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Electron movement in Semiconductors

As we saw on the previous page, electrons at higher orbits that drop back to lower orbits, release photons of light. In LED semiconductors, a similar strategy is at work.

Electrons jump from higher energy states to lower energy states.

LED semiconductors have two layers. The top semiconductor layer, called n-type, is modified to have extra free electrons. The bottom layer, called p-type, is also modified, but to have positively charged “electron holes” (but not really “holes”, more like pockets), which can attract free electrons.

When current is passed through these two layers, the electrons and holes move toward each other – when an electron is close enough to a hole, it falls into it, going from a high-energy state to a low state (hole) and as we already know, this releases a photon.

Blue + Yellow = White

Fluorescent Yellow

Different materials used for the semiconductor will produce different wavelengths of light – however, if LEDs use AsGaAl, the resulting light will be bluish, which would not fare well as a white light. So, on top of this type of semiconductor you will find a yellow film. This film is made from a Yellow Fluorescent Powder – but don’t think of it as just a filter – it makes light by itself. If you remember how Fluorescent Lights work – they use UV light to make light. It’s the similar process with LEDs.

  1. The blue light, emitted from the semiconductor, excites the electrons in the Fluorescent film to a higher orbit. The electrons then return to their natural orbit, emitting a yellow light.
  2. This yellow light mixes with the blue light (which still is coming from the semi-conductor to make white light – this process is called “color mixing”, which we will talk about later.

Early experiments (around 1955) found that low levels of infrared light could be detected when running an electrical cur- rent through Gallium Arsenide, a semiconductor material. Throughout the years, other materials have also been added (e.g. Nitrides, Aluminum) to further upgrade the viability of semiconductors as a practical LED light source

Perfecting LEDs

Actually, pure AlGaAs is a poor conductor of electricity – how- ever, you can add impurities, a process called “Doping”. Doping adds more free electrons to the semiconductor material (n- type). You can also use Doping to create more electron holes in the material (p-type). This entire process of making semi-conductors is not always an exact science, and the final product is not always predictable.

Why not use plastic?

Why not use wood, glass, cotton or plastic? It’s because of the molecular structure. In some materials, it is difficult to manipulate the electron because of the electron’s attachment to its nucleus – if you can’t move an electron, you can’t create light from it. 

Why is the Semiconductor light blue?

LED semiconductors are known to produce a blu-ish light. The color of light from any atom de- pends on how far the electron falls from its higher energy state – the farther the fall, the more energy released and the higher the frequency of light (low frequency = more reds, higher frequencies = more blue). Aside from AlGaAs, other materials such as Indium, Phosphide, Nitrides, Silicon, are used because their molecular structure can pro- duce other colors of light.

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