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Appliance Science: The illuminating physics behind LED lights

LED lights are the latest thing in home lighting, using less energy and lasting longer than their incandescent cousins. How do they work? Find out in the latest installment of Appliance Science.

Colin McDonald/CNET

Where does the light in your house come from? Chances are that it comes from a variety of different types of light sources, but an increasing number of you are bathed in the soothing balm of light generated by light-emitting diodes (LEDs). This new breed of light source is fast replacing incandescent and fluorescent lights as the main way to curse the darkness.

The main reason for the ascendance of the LED light is the holy trifecta of consumer goods: cost, efficiency and reliability. Thanks to recent developments, the price of LED lights is falling -- nearly as low as the old bulbs in some parts of the US. They're also cheaper to run and last a lot longer. All of that is thanks to the work of legions of scientists and engineers who took a 100-year-old scientific oddity and made it into a multibillion-dollar business.

LEDs were first developed early in the last century, when a researcher noticed how applying a current to a certain type of crystal diode (an electrical component that only allows electricity to flow in one direction) made it give off a faint light. The material was not heating up; it was emitting light through another unknown method. This phenomenon (later called electroluminescence) remained a scientific oddity without much practical use until the 1950s, when companies started harnessing it to produce LED lights.

The first were infrared LEDs (or "semiconductor radiant diodes," as the inventors called them), made from a compound called gallium arsenide (GaAs). Later, other researchers used similar materials to create LEDs that gave off red and yellow light, and made these more efficient to create bigger, brighter LEDs.

There was one major problem with these devices, though: they only emit one frequency of light, which means one single color. That's fine for a calculator display, but it doesn't work for the lights that illuminate your house. These need to give off a range of frequencies of light across the visible light spectrum, producing the same white light that the sun does. Incandescent light bulbs do this, but LEDs only give a very small, narrow band of frequencies because of the way the electroluminescent effect works.

Colin McDonald/CNET

Single frequencies

Inside an LED bulb is a small semiconductor chip, which is the part that gives off the light. This is made of two layers of a crystalline material like GaAs that have been contaminated with different materials (chemists call this doping). This contamination means that one layer has lots of high-energy electrons that it wants to give away, while the other layer has spaces for electrons that it really wants to fill at a lower energy level. In the technical language of electronics, one layer is n-type (the one that wants to give them away), while the other is p-type. The combination forms what is called a pn-junction.

To the electron, this junction is like a waterfall: give them a bit of a push one way and they will fall over, but they can't go back up. If you apply voltage so the negative end of the circuit is on the n-type layer, the electrons flow between the two layers easily. If you apply the voltage the other way, the flow is blocked. These layers form a diode (from the ancient Greek di, for two and ode, for way, or path). Like a waterfall, the flow can go one way, but not the other.

Colin West McDonald/CNET

And, like a waterfall, interesting things happen at this point where the flow is controlled. Whereas a waterfall makes noise, electrons flowing across the pn-junction of an LED release light. As the electrons flow from the n to the p layer, the energy level of the electrons falls, dropping from the higher energy level of the p-type layer to the lower one of the n-type. This energy is released as a photon, which we see as light.

How much energy this is depends on the difference between the energy levels of the two layers, called the band gap. To use our waterfall metaphor again, the energy gap is the height between one side of the waterfall and the other. So, an electron moving down this waterfall falls from one energy level to the lower one, releasing the energy difference as a photon of light.

The larger this band gap, the more energy is released, and the shorter the wavelength of light that is emitted. Early LEDs had a small band gap, which meant longer wavelength photons, which you see as red light. As the engineers figured out new ways to make LEDs, they learned how to make a larger band gap, which means the wavelength of the light these LEDs gave off decreases, and the light shifts across the spectrum from red, to orange, green and eventually blue.

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That gets complicated, though, because creating a pn-junction with a large band gap is very hard. The larger the band gap, the less the two layers want to be next to each other: the crystals that they are made of break down more easily. The best material to make them from is gallium nitride (GaN), but this material is fragile, and the doping that makes it an n- or p-type semiconductor interferes with the way the crystals are grown. The bigger the crystals you can grow, the bigger the final LED can be, and the more light it outputs.

Recently, three scientists figured out a new way to make these big crystals for blue LEDs. The Nobel Prize was given to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura (Nobel Prize in Physics, 2014) because they figured out in the late 1990s how to grow these crystals by zapping the growing crystal with electrons. Apparently, they figured this out by accident while examining a growing crystal with an electron microscope. This electron beam stops the doping material from affecting the structure of the crystal as it grows, so it can get larger.

White light, no heat

The blue LED that their process allows is the holy grail of LEDs, because blue light can be converted to other colors by phosphors, chemicals that absorb light and then give it off at a different, higher frequency. There are plenty of phosphors that can convert blue light to green and red, so if you combine a blue LED with some of these phosphors, you get white light, and a white LED.

The spectrum of a white LED, showing the light from the blue LED (left peak) and the converted light from the phosphor (right).Deglr6328 at the English language Wikipedia

The result is a light bulb that is much more efficient than the old incandescent type. While an incandescent bulb may use 60 watts of energy to illuminate a room, an LED bulb can produce the same amount of light using less than 10 watts. And, because there is no heated filament or unusual gases required, the bulb will last longer. LEDs do break down over time (as the doping materials migrate and the crystals gradually break down), but that takes much longer than an incandescent light.

So, LED light bulbs are more efficient than incandescent ones. And now that manufacturers have worked out how to make these bulbs quickly and cheaply, they can replace these incandescent lights for a new era of cheaper, cleaner and more efficient lighting in your home.

Colin West McDonald/CNET

The future of LED lights

As we have seen in our tests, LED light bulbs are far more efficient than incandescent or compact fluorescent ones. They last longer and use less electricity. But they aren't perfect. They can't be dimmed in the same way as incandescent lights, because the light output doesn't change if you reduce the voltage. Instead, the LED light has to fake it, rapidly turning the LED on and off to reduce the light output which sometimes creates an unpleasant flickering effect. Many modern LEDs avoid this by carefully controlling how the individual LEDs inside the bulb are turned on and off, creating a non-flickering light. The latest Phillips 100W replacement bulb, for instance, had no visible flicker in our tests.

Engineers are also looking at new ways to make the light from these bulbs more appealing by using different materials to create the different frequencies of light that make up white light. Most LEDs use a mix of different phosphors to create the mixed white light, but the University of Georgia recently demonstrated a new type of phosphor that creates a wider mix of frequencies, which could make LEDs easier to make and even cheaper.

So, the future of LED lights is cheaper, more efficient and more comfortable lighting. It remains to be seen how many Nobel Prizes they will earn, but the LED light bulb is likely to be the thing that finds its home in your light socket for some time to come.

Correction, Wednesday 10:30 a.m. ET: Edited to clear up confusion between frequency and wavelength in the paragraph beginning, "The larger this band gap". Our thanks to the commenters who pointed out this error.