project on semiconductors

8/11/2019 Project on Semiconductors

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Submitted By

:-

G.Kavi Chandra

Class: XII


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Roll No : 19

Kendriya Vidyalaya No.2 Uppal

CertificateThis is to certify that G.Kavi Chandra

student of Class XII, Kendriya Vidyalaya No.2 Uppal, hascompleted the project titled SemiConductors during theacademic year 2014-2015 and submitted satisfactoryreport, as compiled in the following pages, under mysupervision.

_______________ _________________ _________________


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cknowledgementI would like to express my special thanks of

gratitude to my teacher Mr N.V.N.G.K Rao who

gave me the golden opportunity to do this

wonderful project on the topic SemiConductors ,

which also helped me in doing a lot of Research

and i came to know about so many new things I

am really thankful to them.

Secondly i would also like to thank my parents

and friends who helped me a lot in finalizing this

project within the limited time frame.


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INTRODU TION

Semiconductors :- Most of the solids can be placed in one ofthe two classes: Metals and insulators. Metals are thosethrough which electric charge can easily flow, whileinsulators are those through which electric charge is difficultto flow. This distinction between the metals and the insulatorscan be explained on the basis of the number of free electrons

in them. Metals have a large number of free electrons whichact as charge carriers, while insulators have practically no

free electrons.There are however, certain solids whose electricalconductivity 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 rigidlybound with the atom as in an insulator, nor so loosely boundas in metal. At absolute zero a semiconductor becomes anideal insulator.

Theory and Definition

Semiconductors are the materials whose electricalconductivity lies in between metals and insulator. The


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energy band structure of the semiconductors is similar to theinsulators 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, andthe two energy bands are distinctly separate with nooverlapping. At absolute o0, no electron has any energy evento jump the forbidden gap and reach the conductionband. 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 goto conduction band. Thus at higher temperatures, the crystalbecomes a conductors. This is the specific property of thecrystal which is known as a semiconductor.

Effect of temperature on conductivity ofSemiconductor

At 0K, all semiconductors are insulators. The valence bandat absolute zero is completely filled and there are no freeelectrons in conduction band. At room temperature theelectrons jump to the conduction band due to the thermalenergy. When the temperature increases, a large number ofelectrons cross over the forbidden gap and jump from valence

to conduction band. Hence conductivity of semiconductorincreases with temperature.

INTRINSIC SEMICONDUCTORS

Pur e semi conductors ar e cal l ed int r i nsi c semi -

conductors. In a pure semiconductor, each atom behaves asif there are 8 electrons in its valence shell and therefore theentire material behaves as an insulator at low temperatures.


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(about 1 to 106 parts of the semi-conductor). The process ofadding an impurity to a pure semiconductor so as to improveits conductivity is called doping. Such semi-conductors are

called extrinsic semi-conductors. Extrinsic semiconductorsare of two types :

i) n-type semiconductor ii) p-type semiconductor

n type semiconductor

When an impurity atom belonging to group V of the periodictable 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 siliconatoms, 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 donorimpurity. The electrons so set free in the silicon crystal arecalled extrinsic carriers and the n-type Si-crystal is called n-type extrinsic semiconductor. Therefore n-type Si-crystal willhave a large number of free electrons (majority carriers) andhave a small number of holes (minority carriers).

In terms of valence and conduction band one can think that

all such electrons create a donor energy level just below theconduction band as shown in figure. As the energy gapbetween donor energy level and the conduction band is very

small, the electrons can easily raise themselves to conductionband even at room temperature. Hence, the conductivity of n-type extrinsic semiconductor is markedly increased.

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


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semiconductor. If ni is the number density of electrons isconduction band, then it is proved that

ne nh = n i2

p type semiconductor

If a trivalent impurity like indium is added in pure semi-conductor, the impurity atom can provide only three valenceelectrons for covalent bond formation. Thus a gap is left inone of the covalent bonds.The gap acts as a hole that tends to accept electrons. As thetrivalent impurity atoms accept electrons from the siliconcrystal, it is called acceptor impurity. The holes so createdare extrinsic carriers and the p-type Si-crystal so obtained iscalled p-type extrinsic semiconductor. 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 (majoritycarriers) 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 topof the valance band as shown in figure. The electrons fromvalence band can raise themselves to the accepter energylevel by absorbing thermal energy at room temperature and inturn create holes in the valence band.

Number density of valence band holes (nh) in p-type

semiconductor is approximately equal to that of the acceptoratoms (Na) and is very large as compared to the numberdensity of conduction band electrons (ne). Thus,

nh N a > > n e

electrical resistivity of semiconductors

Consider a block of semiconductor of length l1 area of cross- section A and having number density of electrons and holes as


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The End

project on semiconductors

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