The composition of some other LEDs is a cathode lead, gold wire, heatsink slug, silicon sub-mount chip with ESD protection, solder connection, InGaN semiconductor flip chip, silicone encapsulent and a plastic lens, as well as Indium Gallium Nitride (InGaNi). 

Impurities are introduced later in the manufacturing process; unlike imperfections, they are added on purpose to make the LED work correctly.  This process is called doping.  The impurities generally added are zinc or nitrogen, but silicon, germanium, and tellurium can also been used.  As mentioned before, they will cause the semiconductor to conduct electricity and will make the LED function as an electronic device.  It is because of the impurities that a layer with an abundance or lack of electrons can be created.

To finish the device, it is necessary to bring electricity to it and from it.  So wires must be added onto the substrate.  These wires must adhere well to the semiconductor and be strong enough to handle further processing such as soldering and heating.  Gold and silver compounds are most commonly used for this because they form a chemical bond with the gallium at the surface of the semiconductor. 

LEDs are encased in transparent plastic usually epoxy.  The plastic can be of many varieties and its exact optical properties will determine the output of what the LED looks like.  Some plastics may cause the light to scatter in many directions, some will direct the light straight out in a narrow beam and if the plastic is tinted it will change the color of the light.