Semiconductors and its types

A Semiconductor is a substance which has resistivity in between conductor and insulator .Copper is a good conductor which is resistivity is 1.7*10^-8.Germanium is a semiconductor which is reistivity is 0.6 ohm meter.A Glass is a good insulator which is resistivity is 9*10^ ohm meter.A Nichrome is a resistance material which is resistivity is 10^-4 ohm meter.

Comparing the resistivity of above materials it is apparent that resistivity of germanium is quite high as compared to copper but is quite low when compared with glass.This is shows that resistivity of a semiconductor lies in between conductors and insulators.

Types of semiconductors :

The semiconductors are classifieds as followed

(i) intrinsic semiconductors

(ii)Extrinsic semiconductors

In every elements,the atoms are tied together by the bonding action of valence electrons.Si and Ge atoms contains only 4 valence electrons.These electrons have tendency to fill the outer most orbit.In this way the electrons placed to in the last orbit of an atom share the electrons with their neighbor atoms. Similarly all the electrons are tied together with their neighboring atoms.

(i)Intrinsic Semiconductors :

A pure semiconductor is called intrinsic semiconductor.The silicon and Ge atoms contain only four electrons in the outermost orbit.So they are called tetravalent bonds.In an intrinsic semiconductors even at room temperature hole pair are created.

When electric field is applied across an intrinsic semiconductor ,the current conduction takes same place by two process which is free electrons and holes.Now no electrons get away from the co-valent bonds.So current flows is zero.

At room temperature some valance electrons may acquire sufficient energy.The bonds may be broken,the electrons become free and are shifted to conduction band.

(ii) Extrinsic Semiconductors :

The electrical conductivity of pure semiconductors is increased by adding some impurities in it.The resultant semiconductor is called extrinsic semiconductor.The process of adding impurities to a semiconductor is known as doping.The purpose of adding impurities in the pure semiconductor is to increase the number of free electrons or holes for increasing their conductivity.

The extrinsic semiconductor are divided into two types.They are 

1.n-type semiconductors

2.p-type semiconductors

N-type semiconductors :

N-type semiconductors are formed by adding a small amount of pentavalent impurities to a semiconductor material.The added the impurities are called donar impurities because they will donate electrons.Ge atom has four valence electrons and antimony atom forms co-valent bonds with their surrounding four Ge atoms.The co-valent bond structure and energy band diagram of N-type semiconductor is shown in fig.

This is loosely bound electrons can be easily excited from valence band to the conduction band by the application of small electric field.The extra electron creates impurity because it can donate one electron for conduction.By the addition of pentavalent impurities increases the number of electrons in the conduction band,there by increasing the conductivity of the semiconductor.Now the semiconductor contains more electrons and less holes.Hence it is called N-type semiconductor.

P-type Semiconductor :

The p-type semiconductor is formed by adding a small amount of trivalent impurities to a pure semiconductor material.Three valence electrons in aluminium from co-valent band  with four surrounding atoms of Ge.Now co-valent bond is incomplete,which gives rise to a hole.The co-valent bond structure and energy band diagram are shown in fig   

For more number of holes are generated.The holes increase the conductivity of p-type semiconductor.This impurity is known as acceptor impurities,because the holes can created accept electrons.The number of holes is more than the number of electrons.

In P-type semiconductor holes are majority carriers and electrons are minority carriers.