User:Henry sarker

Classification of semiconductors :- Semiconductors are classified in to two types

(i) Intrinsic semiconductors  (ii) Exterinsic semiconductors n – type                p – type semiconductor       semiconductor Intrinsic semiconductor :-

A semiconductor in an extremely pure form is known as an intrinic semiconductor.

→ An Intrinfic semiconductor, even at room temperature, hole-electron pairs all created. → When electric field is applied across an semiconductor intrinisic semiconductor, the current conduction takes place by two process, namely by free electrons and holes. → Free electrons are produced due to the breeding up of fome co-valent bonds by thermal energy. At the same time holes are created in the co-valent bond itself.

→ When electric field is applied across the semiconductors material electrons will move towards the possitive terminal of supply, holes will move towards negative terminal of the supply. → Thus current conduction inside this intrinific semiconductor material is due to movement of holes & electrons. → But the current in the external wire is only because of electrons. Since while applying electric field, holes are attracted towards negative terminal. There one new electron is introduced. This electron will combine with the hole, thus cancelling them. → At the same time electrons are moving towards positive terminal, while leacing from this intrinisic material it leaves a hole. Again this holes are attracted towards negative terminal.

Extrinisic semiconductor :- → The current conduction capability of intrinisic semiconductor is very low at rom temperature. So we can not use it in electric devices. → Hence the current conduction capability must be increased. This can be achieved by adding impurities to the intrinisic semiconductor. So that it become impurity semiconductor (or) Extrinisic semiconductor. → The process of adding impurity is known as doping. → The amount & type of impurities have to be closely controlled during the preparation of extrinisic semiconductor. Generally, for 108 atoms of semiconductor, one impurity atom is added. → The purpose of adding impurity is to increase either the number of free electrons or holes in the semiconductor crystal. → If the pentavalent impurity is adding to the semiconductor, a large number of free electrons are produced in the semiconductor. → On the other hand if the trivalent impurity is added it introdued large number of holes. → Depending upon the type of impurity added, extrisic semiconductors are classified in to			(i) n – type Semiconductor         (ii) p – type Semiconductor

n – type Semiconductor :- → When a pentavalent impurity is added to a pure semiconductor, it is known as a n – type semiconductor. → Eg ; For pentavalent impurities are Arsenic & Antimony. → Addition of pentavalent impurity provides a large number of free electrons in the semiconductor material. → Such impurities which produce n – type semiconductor material is known as doner impurities.