196183266 Physics Project

8/12/2019 196183266 Physics Project

1/23

Project Prepared By:

Sushruta Dey

XII A

Roll Number :46Boards Roll Number:

Kendriya Vidyalaya Fort William


2/23

AIM

To study and

understand the working

of a semiconductor.


3/23

CERTIFICATE

It is hereby to certify that, the originaland genuine project work has been carried

out to study about the subject matter and

the related data collection and

investigation has been completed solely,

sincerely and satisfactorily by Sushruta

Dey of CLASS XII A, Kendriya Vidyalaya

Fort William , regarding his project titled

N type ,P Type and working theory of

semiconductors.

TeachersSignature


4/23

Acknowledgement

It would be my utmost pleasure to express

my sincere thanks to My Physics Teacher

Mrs. J Sahooand our dearest Mr. A.K Das

Sir in providing a helping hand in this

project. Their valuable guidance, supportand supervision all through this project titled

N type ,P Type and working theory of

semiconductors.are responsible for

attaining its present form.

Sushruta Dey

XII A


5/23

PURPOSEIn recent days, Semiconductors are the most

used things which are used in electronics, so I

wanted to know its working.

Another fact which inspired me to do this

project is that I am in touch with qualitativeanalysis whose knowledge with other factors

helped me to do so.


6/23

CONTENTS

IntroductionTheory and DefinitionEffect of temperature on conductivity

of Semiconductor

Intrinsic SemiconductorsN-type SemiconductorP-type SemiconductorElectrical Resistivity of

Semiconductors


7/23

INTRODUCTION

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 freeelectrons.

There are however, certain solids

whose electrical conductivity is


8/23


9/23

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, noelectron has any energy even to jump

the forbidden gap and reach the

conduction band. Therefore the

substance is an insulator. But when we


10/23

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.


11/23

Effect of temperature on

conductivity of Semiconductor

At 0K, all semiconductors are

insulators. The valence band at absolute

zero is completely filled and there are no

free electrons in conduction band. At

room temperature the electrons jump to

the conduction band due to the thermal

energy. When the temperature

increases, a large number of electrons

cross over the forbidden gap and jump

from valence to conduction band. Hence

conductivity of semiconductor increases

with temperature.


12/23

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.

A semiconductor atom needs energy

of the order of 1.1ev to shake off the

valence electron. This energy becomes

available to it even at room

temperature. Due to thermal agitation

of crystal structure, electrons from a few

covalent bonds come out. The bond

from which electron is freed, a vacancy


13/23

is created there. The vacancy in the

covalent bond is called a hole.

This hole can be filled by some other

electron in a covalent bond. As an

electron from covalent bond moves to fill

the hole, the hole is created in the

covalent bond from which the electron

has moved. Since the direction of

movement of the hole is opposite to thatof the negative electron, a hole behaves

as a positive charge carrier. Thus, at

room temperature, a pure

semiconductor will have electrons andholes wandering in random directions.

These electrons and holes are called

intrinsic carriers.


14/23

As the crystal is neutral, the number

of free electrons will be equal to the

number of holes. 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


15/23

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, however beenormously increased by addition of some

pentavalent or a trivalent impurity in a

very small amount (about 1 to parts

of the semi-conductor). The process ofadding an impurity to a pure

semiconductor so as to improve its

conductivity is called doping. Such semi-

conductors are called extrinsic semi-conductors. Extrinsic semiconductors are

of two types :

i) n-type semiconductor

ii) p-type semiconductor


16/23

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. The

electrons so set free in the silicon crystal

are called extrinsic carriers and the n-

type Si-crystal is called n-type extrinsic

semiconductor. Therefore n-type Si-

crystal will have a large number of free


17/23

electrons (majority carriers) and have 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 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


18/23

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=


19/23

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. The holes so

created are extrinsic carriers and the p-

type Si-crystal so obtained is called p-

type extrinsic semiconductor. Again, as

the pure Si-crystal also possesses a few

electrons and holes, therefore, the p-type


20/23

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 as shown in

figure. The electrons from valence band

can raise themselves to the accepter

energy level by absorbing thermal energy

at room temperature and in turn create

holes in the valence 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


21/23

compared to the number density of

conduction band electrons (ne). Thus,

nh>> Na> > ne


22/23

Electrical Resistivity of Semiconductors

Consider a block of semiconductor of length l1area

of cross-section A and having number density ofelectrons and holes as neand nhrespectively. Suppose

that on applying a potential difference, say V, a currentI flows through it as shown in figure. The electron

current (Ic) and the hole current (Ih) constitute the

current I flowing through the semi conductor i.e.

I=Ie+Ih (i)

It neis the number density of conduction band

electrons in the semiconductor and ve, the drift velocity

of electrons thenIe= eneAve

Similarly, the hole current,Ih= enhAvh

From (i)I = eneAve+ enhAvh

I = eA(neve+ nhvh) (ii)

If is the resistivity of the material of thesemiconductor, then the resistance offered by the

semiconductor to the flow of current is given by :R = l/A (iii)

Since V = RI, from equation (ii) and (iii) we have

V = RI = l/A eA (neve + nhvh)

V= le(neve+nhvh) (iv)


23/23

If E is the electric field set up across the semiconductor,then:

E=V/l (v)

from equation (iv) and (v), we haveE = e (neve+ nhvh)

1/ = e (neve/E + nhvh/E)On applying electric field, the drift velocity

acquired by the electrons (or holes) per unit strength ofelectric field is called mobility of electrons (or

holes). Therefore, mobility of electrons and holes isgiven by :

e= ve/E and h= vh/E

1/ =e(ne e+nh h) (vi)Also, = 1/ is called conductivity of the material

of semiconductor

=e(ne e+nh h) (vii)The relation (vi) and (vii) show that the

conductivity and resistivity of a semiconductor depend

upon the electron and hole number densities and their

mobilities. As neand nhincreases with rise in

temperature, therefore, conductivity of semiconductor

increases with rise in temperature and resistivitydecreases with rise in temperature.

Documents

196183266 Physics Project