenes. Since the white fluorescent lamps have to remain on during TV use, some light tends to bleed through and lighten the picture in dark scenes.

Some of the newest LED displays are made with organic light emitting diodes, or OLEDs. The organic materials used to create these semiconductors are flexible, allowing scientists to create bendable lights and displays – some manufacturers have already started producing smartphones and other devices that use smaller OLED screens. As the technology allows better and larger displays, you may start seeing them in places you would never have been able to find a screen before. OLEDs also allow individual pixels to be turned completely off, letting viewers see true black.