semiconductor

Physics Project Work

Semiconductor

XYZ

Class :- 12th B(Science)

Year:-2012-2013

Board’s roll no:……………………..

2012-2013

                                                            

            Semiconductors

Most of the solids can be placed in one of the two classes: Metals and insulators.  Metals are those through which electric charge can easily flow, while insulators are those through which electric charge is difficult to flow.  This distinction between the metals and the insulators can be explained on the basis of the number of free electrons in them.  Metals have a large number of free electrons which act as charge carriers, while insulators have practically no free electrons.

There are however, certain solids whose electrical conductivity is intermediate between metals and insulators. They are called ‘Semiconductors’.  Carbon, silicon and germanium are examples of semi-conductors.  In semiconductors the outer most electrons are neither so rigidly bound with the atom as in an insulator, nor so loosely bound as in metal.  At absolute zero a semiconductor becomes an ideal insulator.

             

Semiconductors

Theory and Definition

Semiconductors are the materials whose electrical conductivity lies in between metals and insulator.  The energy band structure of the semiconductors is similar to the insulators but in their case, the size of the forbidden energy gap is much smaller than that of the insulator.  In this class of crystals, the forbidden gap is of the order of about 1ev, and the two energy bands are distinctly separate with no overlapping.  At absolute o0, no electron has any energy even to jump the forbidden gap and reach the conduction band.  Therefore the substance is an insulator.  But when we heat the crystal and thus provide some energy to the atoms and their electrons, it becomes an easy matter for some electrons to jump the small (»  1 ev) energy gap and go to conduction band.  Thus at higher temperatures, the crystal becomes a conductors.  This is the specific property of the crystal which is known as a semiconductor.

Effect of temperature on conductivity                                       of Semiconductor

At 0 ⁰K, all semiconductors are insulators.  The valence band at absolute zero is completely filled and there are no free electrons in conduction band. 

INTRINSIC SEMICONDUCTORS

Pure semiconductors are called intrinsic semi-conductors.  In a pure semiconductor, each atom behaves as if there are 8 electrons in its valence shell and therefore the entire material behaves as an insulator at low temperatures.

The vacancy in the covalent bond is called a hole.This hole can be filled by some other electron in a covalent bond. In an intrinsic semiconductor, if ne denotes the electron number density in conduction band, nh the hole number density in valence band and ni the number density or concentration of charge carriers, then,

ne = nh = ni

Extrinsic semiconductors

As the conductivity of intrinsic semi-conductors is poor,

so intrinsic semi-conductors are of little practical importance. 

The conductivity of pure semi-conductor can, be increased by addition of some pentavalent or a trivalent impurity in a very small amount.They are called Extrinsic semiconductors .

They are of two types :

i)                                                n-type semiconductor

ii)                                             ii) p-type semiconductor

n-type semiconductor

When an impurity atom belonging to group V of the periodic table like Arsenic is added to the pure semi-conductor, then four of the five impurity electrons form covalent bonds by sharing one electron with each of the four nearest silicon atoms, and fifth electron from each impurity atom is almost free to conduct electricity.

 As the pentavalent impurity increases the number of free electrons, it is called donor impurity. 

In terms of valence and conduction band one can think that all such electrons create a donor energy level just below the conduction band as shown in figure.  As the energy gap between donor energy level and the conduction band is very small, the electrons can easily raise themselves to conduction band even at room temperature. 

Hence, the conductivity of n-type extrinsic semiconductor is markedly increased.

In a doped or extrinsic semiconductor, the number density of the conduction band (ne) and the number density of holes in the valence band (nh) differ from that in a pure semiconductor. 

If ni is the number density of electrons is conduction band,

then it is proved that

ne nh = ni2

p-type semiconductor

If a trivalent impurity like indium is added in pure semi-conductor, the impurity atom can provide only three valence electrons for covalent bond formation.  Thus a gap is left in one of the covalent bonds.  The gap acts as a hole that tends to accept electrons.  As the trivalent impurity atoms accept electrons from the silicon crystal, it is called acceptor impurity.Again, as the pure Si-crystal also possesses a few electrons and holes, therefore, the p-type si-crystal will  have a large number of  holes (majority carriers) and a small number of electrons (minority carriers).It terms of valence and conduction band one can think that all such holes create an accepter energy level just above the top of the valance band. Number density of valence band holes (nh) in p-type semiconductor is approximately equal to that of the acceptor atoms (Na) and is very large as compared to the number density of conduction band electrons (ne).  Thus,             

nh» Na > > ne