Semiconductor Electronics

The best conductors(silver, copper, gold)have one valence electron, whereas the best insulators have eight valence electrons.

Definition:

A semiconductor is an element with electrical conductivity between those of an insulator and a conductor.

Example of semiconductors

The most useful semiconductors used in laboratory or in solid state devices(i.e.p-n junction,transistor) are germanium(Ge,atomic number 32)and silicon (Si,atomic number 14). They belong to IV group of periodic table.Hence each atom of Ge and Si have four valence electrons in its outer most(i.e. valence shell) orbit.

Characteristics of semiconductor

With increase in temperature ,the resistance of semiconductor is found to decrease .So their electrical conductivity will also increase .Hence all the semiconductors will have negative temperature coefficient.

Types of Semiconductors

Intrinsic semiconductors: The pure semiconductors in which electrical conductivity is due to their own charge carriers like electrons and holes.

Extrinsic semiconductors :The materials (i.e. the semiconductors doped with impurity elements of fifth or third group)whose electrical conductivity is due to charge carriers created or donated by impurity elements.

Crystal structure of intrinsic semiconductors

When two Ge atom are at a certain distance between them ,then positive core or nucleus of one Ge- atom will attract the valance electron of other Ge - atom and vice-versa.so the two valence electrons will be shared by two Ge- atoms forming the attractive covalent bond s. The same process repeats for each pair of Ge-atoms and the crystal structure of Pure Ge formed. Thus in the crystal structure, each Ge-atom acts as if there are 8 valence electrons in the outer shell. So free electrons will not be available for electrical conductivity .Hence at very low temperature ,pure crystal of Ge or Si will not conduct electricity and it will act as insulator or bad conductor.

Conductivity of intrinsic semiconductor

Each atom of pure semiconductor requires energy of 1.1 eV to make free its valence electron .This energy can be available at room temperature .so due to thermal agitation, covalent bonds will break and electrons will be free from covalent bonds .So for broken covalent bond, there will be a vacancy or absence of electron called as hole. This hole is positive in charge and will require an electron from neighbouring covalent bond .Hence hole will be created in the neigbouring bond. So we conclude that holes and electrons will shift alternately from one covalent bond to another .Hence at room temperature , pure semiconductor will have electrons and holes wandering in random directions. They are called as intrinsic charge carriers . If we apply potential difference across such a intrinsic semiconductor ,the electrons will move through it towards positive terminal .Hence the holes will move towards the negative terminal of the battery .So electrical conductivity is observed. Hence the conventional current is in the direction of motion of holes.

Types of Extinsic semiconductor

When the 5th group element or 3rd group element is doped as a impurity in semiconductor crystal ,the four valence electrons of the impurity will form covalent bonds with neighbouring semiconductor atom forming excess free Electrons or holes which causes conductivity while applying potential difference.

n-type extrinsic semiconductor: when 5th group element is doped as an impurity in the semiconductor crystal.

p-type extrinsic semiconductor: when 3rd group element is doped as an impurity in the semiconductor crystal.