Diffusion is a process by which a heavy concentration of particles diffuse into a surrounding region of lower concentration. The main difference between the diffusion and alloy process is the fact that liquefaction is not reached in the diffusion process. In the diffusion process heat is applied only to increase the activity of elements involved. For formation of such diodes, either solid or gaseous diffusion process can be employed. The process of solid diffusion starts with formation of layer of an acceptor impurity on an N- type substrate and heating the two until the impurity diffuses into the substrate to form the P-type layer, as illustrated in figure. A large P-N junction is divided into parts by cutting process. Metallic contacts are made for connecting anode and cathode leads.
In the process of gaseous diffusion instead of layer formation of an acceptor impurity, an N- type substrate is placed in a gaseous atmosphere of acceptor impurities and then heated. The impurity diffuses into the substrate to form P- type layer on the N- type substrate. Though, the diffusion process requires more time than the alloy process but it is relatively inexpensive, and can be very accurately controlled. The diffusion technique leads itself to the simultaneous fabrication of many hundreds of diodes on one small disc of semiconductor material and is most commonly used in the manufacture of semiconductor diodes. This technique is also used in the production of transistors and ICs (integrated circuits).