Light-emitting diodes , commonly referred to as LEDs, are true unsung heroes in the world of electronics. They serve many different functions in all sorts of devices.
They form the numbers on digital clocks, transmit information from remote controls, light up watches and tell you when your devices are on. Together, they can form images on a giant television screen or illuminate a traffic light.
In fact, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike incandescent bulbs, they don't have a filament to burn out, they use less electricity, and they don't get particularly hot.
They are lit only by the movement of electrons in a semiconductor material, and they last as long as a standard transistor.
The lifespan of an LED exceeds the short lifespan of an incandescent bulb by several thousand hours . Because of these advantages, small LEDs are one of the most popular technologies used to illuminate LCD TVs.
LEDs have several advantages over traditional incandescent lamps, but their main advantage is efficiency. In incandescent bulbs , the process of producing light involves generating a lot of heat (the filament must be heated to light up).
This energy is completely lost, unless you use the lamp as a heater, because much of the available electricity is not used to produce visible light.
LEDs generate very little heat, relatively speaking. A much higher percentage of the electrical energy goes directly to producing light, which significantly reduces electricity requirements.
Per watt, LEDs produce more lumens (or amounts of visible light) than regular incandescent bulbs.
Light-emitting diodes have a higher luminous efficacy (the efficiency with which electricity is converted into visible light) than incandescent bulbs.
A 60-watt incandescent bulb can produce between 750 and 900 lumens, but an LED bulb only uses 6 to 8 watts. And that same LED bulb can last 25,000 hours, while the 60-watt incandescent bulb is only likely to last about 1,200 hours.
In other words, one LED bulb can last as long as 21 60-watt incandescent bulbs lit consecutively [source: EarthEasy].
Until recently, LEDs were too expensive to use in most lighting applications because they are constructed from advanced semiconductor materials.
However, the price of semiconductors dropped after the year 2000, making LEDs a more cost-effective lighting option for a wide range of situations. While they are more expensive than incandescent bulbs initially (about $5 versus $1 for incandescent bulbs), their lower long-term cost can make them a better buy.
Several companies have started selling LED bulbs designed to compete with incandescent bulbs and compact fluorescent bulbs. These bulbs promise a long life of bright light and amazing energy efficiency.
In this article, we'll look at the technology behind those ubiquitous turn signals, shedding light on some of the principles of electricity and light.
What is a diode?
A diode is the simplest form of semiconductor device. Basically, a semiconductor is a material that varies in its ability to conduct electric current.
Most semiconductors consist of a poor conductor to which impurities (atoms of another material) have been added. The process of adding impurities is called doping. In the case of LEDs, the conductive material is usually aluminum gallium arsenide (AlGaAs).
In pure aluminum-gallium-arsenide, all atoms bond perfectly with their neighbors, leaving no free electrons (negatively charged particles) to conduct electric current.
In a doped material, additional atoms change the balance, either by adding free electrons or by creating holes where electrons can go. Either of these changes makes the material more conductive.
A semiconductor containing extra electrons is called an N-type material , because it contains extra negatively charged particles.
In an N-type material, free electrons move from a negatively charged area to a positively charged area. A semiconductor with extra holes is called a P-type material , because it effectively has extra positively charged particles.
Electrons can jump from one hole to another, moving from a negatively charged area to a positively charged area. Therefore, 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 at 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 in the P-type material along the junction between the layers, forming a depletion region.
In a depletion region, the semiconductor material returns to its original insulating state: all the holes are filled, so there are no free electrons or empty spaces for electrons, and electricity cannot flow.
To eliminate the depletion region , electrons must move from the N-type region to the P-type region and holes must move in the opposite direction. This is done by connecting the N-type side of the diode to the negative end of a circuit and the P-type side to the positive end.
Free electrons in the N-type material are repelled by the negative electrode and attracted to the positive electrode. Holes in the P-type material move in the other direction.
When the voltage difference between the electrodes is high enough, the electrons in the depletion region are forced out of their holes and begin to move freely again. The depletion region disappears and charge moves through the diode.
If you try to pass current in the other direction, with the P-type side connected to the negative end of the circuit and the N-type side connected to the positive end, the 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 through the junction because the holes and electrons are each moving in the wrong direction.
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 packets of particles that have energy and speed, but no mass.
These particles, called photons , are the most fundamental units of light. Photons are released as a result of electrons moving. In an atom, electrons move in orbit around the nucleus.
Electrons in different orbits have different energies. In general, the more energetic electrons move into orbits farther from the nucleus.
For an electron to move from a lower orbital to a higher orbital, something must increase its energy level. Conversely, an electron releases energy when it moves from a higher orbital to a lower orbital.
This energy is released in the form of a photon. A larger energy drop releases a higher energy photon, which is characterized by a higher frequency.
As we saw earlier, free electrons moving through a diode can fall into the empty holes in the P-type layer. This involves a drop from the conduction layer to the P-type layer.
This involves a drop in the conduction band to a lower orbital, so that 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 made of a certain material.
The atoms of a standard silicon diode , for example, are arranged in such a way that the electron falls over a relatively short distance. As a result, the frequency of the photon is so low that it is invisible to the human eye, it lies in the infrared part 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 those that illuminate the digits on a digital clock, are made from 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 digital displays in electronic devices, visible LEDs are popular due to their long life and miniature size.
Depending on the materials used in LEDs, they can be built to glow in infrared, ultraviolet, and every color of the visible spectrum. While all diodes emit light, most don't do so very efficiently.
In a regular diode , the semiconductor material itself ends up absorbing a lot of the light energy. LEDs are specially designed to release a large number of photons outward. Additionally, they are housed in a plastic bulb that focuses the light in a particular direction.
Most of the light from the diode bounces off the sides of the bulb and then passes through the rounded end. LED bulbs vs. CFLs and incandescent bulbs.
For decades, 100-watt incandescent bulbs have illuminated hallways and bedrooms; 60-watt incandescent bulbs provide a softer light in reading lamps and closets.
But incandescent lamps are inefficient, waste a lot of energy as heat, and have a shorter life than fluorescent lamps.
Why choose a LDC?
Recently, more efficient compact fluorescent light bulbs (CFLs) have become popular alternatives. While incandescent bulbs last an average of 1,200 hours, CFLs can last 10,000 hours [source: EarthEasy].
Unfortunately, CFLs contain toxic mercury, making them potentially dangerous and difficult to dispose of. Along came the LED bulb. LEDs offer the benefits of CFLs: lower energy consumption and longer life without the downside of toxic mercury [source: Scheer and Moss].
For example, a 60-watt incandescent light bulb uses more than $150 in electricity per year and provides about 800 lumens of light; an equivalent compact fluorescent bulb uses less than 15 watts and costs only about $35 in electricity per year.
LED bulbs are even better, using about 8.5 watts of electricity, costing about $21 per year, and lasting 25,000 hours or more [source: EarthEasy]. A full year is only 8,760 hours.
Imagine the lifespan of an LED bulb in an average home! So LEDs sound great and they are but there are reasons why incandescent bulbs and compact fluorescent bulbs are still around.
Why choose an LED bulb?
The main reason is price: LED bulbs cost more than other options, although LED bulb prices have come down in recent years.
However, their longer lifespan and significantly lower power consumption offset the higher barrier to entry. In addition to the high cost barrier, LEDs are vulnerable to high temperatures.
If the LED circuits get too hot, more current will flow through the junction mentioned earlier in this article. When the junction gets too hot, the bulb will burn out [source: LEDs Magazine].
And while LEDs don't contain mercury (unlike CFLs), they can contain other hazardous elements like lead or arsenic [source: Scheer and Moss]. Additionally, one study found that light pollution increased with the use of LED bulbs [source: Brady].
Some people also prefer the look of incandescent light , finding it warmer than the yellowish-looking CFLs and the bluish glow of LEDs.
The difference between lighting types may take some adjustment, but LEDs also come in a variety of hues.
LEDs can be dimmed (unlike CFLs) and are great for promoting plant growth because they efficiently provide tons of light without producing heat that could be harmful to plants.
While CFLs and incandescent bulbs are limited to the color of their housing, some LED bulbs can be modified to emit millions of colors.
These bulbs are typically more expensive than their white LED counterparts, but they can be programmed to change color depending on the time of day , giving people more control over their environment.
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