pn Junction

When a p-type semiconductor is suitably joined to n-type semiconductor, the contact
surface is called pn junction.

Most semiconductor devices contain one or more pn junctions. The pn junction is
of great importance because it is in effect, the control element for semiconductor
devices. A thorough knowledge of the formation and properties of pn junction can
enable the reader to understand the semiconductor devices.

Formation of pn junction. In actual practice, the characteristic properties of pn junction
will not be appar if a p-type block is just bring in contact with n-type block. In fact,
it is fabricate by special techniques. One common method of making pn
junction is call alloying. In this method, a small block of indium (trivalent impurity) is
placed on an n-type germanium slab as shown in Fig. 1. The system is then
heated to a temperature of about 500 0 C.

The indium and some of the germanium melt
to form a small puddle of molten germanium-indium mixture as shown in Fig.1.
The temperature is then lower and puddle begins to solidify. Under proper conditions,
the atoms of indium impurity will be suitably adjusted in the germanium slab to form a
single crystal. The addition of indium overcomes the excess of electrons in the n-type
germanium to such an extent that it creates a p-type region.
As the process goes on, the remaining molten mixture becomes increasingly rich
in indium. When all germanium has been redeposit, the remaining material appears as
indium button which is frozen on to the outer surface of the crystallized portion as
shown in Fig.1. This button serves as a suitable base for soldering on leads.

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