LEDs or Light Emitting Diodes are made from very thin layers of semiconductor material. One layer has an excess of electrons, while the next would have a deficit. This difference causes the electrons to move from one layer to another, thereby generating light. Manufacturers now have the technology to make these layers as thin as .5 micron or less. A micron is equivalent to a ten-thousandth of an inch.
Impurities within the semiconductor create the required electron density. A semiconductor is a crystalline material that conducts electricity only when there’s high density of impurities. A slice or wafer of semiconductor is a single uniform crystal, and the impurities are later introduced during the process or manufacture. The particular semiconductors used for LED manufacture are gallium arsenide (GaAs), gallium phosphide (GaP), or gallium arsenide phosphide (GaAsP). The different semiconductor materials called substrates and different impurities result in different colours of light from the LED.
Impurities unlike imperfections are introduced deliberately for the desired LED function, a process is called doping. The impurities commonly introduced are zinc or nitrogen, but silicon, germanium, and tellurium have also been used. They cause the semiconductor to conduct electricity and make the LED function as an electronic device. It’s through impurities that a layer with an excess or a deficit of electrons is created.
To complete the function, it is necessary to introduce electricity to it and from it for which, wires must be attached onto the substrate. These wires need to stick well to the semiconductor and give enough strength to withstand subsequent processes such as soldering and heating. Gold and silver compounds are the most commonly used for this purpose as they form a chemical bond with the gallium at the surface of the wafer.
One method of adding the necessary impurities to the semiconductor crystal is to grow additional layers of crystal onto the wafer surface. In this process, known as ‘Liquid Phase Epitaxy’, the wafer is put on a graphite slide and passed underneath reservoirs of molten GaAsP. Contact patterns are exposed on the surface of the wafer using photoresist, after which the wafers are put into a heated vacuum chamber. In the vacuum chamber, molten metal is evaporated onto the contact pattern on the wafer surface.
LEDs are encased in transparent plastic, similar to the lucite paperweights that have objects suspended within. The plastic could be of multiple varieties, and its exact optical properties would determine the output of the LED. Some plastics are diffusive, which means the light will scatter in many directions. Some are transparent, and can be shaped into lenses that would give a focussed light directed straight out from the LED in a narrow beam. The plastics can be tinted, which would change the colour of the LED by allowing more or less of light of a particular colour to pass through.
Newer materials are now being developed that would someday allow fabrication of blue and white light LEDs. Additional colours would certainly open up new applications.