How LEDs work, the cheapest and most resistant light source on the market

Credits: Zishan khan, via Unsplash.

Miniaturizablelight, resistant and powerful: from the screen of our phones to the street lighting, i LEDs (Light Emitting Diode) have now ousted every other light source, providing intense lighting and vivid colors with low consumption.

The interest in this technology is such that even institutions, such as the European Union, have decided to encourage its diffusion, banning the sale of old incandescent light bulbs already in 2012 and following the same path for halogen lamps, partially banned in starting from 2018.

Where do these virtues of LEDs come from and why did their diffusion only begin in the 2000s?

The origins of LEDs

Electric lighting was born in the early 1800s but only took hold in 1879with the entry into the market of incandescent light bulbs Of Thomas Edison. In the early 1900s new light sources, such as neon or LED lamps fluorescencethey brought a more efficient alternative, a energy saving and long-lived compared to traditional light bulbs, but they have never managed to replace them due to defects such aslight emission (measured in lumens) reduced and more “artificial” light to the human eye.

Also at the beginning of the twentieth century, the English engineer Henry Joseph Round discovered theelectroluminescence of the solid state diodesi.e. the ability of some materials to emit light if subjected to one electric current. LEDs are particular diodes developed to exploit this phenomenon and were born in 1961 with the introduction of led a infraredstill widespread today in remote controls or photocells.

Already the following year, Nick Holonyak Jr. invented the first one red ledtherefore capable of emitting visible light to the naked eye. Studies on these diodes and possible applications (for example in laser) focused on the materials used in order to obtain different colors or shades, working with multiple sources at a time.

Among the main protagonists of these studies, the American engineer M. George Craford (Holonyak's former student at the University of Illinois) in 1972 managed to create i first yellow LEDs commercially distributed by the company he worked for, the Monsanto. Crafrord dedicated his entire career to the world of LEDs: it is also thanks to his studies that we owe the first great diffusion of these light sources, used for example in traffic lights and road light signals.

LEDs have been used for decades in road signs, and are also becoming widespread on our cars. Credits: Mark Boss, via Unsplash.

The last decisive turning point came in 1994 by the Japanese engineer Shuji Nakamura and his studies on high intensity blue LED: improving the light source and using it to trigger the phosphorescence of substances deposited on the diode, LED bulbs suitable for lighting buildings have finally been created, often reducing the energy needed by 7 or 10 times.

How the LED works

But how is an LED made? Since their birth, these devices have been based on the use of semiconductorsi.e. materials (such as Silicon ol'Gallium Arsenurium) That normally not I am good current conductors but they can still become so thanks to external energy sources such as heat or light.

What makes a material a conductor is the characteristic it has “free” electrons“, capable of moving along the material following a potential difference (more simply referred to as ddp). The electrons possessed by each atom have in fact different energies, higher as they move away from the nucleus: when one of these has an energy high enough to reach the conduction bandbecomes free to circulate and no longer bound to its atom.

In the metalseven without external intervention the electrons of valence band they can move on to conduction band because these these are superimposable. Conversely, a insulating has important energy leaps, therefore it does not conduct electricity under normal conditions.

In semiconductors, a small amount of energy is enough to make the valence electrons “jump” to the higher band energy. The reverse path will see the electron shed excess energy, such as light or heat depending on the jump.

In the case of semiconductors, the energy leap And Bass. The energy required for the jump depends on the material used, but also on the doping, i.e. the introduction of elements such as phosphorus or boron into the crystalline structure of the material: this causes an abundance of negative (n) or positive (p) charges in the “drugged” material.

Posing two semiconductors in contact negative and positive charges accumulate at the junction: The electrons in the conduction band they pass from the material richer in electrons (n) to the positively charged one (p) and here they decay at the valence bandreleasing excess energy in the form of light.

This process is much more efficient than that of incandescent light bulbs, because strongly reduces losses in heat or infrared rays invisible to the naked eye: if a traditional light bulb the energy transformed into light is a maximum of 5%, in the case of LEDs theefficiency is typically 15% or greater, always starting from already lower consumption.

The losses in the form of heat are however high: this means that for higher power LEDs (such as those of car or motorbike headlights, which can consume tens of W) measures are necessary to dissipate it, usually with metal fins or fans to force cooling. A poor dissipation door to reduction of light emission it's at faultsand is the first cause of failure in the cheapest light bulbs, given the simplicity and robustness of LEDs.

The energy saving of LEDs

The characteristic low consumption is the characteristic that most of all has interested the institutions, interested in energy saving for environmental reasons but also infrastructural: this is the reason that led the EU to ban incandescent light bulbs.

Despite a higher but constantly decreasing price, an LED lamp consumes up to 60% less than a fluorescent lamp and up to 95% less than incandescent lamps: certainly an important saving for us consumers, but even more significant for the Municipalities, for which street lighting can be responsible for up to 25% of energy consumption.

In a world of energy crises and waste, LEDs now represent a precious ally for reducing greenhouse gas emissions and electricity consumption, helping us get closer to the Kyoto objectives and strengthening our efforts against climate change.