Anyone who has been fortunate enough to have a clear sky in the last few days has seen a rare astronomical phenomenon with special symbolism: the connection between Jupiter and Saturn. The planets were so close to the sky, when viewed from Earth, that they looked like a single star. For some experts, such a connection is the origin of the legend of the Star of Bethlehem, the leader of the kings to the newborn Jesus Christ.
But even if this is not the Christmas star – and although there never was a specific star – the fact is that the legend, along with the idea of Jesus as a light that would have come to enlighten mankind, is the origin of the Christmas tradition of lighting trees, houses and streets.
Christmas lights date back to before electricity was discovered. In 1882, Edward Hibberd Johnson replaced the previously used candles with incandescent lamps in order to advertise the latest invention of his friend and partner Thomas Edison. Edison’s lamps eliminated fire hazards, and since the 2000s, wires with hundreds of LEDs have replaced incandescent lamps with energy savings and longer life.
LED is an abbreviation for light emitting diode. They are semiconductor materials that emit light when exposed to an electrical current, a property known as electroluminescence. For this reason, the starting point in the history of LEDs was laid in 1907 when the Englishman Henry Joseph Round demonstrated electroluminescence. But it wasn’t until 1962 that the American Nick Holonyak Jr., who worked in the laboratories of General Electric, produced the first LED with emitting visible light and a brightness that could be applied. These first LEDs emitted red light, a signature of the semiconductor, which is essentially the combination of gallium and arsenic.
The light emission on the LEDs – in addition to the visible spectrum, also in the infrared and ultraviolet – occurs through the interaction between electrons and holes, a part of physics that, at least in my time, emerged from science classes. To get an idea of what it is, we can use a more familiar analogy of the atom as a planetary system.
In this model, electrons orbit a nucleus made up of protons and neutrons at defined energy levels, the orbitals, and between them we have forbidden levels where the electron cannot be. There are a limited number of electrons that can occupy a given level of energy, and they always occupy the lowest possible level first, closest to the nucleus.
When we move from isolated atoms to solids made up of several organized atoms – from particle physics to so-called solid state physics or to condensed matter – the energy levels of these atoms interact and form energy bands. Here, too, electrons can circulate in different bands, but there are forbidden bands. Another important classification is the valence band – the highest band that is completely filled with the corresponding electrons, inert – and the conduction band – the band directly above that where free electrons are located.
The fundamental difference between conductive and insulating materials lies in the energy that electrons need to transpose this barrier between the valence and conduction bands, i.e. to cross the forbidden band (also known from English as the gap).
Semiconductor materials are again in the middle of the path and behave like conductors or insulators depending on the conditions. In them the electrons leave the absence band in the valence band when passing from one band to the other, and it is a so-called hole. Holes behave like a positively charged particle and move like the electron and contribute to the current.
LEDs are a specific type of semiconductor material called a diode. The diodes combine semiconductors mixed with other elements (the word used for this mixture is doping), which results in one side being full of free electrons and the other being filled with corresponding holes. When an electrical current is applied, it is the interaction between electrons and holes that causes light to be emitted, and the color and brightness of that light depends on the energy the electrons need to cross the forbidden band.
This energy in turn depends on the material used. After the first red and not very bright LED, in the years that followed, new materials and combinations between them were tested in search of more colors and brightness.
Despite the joy of Christmas lights, this was not the application that awarded the inventors of the first blue LED the Nobel Prize in Physics in 2014. LED applications go much further and are now replacing incandescent lamps in private homes and even in public lighting for entire cities.
To reach this point, white light was needed, which was obtained by combining red, green and blue light emitting LEDs. Red and green LEDs have been around since the 1960s, but it wasn’t until 1990 that the Japanese Isamu Akasaki, Hiroshi Amano and Shiji Nakamuro synthesized a blue light-emitting gallium nitride diode.
From then on, the Christmas lights became more colorful and the cities with less energy consumption were illuminated, but light pollution also increased. Therefore, at this moment, I do not wish the readers of Syntheses a Christmas party with lots of light, but rather the balance between the joy of the Christmas lights and the darkness that is necessary so that we can see the stars above us.