LEDs - Light Emitting Diodes
A light emitting diode, or LED, is a device that emits light when a current is flowing through it. The diode itself is a form of semiconductor - a material that can conduct electrical current - and they often have impurities added to them (a process called doping) to make them more conductive. The semiconductor material in LEDs is usually aluminium gallium arsenide (AlGaAs) or indium gallium nitride (InGaN) while the impurities are atoms from another material. These impurity atoms can add extra ‘free’ electrons. Or they can create gaps or holes where the electrons can move to.
A semiconductor with extra electrons is called an n-type material because it has extra negatively charged particles. A material with extra holes is called a p-type material because it has extra positively charged particles. A diode consists of a piece of n-type material bonded to a section of p-type material, with electrodes at each end. When there is no voltage, free electrons from the n-type side fill the holes in the p-type material and create an insulating layer in the middle called the depletion zone. The depletion zone disappears when the negative end of a battery is attached to the positively charged p-type material. A current also flows across the junction in one direction only.
When a circuit is made such that the positive end of a voltage, or battery, connects to the n-type layer and the negative to the p-type layer, no current flows. But you get a collection of free elections at one end of the diode and holes on the other and the size of the depletion zone in between increases. When current flows through the LED the electrons move across the diode, and light energy is released in the form photons or light. Whether that light is visible or not depends on the exact material from which the diode is made.
LEDs are made from semiconductor materials such as GaN and InGaN that have what is called a wide energy band gap, which produces photons of higher of wavelengths in the visible light range and where the exact ratio of the different materials frequencies and determine the colour of the light.
The first solid state LEDs were limited to emitting red light as the photon frequency was low but now LEDs are available in all colours and also in ultraviolet light. They last much longer (around 30 times) than conventional filament light bulbs and fit easily into electronic circuits. They generate little heat, compared to incandescent bulbs and so more electrical power goes directly into generating light rather than heat. This makes them extremely efficient. Today’s visible LEDs produce more lumens of light per Watt than regular incandescent bulbs and they have significantly longer lifetimes. Also, unlike certain fluorescent bulbs, they do not contain mercury - which is toxic and produces disposal issues.
LEDs were originally used as indicators, such as red warning lights. The big breakthrough came when it was realised that by changing the semiconductor material and its composition, the exact colour of the light emitted from the diode could be tuned enabling blue LED emitters. As a mixture of at least the three basic colors red, green and blue are necessary to generate white light, LEDs were more widely adopted for traffic and automobile lighting, street lights, TV’s and of course they became more cost effective with improvements in manufacturing.
Commercialising LEDs wasn’t an easy process. The main challenge was to improve the efficiency of converting electrical energy to light. This year’s laureates played important roles in not just pioneering the invention itself but developed technologies and manufacturing techniques to mass-produce LEDS at affordable costs.