||Thorn Lighting was one of the early pioneers of LED research and manufacturing. Its first products appeared in the 1972 catalogue and a large range quickly developed, but by the later 1970s these operations were abandoned. It soon became clear that the extremely low light output was not really compatible with the company's lighting business, and the much greater resources of the larger semiconductor companies were required to make significant improvements.
All Thorn LEDs are based on Gallium Arsenide Phosphide, the As:P ratio being adjusted to influence the colour of light from red through yellow to green. The base crystal is made of GaAs upon which an n-type GaAsP epitaxial layer of the desired As:P ratio is grown. Its surface is doped with zinc to form the p-type layer. Individual diodes are constructed by etching away part of the surface to leave an array of raised circular plateaus called mesas. Gold contacts are metallised across the back face of the array of diodes, and a small dot is deposited at the centre of each mesa. The crystal is finally sliced into multiple small dice. Each die is brazed onto a TO-18 transistor header, and a gold wirebond makes the connection from the top metallisation to the terminal post - note the crude early weld here. The whole is encapsulated in transparent epoxy to maximise light extraction and afford the necessary mechanical protection.
The mesa construction marks a considerable improvement over the ring-shaped metallisation on earlier devices such as Monsanto's MV-1. Energy in the older design was wasted by light emission even in the area under the metallisation, but with the mesa arrangement, the only light emission that is blocked is the tiny area under the gold wirebond.