Why do so many old displays/indicators particularly use red lights? Are/Were white diodes harder to create?
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Hi, I did my masters degree research on the fabrication and testing of blue and white (polymer) LEDs and then subsequently did doctoral research on thin-film solar panels (which are like an LED in reverse).
As many others have said, the reason that old tech uses red LEDs over other colours is simply that they were invented first.
We now ask, “why were they invented first?” And the answer to that, is that they were the easiest ones to invent.
You probably know that some things conduct electricity (like metals) and some things don’t (like wood and plastic). Electricity is caused by the movement of a tiny particle called an electron.
Think about going to the cinema or theatre. You have a row of seats. If the row of seats is full of people, it’s very difficult, if not impossible to get along it. This is like electrons trying to move through plastic. All the electrons in plastic are held in place like people in cinema seats. Moving them around means that people have to stand up and sit down so it’s very difficult.
In metals, though, there’s like a walkway *above* the seats like a bridge. The seats are all completely full, but you can walk along easily by going on the walkway. Plastics also have this walkway, but it is just far to high for you to get on to.
Now we come to a special material that’s halfway between a conductor and an insulator. They have *quite* full rows (not completely), with a few gaps, but also a walkway above that’s a little bit higher than in metals, but not quite as high as insulators, so it’s a bit more difficult to move around than in metals, but not as difficult to move around as in plastic. These materials are called “semiconductors”.
In a semiconductor there’s two ways to move around. Either you get the people in the seats to budge along one into the empty seats OR you get a load of energy and jump up onto the walkway from their seat and walk along. This lets them move freely, and makes a space in the seats that lets the electrons down below move a little bit easier too.
So, how do electrons get energy? Well, they can absorb light, for one, and they have to absorb one “photon” of light at a time, because that’s how light comes – in little packets called photons. Think about a rainbow, red orange yellow green blue indigo violet. As you go through that list, you are getting photons that have more and more energy. Red has the least, violet has the most.
So an electron can absorb a photon and jump up to the higher level. The further to the blue end of the rainbow that the photon is, the higher the electron can jump. If you have a material with a big gap between the floor and the walkway, it will only absorb blue, indigo and violet photons. If you have a material with a small jump, it will absorb all of them – as long as they have enough energy to jump higher than the walkway, they get absorbed.
The height of the walkway is called the “band gap” by scientists.
This is the basic principle behind solar panels that absorb light and give out electrical energy. They have a low band gap, so that they can absorb all the different light photons and make lots of current.
Now what about LEDs? They’re like solar panels, but backwards. You put in electrons on the walkway and let them fall down and *give off* light. Now, the colour of the light corresponds to the distance they fall. Fall a little way, red light (low energy) given off. Fall a long way, blue light (high energy) given off.
So why did we get red LEDs first? Well basically it is really difficult to make materials with a high band gap and consistently get them to walk along it to the right place, then fall into an empty seat below. A low walkway is much more stable and easier to get the electrons to walk along without falling off in the wrong place. A big band gap is inherently difficult to make – which is also why we invented solar panels with their low band gaps before we invented LEDs. In fact, if you take a solar panel and put it in a dark room, you can use it as an LED by forcing current through it and making it run backwards. Usually the band gap is pretty low. So the light is either red or infra red (even lower energy than red light and invisible to humans).
Edit: I realised that there is an inaccuracy in my description: for metals the walkway is at the same level as the seats, so you don’t have to jump to get up there at all. It’s sort of like the row is just wide and you can walk past all the other movie patrons without bumping into their knees very much (you do a little bit).
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