By Stephen B. Schott
The Nobel Prize in Physics is usually awarded for a Star Trek-sounding discovery or invention like Giant Magnetoresistance. But this year’s award recognizes an invention that we use every day: the blue LED. You may have one in your pocket or nearby right now. In fact, you’re probably staring at millions of them as you read this.
Before LEDs Part 1: The Incandescent Bulb
Before exploring the LED and blue LED, let’s visit the incandescent bulb that lit the 20th century using the simple principle that things glow when they get hot. In an incandescent light bulb, electricity passes through a conductive wire to a tungsten filament 6 feet long wound up to an inch or so length in a typical bulb. As electricity passes through this filament, electrons move between higher and lower energy states—and on each movement to a lower state, the electrons give off photons, some of which result in significant visible light.
All of that creates a few problems. One problem is that to get significant visible light from a light bulb requires that the tungsten be heated to about 2,200C/4,000F and at such extreme temperatures, tungsten combusts. The air-tight bulb prevents combustion because it seals the bulb from oxygen. No oxygen, no combustion. One problem solved.
The next problem is that all the electricity passing through the tungsten filament causes vibration—this vibration releases tungsten atoms and degrades the filament, making it thinner and thinner until it breaks. The next bit of genius is that the gas inside the bulb is an inert gas called argon.
Remember the periodic table—argon was one of the special noble gasses way out on the right that don’t react with other elements. As tungsten atoms shake free of the filament, they have nowhere to go because the inert argon gas won’t react to it—having nowhere to go, most of the tungsten atoms return to the tungsten filament, slowing its breakdown.
The problem with the incandescent bulb is that only 10% of the energy it uses results in visible light. The remaining 90% of the energy is given off as invisible light and heat, which makes the incandescent bulb an effective but inefficient light source.
Before LEDs Part 2: The Fluorescent Bulb
Enter the fluorescent light bulb, which is probably above you if you’re reading this in an office. These are those long tubes usually paired together in drop ceilings.
Fluorescent bulbs act on the same principle of exciting atoms and jumping electrons between higher and lower energy states. The fluorescent light bulb has two electrodes, one at each end of the tube, that release free electrons into the tube. The free electrons collide with the mercury atoms in the tube, causing the mercury atoms’ electrons to move between higher and lower energy states. Just as with the incandescent bulb, this movement between states releases photons.
But the genius in the fluorescent bulb is that not only does it release visible light from the photons, it makes certain invisible ultraviolet light that we normally cannot see visible by the use of a phosphor coating on the inside of the tube. When a photon hits a phosphor atom, one of the phosphor’s electrons jumps to a higher energy level. When the electron falls back to its normal level, it releases energy in the form of photons that are visible as white light.
Fluorescent bulbs are 4-5 times more efficient than incandescent bulbs but they still release significant amounts of heat and invisible light. Fluorescent bulbs also use mercury, which is toxic.
Part 3: Enter the LED
An LED works using a semiconductor material to transmit electricity across it. Semiconductors are just what their name suggests: They have varying degrees of ability to conduct electricity, and usually are made from a poor conductor with intentional impurities introduced into it in a process called doping.
The typical conductor material used in LEDs is aluminum-gallium-arsenide, which when pure, has no free electrons across that conduct electrical current. But when engineers introduce impurities into the material, the impurities create free electrons (negative charges) and also positive holes where the electrons can go. The free electron material is called an N-type material, and the positive hole area is called a P-type material.
When a battery applies voltage across the LED from the N-type material to the P-type material, electrons begin to move from a depletion zone between the N- and P-type materials to either end of the charged LED (each end having a positive or negative charge). As the electrons and positive holes move, elections “find” the positive holes and when they do, they fall to a lower energy state and give off photons in the form of visible light.
LEDs produce light at about 10 times the efficiency and have a lifetime of about 50 times as long as an incandescent bulb.
Part 4: Enter the blue LED
As you may recall from above, when an electron falls from a higher to a lower energy state, it gives off photons and given the right materials, it gives off photons that have a frequency visible to the human eye. Different light frequencies produce different colors but before this year’s Nobel Laureates came along, LEDs came in just two colors: red and green, which were the results of semiconductor material choice. And while red and green can be combined to make a lot of different colors, they can’t be combined to make white light, which is what we humans are most comfortable with. Imagine reading by red light: You could do it but you wouldn’t be happy.
Scientists knew that the way to make a reliable white light was to make a blue LED. Professors Akasaki, Amano, and Nakamura knew that the solution lay in using gallium nitride as their semiconductor material in the diodes and they tackled the problem of making gallium nitride crystals of a size large enough for practical use. Professors Akasaki and Amano from Nagoya University in Japan solved this problem by designing a micro-scaffold made partly from sapphire. Professor Nakamura made a similar breakthrough while he was working at the chemical company Nichia. where he created a temperature manipulation process to grow the crystals.
From their discovery, the blue LED became a possibility, and using the blue LED, many people are now able to experience white LED lights, like the kinds in LED displays, long-life flashlights, camera flashes, and even the light on the back of your phone.
If you have questions, contact me.
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