Fabrication of a p-n junction diode

In the earlier description, a p-n junction was described, for simplicity, as though it were made by physically fusing together two separately-grown p-type and n-type crystals. In practice, that is not how it is done: a p-n junction is instead grown from a SINGLE crystalline structure of silicon or germanium, into which carefully controlled amounts of donor impurity are diffused in from one side and acceptor impurity from the other, while the crystal is heated in a furnace. As these impurities diffuse in from either side, electrons and holes combine near the centre of the crystal, and the depletion region -- and with it, the junction -- develops naturally at that boundary. Once the junction has formed, electrodes are attached after the crystal is cut into thin transverse sections, and hundreds of individual diodes can be prepared this way from a single grown crystal; in fact, essentially all semiconductor devices, including entire integrated circuits, are fabricated by 'growing' junctions at the locations where they are needed, rather than by physically joining separate pieces. …