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What is a PN Junction and how is it formed?

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- 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 apparent if a p-type block is just brought in contact with n-type block. In fact, pn junction is fabricated by special techniques. One common method of making pn junction is called alloying. In this method, a small block of indium (trivalent impurity) is placed on an n-type germanium slab as shown in Fig. 2.18 (i). The system is then heated to a temperature of about 500ºC. The indium and some of the germanium melt to form a small puddle of molten germanium-indium mixture as shown in Fig. 2.18 (ii). The temperature is then lowered 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 redeposited, the remaining material appears as indium button which is frozen on to the outer surface of the crystallized portion as shown in Fig. 2.18 (iii). This button serves as a suitable base for soldering on leads.

Wilfykil answered the question on August 14, 2019 at 05:28

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What is a PN Junction and how is it formed?

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