Let there be Light-- Emitting Diodes (LEDs)
M.V.N. Murthy, The Institute of Mathematical Sciences, Chennai

Until this year's Nobel prize, LED bulbs were the unsung heroes of the electronic age. Almost without noticing we have been using them, whether in remote controls, digital clocks, traffic lights, etc. They even light up the big television screens now a days.
On October 7, 2014, the Nobel Prize in Physics was awarded to Isamu Akasaki, Hiroshi Amano from Japan and Shuji Nakamura from USA, for their contribution to the invention of blue light-emitting diodes or LED in short. This invention further revolutionised the use of LED bulbs by paving the way for a very wide range of applications.
At one level, LED's are simple tiny light bulbs that sit on any electrical circuit. In this sense they are no different from an incandescent bulb, tube light or any other bulb. But unlike these other bulbs, LED's are extraordinarily efficient in converting the electrical energy in to light apart from being very tiny. This is the reason for the revolutionary changes that we are seeing in lighting systems.
To see how this comes about and to understand the importance of LEDs, we need to understand how we produce light in different systems.

Sources of light
People have discovered various ways of producing light from time immemorial: The earliest invention is burning, say wood or some such material like a match-stick. This is direct conversion of heat to light and remained the basic process for producing light for a long time.
Incandescent light bulbs using electricity have been in use for more than a century. A light bulb produces light when a filament of wire (kept in an evacuated glass chamber) is heated to high temperature by passing an electrical current through it. In 1802 Humphry Davy used an electrical battery to pass electrical current through a thin strip of platinum which has very high melting point. Later improvement were made by Joseph Swan and Thomas Edison towards the end of the 19th century and then commercial production began. The material used for the filament went through many changes and finally ended up with tungsten filaments. These are used even today though they use an inefficient method to convert electrical energy to heat and then light.
In the beginning of the 20th century, the fluorescent lamp or what is called tube light was invented. These are more efficient than incandescent lamps. The lamp consists of a tube filled with a gas containing mercury vapour at low pressure (0.3% of atmospheric pressure) along with some other gasses. Typically the tube is straight with electrodes made of tungsten. A voltage is maintained between the electrodes. When the voltage is switched on, the tungsten cathode heats up to emit electrons. These electrons then further ionise the gas, emitting more electrons. A fluorescent coating along the interior of the tube emits light when these electrons hit the surface. This method is more efficient than the incandescent lamps and there is more light produced per unit watt.
An improvement over these tube lights is the compact fluorescent lamp (CFL). While using the same principle, they are compact (as the name indicates) like the incandescent lamp with smaller tubes which are bent to make them compact. The electrons emitted by the cathode first excite the mercury atoms which radiate ultraviolet light which in turn is converted to visible light by the impact on the fluorescent coating on the bulb's inside surface. These bulbs consume about one-fifth of the electrical energy compared to the incandescent lamp and also last much longer.
One main problem with these fluorescent lamps is the presence of mercury which is toxic and therefore their disposal is complicated with environmental concerns.
Enter the Light Emitting Diodes (LED) which produce light when the electrons move around in semiconductors. The basic unit of an LED is a combination of semiconductors called a diode.

BOX on Semiconductors
Not all electrons in solids participate in conducting electricity. Metals conduct electricity because the outermost electrons of the atoms are quasifree (free to move freely through the metal but not to leave it) and the movement of these under an electric field causes current to flow. Semiconductors, as the name indicates, behave about halfway between conductors like metals, and insulators which do not conduct electricity. The exotic thing about semiconductors is that they can have excess of almost-free electrons (like metals, which have large numbers of electrons, thus becoming good conductors when the electrons move under an electric potential). But semiconductors can also have an equal shortage of these electrons; the absence of these electrons leaves a gap in the electronic arrangement which are called holes. This happens because pure semiconductors like Silicon or Germanium are rarely used. Instead, semiconductors are doped with small number of impurity atoms such as phosphorus or gallium which cause an excess or shortage of electrons that can conduct electricity. Semiconductors which have an excess of electrons are called n-type while those with a shortage are called p-type. Most electronic equipment is made with semiconductor devices that have been constructed by a combination of such n- and p-type junctions.

END OF BOX -----------------------------------------------------------

A diode is made up of a junction of two layers which has more free electrons on one side and less electrons, called holes, on the other. (See box on semiconductors). There may be several layers in an LED. When an electron strikes the n- or negative side (with excess electrons), the struck electron moves to the p- or positive side (with excess holes) and recombines with a hole to produce energy in the form of light. In an ordinary diode (used in electronic circuits), the energy is emitted in the form of heat, not light. The special nature of the material in an LED allows it to emit useful light instead of heat.
So basically the LEDs produce light when the electrons move around in the semiconductor structure. There is no heating of the filament or ionisation of gas involved. In fact there are no moving parts except for electrons hopping from one side to another. This is in fact the most efficient conversion of electrical energy to light as there is no heating, hence no energy loss involved. In addition each LED is the size of approximately a grain of sand; combining many such LEDs one can produce an LED lamp producing enough light.
Nick Holonyak came up with the idea of the light-emitting diode in 1962, more than half a century ago. These were red light emitting diodes at first. These were used in digital watches, calculators and as indicators in many electronic gadgets. We know that all colours of the spectral rainbow can be produced by suitable combinations of the three basic colours, red, green and blue. Large scale use in lighting buildings and TVs was not yet possible since in order to produce all colours, especially white light, we also need to have green and blue light emitting diodes. This required the emission of light of higher frequencies and therefore higher energy. Understanding the type of material used in semiconductors was crucial.
This is the challenge that the 2014 Nobel prize winners undertook. Akasaki and Amano worked at Nagoya University in Japan while Nakamura worked at Nichia Chemicals. They arrived at Gallium nitride (GaN) as the material of choice for producing blue LEDs. This was difficult since one has to grow gallium nitride crystals of high enough quality, which was considered hopeless in the beginning. The Nobel Laureates succeeded in this difficult task. The moment they invented the blue light emitting diodes, the flood gates opened for a fundamental transformation of illumination technology. Today, most white-light-emitting LEDs are made by using blue LEDs and converting the blue light into white using phosphors. The box shows the materials used in different colours of LEDs today.

BOX on LED colours
Types of Light Emitting Diode

o Gallium Arsenide (GaAs) - infra-red
o Gallium Arsenide Phosphide (GaAsP) - red to infra-red, orange
o Aluminium Gallium Arsenide Phosphide (AlGaAsP) - high-brightness red, orange-red, orange, and yellow
o Gallium Phosphide (GaP) - red, yellow and green
o Aluminium Gallium Phosphide (AlGaP) - green
o Gallium Nitride (GaN) - green, emerald green
o Gallium Indium Nitride (GaInN) - near ultraviolet, bluish-green and blue
o Silicon Carbide (SiC) - blue as a substrate
o Zinc Selenide (ZnSe) - blue
o Aluminium Gallium Nitride (AlGaN) - ultraviolet

END OF BOX

An LED produces light as much as an incandescent lamp of 100 watts power by using only about 10 watts of power. This is only one part of the story, the other important part is that it lasts 40-50 times longer than an incandescent bulb. Thus the LED lamp holds a great promise in lighting up billions of homes which currently lack access to electricity grids since they can be powered by solar power directly. Such a direct use reduces the demand for power which is produced using pollution-causing power sources which burn coal and natural gas. The table shows a comparison of light emitting efficiencies per watt of various light sources. (Lumen or lm is a unit of brightness.)
In the long run LEDs may hold the solution for the potentially disastrous climate changes due to the emission of green house gasses the world over.
Sources:
The Nobel Prize Web-site: www.nobelprize.org
Electronics Tutorials: http://www.electronics-tutorials.ws