Thursday, February 17, 2011

Steph’s Science Corner: Why We Love LEDs

from www.treehugger.comThese days, you can’t find many pieces of technology without LEDs embedded somewhere in their design. It’s hard to be a functioning member of society without at least recognizing the acronym, even if we know comparatively little about its implication. LED stands for Light Emitting Diode, and most of us are typically used to discussing them in the context of high-definition TVs, though they can also be found in the faces of digital watches and in the invisible emissions of your remote control. They’re low-heat, high-efficiency, and with the advancements that we’ve made in materials science, extremely versatile in the range of colors that they’re capable of emitting.

If you’ve ever looked at one of these tiny bulbs up close, you might have noticed the absence of a traditional piece of metal in the center called a filament. In traditional incandescent bulbs that we use to illuminate rooms, light is generated by pumping electricity through a thin strip of metal – usually tungsten. This metal filament has a resistance to the electricity, so it dissipates electrical energy in the form of heat. Eventually, it gets so white-hot that it glows. But this process wastes a lot of electricity generating all that excess wasted heat that we don’t use. By comparison, LEDs generate minimal heat, because the process by which they produce light particles (photons) is a completely different beast.

The semiconductor revolution

Beyond their visual application, diodes are essential in most modern electronic systems, and they were made possible by the silicon revolution, a term that comes from the fact that most semiconductors are made primarily from silicon. Diodes are the simplest form of semiconductor.

Bohr model of siliconIn order to understand what a semiconductor is, we must briefly recall that a chunk of material is made up of millions of tiny atoms. They each have a positively charged nucleus and electrons that orbit around them at different energy levels [quantum mechanics has shown us that this isn’t actually correct, but we can use this model to simplify things]. In the bulk, these atoms are mashed together, so the electron orbitals overlap and blend together. Electrons that aren’t very tightly held in, like those in the outermost rings, can easily hop over onto nearby neighbors. The purposeful movement of these electrons across a material is electricity. If these electrons move easily, the material is a conductor. If they’re stubbornly stuck in place, it’s an insulator.

Naturally, a semiconductor sounds like a half-conductor, and that’s almost accurate. When we classify new semiconductor materials, all we have to do is make sure they have a conductivity that falls within a certain range. But their design, their purpose, and their use is far more complicated.

taken from my all-time favorite physics reference website: Hyperphysics, maintained by Georgia State University. Since many semiconductors are made out of silicon, its easiest to start there. Silicon – and all other elements in its column of the periodic table – have four outer electrons, which means they use those electrons to make four bonds, if possible. That allows them to form rigid, tightly-bound crystal structures. While it seems like all these bonded electrons are tightly held, there is a small possibility that one of these electrons will get knocked loose, leaving behind a hole. This hole (or, more accurately, this loose electron) can move around as it tries to recombine. But with a force behind it, this hole and electron will separate, and the silicon crystal will conduct a very small amount of electricity.

We can encourage this process by adding tiny amounts of an impurity. If we add a few elements out of the column to the right of silicon – those with five outer electrons instead of four, such as phosphorus or arsenic – they still form four bonds with their neighboring silicon atoms, but the fifth electron will be free to move around. Conversely, if a few atoms with just three outer electrons were added instead – those found one column to the left, such as boron or gallium – they would silicon dopingform only three bonds, leaving a silicon that has nothing to bind to. These “holes” as they are called will greedily steal loose electrons from nearby neighbors, sharing the burden. We call this introduction of an impurity “doping.” The first scenario introduces an extra electron, so we call it N-type doping (for the extra electrons that create a negative charge). We call the second scenario P-type semiconductors for the absence of and electron, creating a positive-charge effect.

What is a diode and what is it used for?

Semiconductors can be found at the heart of computer chips, transistors, solar panels, and a host of other applications, but their simplest form is the diode. depeletion layerA diode is, very simply, P-type material bonded to N-type material. When we allow these two pieces to touch, electrons will instantly want to fill the holes near the contact area. As electrons and holes combine, they neutralize, and form an insulating layer in the middle, called the depletion zone. Current cannot flow across this diode in this state.

But imagine that we hook up a battery to this diode. As a quick note, electrons are charge carriers in electricity, there is no such thing as mobile positive charges. However, when we talk about the positive end of a battery, it’s easier to say that “positive charge flows” from the positive end, even though that’s not actually what is taking place. Negative charge is flowing away from the positive end. Keeping that in mind, if we connect the The depletion region grows in this diagramnegative end of the battery to the P-type side and the positive end of the battery to the N-type, then electrons will flow out of the negative end into the positively charged, hole-heavy side. Similarly, “positive charge” flows into the negative end. This creates more opportunities for recombination, increasing the size of the depletion zone.

From But if instead we connected the negative end of the battery to the N-type side, we get electric repulsion. Electrons come flooding in from the wire, pushing a cascading river of electrons towards the P-type material. So many electrons continuously flow in that others are kicked out faster than they can recombine. Electric charge flows across the junction as these electrons push onward towards the opposite wire, and eventually, the positive end of the battery. In this way, diodes only allow current to flow through them in one direction, called a “bias.” If we connected this diode to an alternating current that constantly switches directions (AC power), they would flip on and off, allowing current and then blocking it.

How do we get light out of diodes?

In the previous section I mentioned that electrons exist in orbitals around the nucleus. The electrons closest to the nucleus are those that are held in the most tightly. In electromagnetism, the force pulling two oppositely charged objects together gets stronger as they get closer. So those electrons that are nearest to the nucleus are held in much more tightly. Tightly held electrons are very energetically stable – they like to be where they are. We say then that inner electrons exists at a lower energy state; they are more stable, and stable electrons are easy to leave alone, harder to pluck off. Outer electrons however -- which can be easily whisked away -- take a lot more energy to hold onto. They’re more unstable, so they’re at a higher energy state.

Bohr_model_Balmer_32 When an electron drops from a higher energy state to a lower one, it loses that extra energy, sending it off in the form of a photon – a particle of light. This photon has exactly the same amount of energy as the gap between energy levels, which is distinct and constant for unique materials. A photon with high energy has higher frequency, while low energy photons have low frequency.

Nature wants to be neutral, and a neutral system has less energy than a separated electron-hole pair. When free electrons drop from a free conducting state to a bound one (falling into a hole) they give off that extra energy as a photon. So in any diode, rapid splits and combinations produce a steady stream of photons, all of the same energy level (aka color). When the gap between these two levels is relatively low, such as in a silicon diode, they give off a very low-energy photon, which falls in the infrared region. But we can create semiconductors out of all kinds of materials or combinations thereof, and we can tune the size of the energy gap to be in the visible region.

pretty LED lights These special light-emitting diodes don’t produce any excess waste heat, and they can be tuned to a wide variety of colors just by changing the type of material. We can make them blink on and off by changing the direction of current flow. We can also maximize their intensity by putting them in plastic cases that help to direct the light in a single direction. They also don’t have a filament to burn out, so they last for an extremely long time. Perfect for TVs!

LED advancements and applications – watch them take over your town!

You can find LEDs and other types of diodes in just about everything. They’re getting cheaper, brighter, and replacing nearly all other light sources as we work to conserve and grow more efficient. Many cities across the U.S. have been replacing street lamps and stop lights with LEDs to save money. Recently, we’ve been pushing forward on new devices called OLEDs (organic-LEDs) that are made from carbon-based dye chemicals that can be rolled out thin sheets of film. In theory, they could be brighter, crisper, and more efficient that semiconductor LEDs, and they would be flexible, too!

The message here? LEDs were first created in 1927, and were prohibitively expensive until 1968, but it hasn’t been until the last ten years that we have become better at tuning and shrinking them, and thus being able to use them for applications that are far more exciting that just digital watches. Simply put, they are the immediate future, and they’re going to become one of the most important uses that we have for natural minerals and materials science advancements. Appreciate them. They’re pretty freaking cool.

Only one week left to direct all suggestions and topic requests to After that I’ll be taking a little vacation, but I’ll be back! I know, I know, don’t miss me too much while I’m gone. Thanks for reading!