There are several types of semiconductor diode fabrication: 1. Grown Junction Diode:Diodes of this type are formed during the crystal pulling process. P and N-type impurities can be alternately added to the molten semiconductor material in the crucible, which results in a P-N junction when crystal is pulled. 2.Alloy Type or Fused Junction Diode:Such a diode is formed by first placing a P- type impurity (a tiny pellet of aluminium or some other P- type impurity) into the surface of an N-type crystal and heating the two until liquefaction occurs where the two materials meet. An alloy will result that on cooling will give a P-N junction at the boundary of the alloy substrate.
3. Diffused Junction Diode: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.
4. Epitaxial Growth or Planar Diffused Diode. To construct an epitaxially grown diode, a very thin (single crystal) high impurity layer of semiconductor material (silicon or germanium) is grown on a heavily doped substrate (base) of the same material. This complete structure then forms the N- region on which P- region is diffused.
5. Point Contact Diode.A barrier layer is formed round the point contact by a pulsating current forming process. This causes a P-region to be formed round the wire and since pure germanium is N-type, a very small P-N junction in the shape of a hemisphere is formed round the point contact. The forming process cannot be controlled with precision. Because of small area of the junction, point contact diode can be used to rectify only very small currents.