mayank's presentation of transistor as a switch 1

8/10/2019 Mayank's Presentation of Transistor as a Switch 1

1/33

CERTIFICATE

This is to certify that the investigatory Project entitled

WORKING OF A TRANSISTOR AS A SWITCH is an original

work done byAnuj Pandey

During 2004-05 under the supervision and guidance of

f

!r" #dwin !rs" $arita %nil

&enue' $t" !ary(s )nter *ollege

Date'


2/33

ACKNOWLEGEMENT

) %nuj Pandey of standard +)) of $cience $trea, $t" !ary(s

)nter *ollege #tawah is genuinely thankful to ,y teacher !r"

#dwin for their help and able guidance which ) received for the

co,pletion of this project"

./1)3 T%$)$T %$ $/)T*6

) a, also thankful to the principal ,y co-student and parents

who have always given ,e their support which was of i,,ense

help towards the co,pletion of this project"


3/33

DECLARATION

) here by declare that the project entitled /1)3

T%$)$T %$ % $/)T* is an original work done by ,e

during the session 2004-05 under the guidance and supervision of

!r" #dwin and !rs" $arita %nil

$ignature of the supervisor

7"

2"


4/33

INDEX

CONTENTS PAGE NO.

1) )TD8*T) 7

2) %)! 7

3) *!P#T$ 7

4) *)*8)T D#T%)9$

4"a" $e,iconductors 2

4"a"7 or,ation of holes in se,iconductor :

4"a"2 #lectrical conductance in se,iconductors 5

4"a": )ntrinsic $e,iconductors ;

4"a"4 9i,itations of developing pure ;

se,iconductor based devices

4"a"5 Doping

4"a"< -type $e,iconductors >

4"a"> P-type $e,iconductors 70

4"a"? 3eneral )nfor,ation 72

4"a"70 Distinction between )ntrinsic se,iconductors 74

and #=trinsic se,iconductors

4"a"77 Distinction between P-type se,iconductors 75


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and -type se,iconductors

4"b Transistors

4"b"7 Principal and working of junction Transistor

4"b"2 !odes of study of junction Transistors

4"b": %dvantages of Transistor

4"b"4 Drawbacks of Transistor

4"c *arbon esistor @70kA - 0"5kAB

4"d &ariable esistor @70kA 700kA - 0"5wB

5" P)*)P9# C /1)3 T%$)$T %$ % $/)T*

;" **98$)


6/33

INTRODUCTION

$witch has its own i,portance in the electricity because it is

the factor which decides the passage of current through circuit" )f

on than current passes through otherwise not" There are different

types of switches such as ,echanical electrical electronic etc"""

ut of these electronic switches best because of their high speed of

operation and absence of sparking" % transistor can be used as

switch by turning it back and forth between on and off stage"

AIM

To ,ake a circuit showing the use of a transistor as a switch

and to study the working of a transistor as a switch"

@7B y using an n-p-n transistor and varying the current in base

circuit"

@2B y using an n-p-n transistor and varying the potential in base

e,itter circuit"

COMPONENTS

1. n-p-n transistor @* 70> or 2T+ :00B

2. carbon resistor @701A - 0"5wB

3. variable resistor @701A 700kAB- 0"5w


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4.battery @;&B

5.bulb @;& F 0"0;%B

SEMICONDUCTORS

The energy band diagra, of se,iconductors is shown in the

figure given below"

ere also the valence band is totally filled and conduction

band is e,pty but the forbidden gap between conduction bands is

Guite s,all" )t is about 7 e&" or e=" The forbidden gap for

ger,aniu, is of 0"


8/33

Formation of hol! in !mi"on#$"tor

This can be understood in two ways"

%. From th nr&' (an# #ia&ram of th !mi"on#$"tor.

)n the energy band diagra, of the se,iconductor there is an

energy gap of about 7 e& between the valence band and the

conduction band" %t Hero 1elvin the se,iconductor behaves as an

insulator as no electron for, the valence band can cross this

energy gap and go to the conduction band" ut at higher

te,perature so,e of the electrons gain energy due to ther,al

agitation and ,ove for, the valence band to the agitation and

,ove for, the valence band to the conduction band" %s a result of

it a vacancy is created in the valence band at a place where the

electron was present before ,oving to the conduction band"

This vacancy is called a hole" $ince the absence of a

negatively charged electron is eGuivalent to the presence of an

eGuivalent a,ount of positive charge therefore a hole is

considered as a seat of positive charge therefore a hole is

considered as a seat of positive charge having a charge eGual to

that of an electron" The hole is considered as an active particle in

the valence band as an electron in the conduction band" The ,otion

of the electrons in the conduction band and also the ,otion the

electrons of holes in the valence band are responsible for the

electrical conduction in se,iconductors"


9/33

). From th *aln" (on# !t$#' of th !mi"on#$"tor

*onsider a se,iconductor crystal of silicon or

ger,aniu, under study" /e know that the $i or 3e have four

valence electrons"

#very ato, of 3e tends to share one of its four valence

electrons with each of its four nearest neighboring ato,s and also

to take share of one covalent bond shares two electrons one for,

each ato," y for,ing such covalent bonds each of 3e ato,s in

the crystal behaves as if the outer,ost orbit of each ato, is

co,plete with eight electrons in the 3e structure"

%t te,perature 01 for 3e structure the valence bond is full"

The energy gap is 0"


10/33

crossing over to the conduction band leaving behind eGual nu,ber

of holes in the valence band" )n intrinsic se,iconductor at roo,

te,perature er,i level is about half the way in the forbidden gap"

/hen an electron break away fro, a covalent bond the

e,pty place or vacancy left in the bond is called hole shown by

hollow circle in the ig" /hen an e=ternal electric field is applied

these free electron and hole s ,ove in opposite direction and

constitute a current flow through 3er,aniu, crystal" The nu,ber

of free electron @in conduction bandB and holes @in valence bandB is

e=actly eGual in an intrinsic se,iconductor"

Thus in intrinsic se,iconductor

neI nh I ni

where ne nh are nu,ber density of electrons in band and

nu,ber of density of holes in valence band n i is the nu,ber

density of intrinsic carriers @electron or holesB in a pure

se,iconductor"

The nu,ber of electrons or holes in a pure se,iconductor at a

given roo, te,perature T1 is given byneI nhI %T

:J2eF #gJ2 1T


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/here k is a oltH,ann constant and #g is the energy gap

between conduction band and valence band" )t ,eans on increasing

te,perature the nu,ber of current carriers increases"

)t is very difficult to ,ake an intrinsic se,iconductor because of

the difficulty in preparing e=tre,ely pure ,aterial"

El"tri"al "on#$"tan" in !mi"on#$"tor!

)n a pure se,iconductor each ato, behaves as if there

are eight electrons in its valence shell @due to the for,ation of

covalent bondsB and therefore the entire ,aterial behaves as an

insulator at low te,peratures"

% se,i-conductor ato, needs energy of the order 7"7 e& to

shake off the valence electron" This energy beco,es available to

the se,iconductor even at roo, te,perature" Due to ther,al

agitation of the crystal structure electrons fro, few covalent

bonds co,e out" The bond fro, which electron is freed a vacancy

is created there" The vacancy is covalent bond @where there should

have been an electronB is called ahol.

This can be filled by so,e other electron in a covalent bond"

%s an electron fro, which electron has ,oved" The holes and

electrons are called intrinsic carriers and these are only the current

carriers in a $e,i- conductor"


12/33

INTRINSIC SEMICONDUCTOR

% se,iconductor free fro, all types of i,purities is called a

intrin!i" !mi"on#$"tor" %t 0 1 a se,iconductor is an insulator

i"e it possesses Hero conductivity" /hen te,perature is increased a

few covalent bonds break up and release the electrons" These

electrons ,ove to conduction band leaving behind eGual nu,ber

of holes in valence band" The conductivity of an intrinsic

se,iconductor is due to both electrons and holes"

LIMITATION OF DE+ELOPING PURE

SEMICONDUCTOR ,ASED DE+ICE

)n pure se,iconductor at roo, te,perature the nu,ber of

intrinsic charge carriers @electron and holesB is very s,all @K

707;

,-:

B" That is why the pure se,iconductor is low

conductivity ,aterial"

)n pure se,iconductor" The intrinsic charge carriers are

always produced due the breakage of covalent bond by virtue

of ther,al ,otion" ence enough fle=ibility is not available

to control their nu,ber in pure se,iconductor"

)n pure se,iconductor the nu,ber of electron @neB is always

eGual to nu,ber of holes @nhB" )t is never possible in pure

se,iconductor to have large nu,ber of conduction electron


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only on large nu,ber of conduction holes only" That is why

pure se,iconductor is not of ,uch use"

DOPING

Doping is the process of deliberation addition of a desirable

i,purity ato, to a pure se,iconductor to ,odify its properties in a

controlled ,anner" The i,purity added ,ay be I 7 part per

,illion@pp,B"

)n doping process it is reGuired that

The dopant ato, should take the position of

se,iconductor ato, in the lattice"

The presence of the dopant ato, should not distort the

crystal lattice"

The siHe of the dopant ato, should be al,ost sa,e as that

of crystal ato,"

The concentration of dopant ato, should not be large@ not

,ore than 7L of the crystal ato,B"

)t is to be noted that the doping of se,iconductor increases its

electrical conductivity to a great e=tant"

MET-OD OF DOPING

Doping is achieved in ,any waysM for this we have


14/33

%dd the i,purity ato,s in the ,elt of the se,iconductor"

eat the crystalline se,iconductor in an at,osphere

containing dopant ato,s or ,olecules so that the latter

diffuse into the se,iconductor or

),plant dopant ato, by bo,barding the se,iconductor

with the ions"

EXTRINSIC SEMICONDUCTOR

% dopant se,iconductor or a se,iconductor with suitable

i,purity ato,s added to is called e=trinsic se,iconductor"

#=trinsic se,iconductors are of following two types'

- type se,iconductor

P- type se,iconductor

N T'/ Smi"on#$"tor

/hen a pure se,iconductor of silicon @$iB in which $i ato,

has four valence electron is doped with a controlled a,ount of

Pentavalent ato, say arsenic @%sB or phosphorus@PB or anti,ony

or bis,uth which have five valence electron the i,purity ato,


15/33

will replace the silicon ato, as shown in fig" The four of the five

valence bond by sharing the electron with adjoining four silicon

ato, while the fifth electron is very loosely bound with the parent

parent i,purity ato, and is co,paratively free to ,ove" Thus each

i,purity added donates one free electron to the crystal structure"

These i,purity ato,s which donate free electron for conduction

are called donor ato,s" $ince the conduction of electricity is due to

the ,otion of electron i"e" negative charge of n type carriers

therefore the resulting se,iconductor is called donor type or n-

type se,iconductor" n giving up their fifth electron the donor

ato,s beco,e positively charged" owever the ,atter re,ains

electrically neutral as whole" The e=tra electron of donor ato,

orbits around donor nucleus in hydrogen like ,atter" )t has been

found that 0"05 e& energy in $i and 0"07 e& energy in 3e are

reGuired to re,ove this electron fro, i,purity ato, and ,ake it a

free electron"

%t roo, te,perature so,e of the covalent bond ,ay get

ruptured producing there by free electron and eGual nu,ber holes

in the n- type se,iconductor" ut over all the total nu,ber of holes

in n-type se,iconductor is relatively low hence in n- typese,iconductor electron are ,ajority carriers and holes are

,inority carriers"


16/33

#nergy band diagra, is shown in fig" or a silicon

se,iconductor with i,purity ato, of arsenic or phosphorus the

energies of the free electron in the lowest energy levels @called

donor energy levelsB between the valance and conduction band and

the lowest donor electron energy level lies at 0"05 e& below the

botto, of the conduction band"

/hen we add pentavalent i,purity in a pure se,iconductor

of 3e or $i the final level in forbidden gap shift very close to

conduction band" )f doping is very large the final level ,ay ,ove

in to the conduction band"

)t is to be noted that this energies is co,parable in to the

ther,al energy of electron at roo, te,perature @I 0"0: e&B" Thus

very s,all energy supplied can e=cite the electron fro, donor

levels to conduction band" Due to it se,iconductor shows the

conductivity re,arkably i,proved"

P T'/ Smi"on#$"tor

/hen a pure se,iconductor of 3er,aniu, @3eB in which

each ato, ha s four valence electron is doped with a controlled

a,ount of trivalent ato,s say galliu, or )ndiu, @)nB or oron @B

or %lu,inu, @%lB which will replace the 3e ato,s as shown in the

figure" The three valence electrons of the i,purity ato, will for,


17/33

covalent bonds by sharing the electrons of the adjoining three

ato,s of 3e while there will be one inco,plete covalent bond

with a neighboring 3e ato, due to the deficiency of an electron

fro, one of the 3e-3e bonds thus co,pleting the )n F 3e bond"

This ,ake )ndiu, ioniHed @negatively chargedB and creates a

Nhole(" %n electron ,oving fro, a 3e-3e bond to fill a hole leaves

a hole behind" That is how holes ,ove in the se,i-conductor

structure" That is how holes ,ove in the se,i-conductor structure"

The trivalent ato,s are called acceptor ato,s and the conduction

of electricity occurs due to ,otion of holes i"e positive charges or

p-type carriers" That is why the resulting se,iconductor is called

acceptor type or p-type se,iconductor"

%lso at ordinary te,perature so,e of the covalent bonds

,ay get ruptured releasing eGual nu,ber of holes and electrons"

Therefore the total nu,ber of electrons is relatively s,all as

co,pared to the nu,ber of holes in the p-type se,iconductor

electrons are ,inority carriers and holes are ,ajority carriers"

or a 3e or $i se,iconductor the doping of i,purity ato,s

of )ndiu, or boron produce so,e allowed energy levels which are

situated in the forbidden gap slightly above the valence band"%t roo, te,perature due to ther,al energy the electrons

fro, the valence band are easily transferred to the acceptor level

until these levels are filled" This produces a large nu,ber of holes


18/33

in the valence band thereby the valence band beco,es a hole

conducting band" /hen an e=ternal electric field is applied to a p-

type se,iconductor these holes will act as carriers of current" Due

to it the p-type se,iconductor shows its electrical conductivity

,uch i,proved than what it was for pure se,iconductor"

GENERAL INFORMATION

)n pure ger,aniu, se,iconductor the er,i level is about

half way in the forbidden gap"

)n n-type se,iconductor the er,i level lies in the forbidden

gap very close to conduction band"

)n p-type se,iconductor the er,i level lies in the forbidden

gap very close to valence band"

/ith rise in te,perature the er,i level ,oves toward the

centre of the forbidden gap for both types of se,iconductors"

n-type se,iconductor is better than the p-type se,iconductor

as electrons have ,ore ,obility than holes" or a given

voltage applied conduction band current in n-type

se,iconductor is ,ore than that of p-type se,iconductor"

)f the light of energy greater than the forbidden energy gap is

incident on an intrinsic se,iconductor the electrons for, the


19/33

valence band cross over to conduction band" Due to which

electrons and holes pair are created" Due to increase in the

nu,ber of current carriers the conductivity of se,iconductor

increases" This property of se,iconductor is called

photoconductivity"

)n a doped se,iconductor the nu,ber density of electrons

and holes is not eGual" ut it can be established that

nenhI ni2

/here ne nhare the nu,ber density of electrons

and holes respectively and ni is nu,ber density of intrinsic

carriers @i"e electrons or holesB in a pure se,iconductor"

)n n-type se,iconductor the nu,ber density of electrons is

nearly eGual to the nu,ber density of donor ato,s dand is

very large as co,pared to nu,ber density of holes" ence n e

K d O O nh

)n p-type se,iconductor the nu,ber density of holes is

nearly eGual to the nu,ber density of acceptor ato,s aand

is very large as co,pared to nu,ber density of electrons"

encenh K a O O ne


20/33


21/33

)T)$)*

$#!)*D8*T

#+T)$)*

$#!)*D8*T

7" )t is a pure se,i conducting

,aterial and no i,purity ato,s

are added to it"

2" #=a,ple is crystalline for,s

of pure silicon and ger,aniu,"

:"The nu,ber of free electrons

in the conduction band and the

nu,ber of holes in valence band

is e=actly eGual and very s,all

indeed"

4" )ts electrical conductivity is

low"

5" )ts electrical conductivity is a

function of te,perature alone"

7" )t is prepared by doping a s,all

Guantity of pure se,i conducting

,aterial"

2" #=a,ples are silicon and

ger,aniu, crystals with i,purity

ato,s of arsenic anti,ony

phosphorous etc""" or indiu, boron

alu,inu, etc"""

:" The nu,ber of free electrons and

holes is never eGual" There is e=cess

of electrons in n-type

se,iconductors and e=cess of holes

in p-type se,iconductors"

4" )ts electrical conductivity is high"

5" )ts electrical conductivity

depends upon the te,perature as

well as on the Guantity of i,purity

ato,s doped in the structure"


22/33

n-TEP# $#!)*D8*T p-TEP# $#!)*D8*T

7" )t is an e=trinsicse,iconductor which is

obtained by doping the

i,purity ato,s of &thgroup

periodic table to the pure

ger,aniu, or silicon

se,iconductor"

2" The i,purity ato,s added

provide e=tra electrons in thestructure and are called donor

ato,s"

:" The electrons are ,ajority

carriers and holes are ,inority

carriers"

4" The electrons density @neB is

,uch greater than the hole

density @nhB i"e" nh"OO nh"5" The donor energy level is

close to conduction band and

far away fro, the valence

band"

;" The er,i energy level lies

in between the donor energy

level and conduction band"

7" )t is an e=trinsicse,iconductor which is

obtained by doping the

i,purity ato,s of group ))) of

periodic table to the pure

ger,aniu, or silicon

se,iconductor"

2" The i,purity ato,s added

create vacancies of electron @i"eholesB in the structure and are

called acceptor ato,s"

:" The holes are ,ajority

carriers and electrons are

,inority carriers"

4" The hole density @nhB is

,uch greater than the electron

density @neB i"e nh" OO ne"5" The acceptor energy level is

close to valence band and is far

away fro, conduction band"

;" The er,i energy level lies

in between the acceptor energy

level and valence band"

Tran!i!tor

% three section se,iconductor devices for,ed by growing

a thin layer of p-type se,iconductor between two co,paratively

thick layers of n-type se,iconductor and vice-versa"


23/33

The thin layer at the centre is lightly doped and is called

base@B" The other two layers serve as e,itter @#B and

collector@*B" /hen the e,itter is forward biased the ,ajority

carriers ,ove across the base so as to reach the collector biased

in reverse ,anner" % s,all reco,bination of holes and electrons

takes place in the base region"

0UNCTION TRANSISTOR

% junction transistor is obtained by growing a thin

layer of one type se,iconductor in between two thick layers of

other si,ilar type se,iconductor" Thus a junction transistor is a

se,iconductor or device having two junctions and three

ter,inals"

The two types of junction transistor are p-n-p transistor

and n-p-n junction transistor"

% p-n-p junction transistor is obtained by growing a

thin layer of n-type se,iconductor in between two relatively

thick layers of p type se,iconductor"

%n n-p-n junction transistor is obtained by growing a thin

layer of p-type se,iconductor in between two relatively thick

layers of n type se,iconductor"

The thin layer of junction diode transistor is said to for,

the base @B" ne of the thick layers serves as the e,itter @#B

and the other thick layer serves the collector @*B"


24/33

The function of e,itter is to e,it the ,ajority carriers"

unction of collector is to collect the ,ajority carriers" ase

provides the proper interaction between the e,itter and the

collector"

$y,bolically the two types of transistor are represented in

the given figure" The direction of arrowhead indicates the

direction of flow of positive charge"

)n case of p-n-p transistor in the figure the arrow head

inwards because ,ajority carriers are holes" )n case of n-p-n

transistor in the figure the arrow head is outwards because

,ajority carriers are electrons"

% junction transistor is a transfor,er of resistance which

can be achieved by inter changing the biasing across the

junction triode hence its na,e is junction transistor" The tran!i!tor i! a "$rrnt #ri*n #*i" in which the

e,itter controls the collector current"

Wor1in& of 2$n"tion Tran!i!tor

a3 /n/ Tran!i!tor. The e=peri,ental arrange,ent isshown in figure" The e,itter base junction is forward

biased" )t ,eans the positive pole of e,itter base battery

&is connected to e,itter and its negative pole to the

base" *ollector base junction is reverse biased i"e" the

negative pole of the collector base battery &** is

connected to collector and its positive pole to the base"


25/33

The resistance of e,itter base junction is very low"

$o the voltage of &@i"e B is Guite s,all @K 7"5 voltB"

The resistance of collector base junction is very high" $o

the voltage of &**@i"e &*B is Guite large @K 45 voltB"

oles which are ,ajority carriers in e,itter @p-

type se,iconductorB are repelled towards base by positive

potential on e,itter due to battery & resulting e,itter

current )e" The base being thin and lightly doped @n-type

se,iconductorB has low nu,ber density of electrons" /hen

holes enter the base region then only few holes @says 5L

)e I 0"05 )eB" The re,aining ?5L holes pass over to the

collector on account of high negative potential of collector

due to battery &** resulting collector current )e @I?5L)e I

0"?5 )eB"

%s one hole reaches the collector it is neutraliHed by the

flow of one electron fro, the negative ter,inal of the

battery &** to collector through connecting wire" %t the

sa,e ti,e a covalent bond is broken in the e,itter the

electron goes to the positive ter,inal of the battery &

through the connecting wire and hole produced begins to

,ove towards the base" Then one electron flows fro,

negative ter,inal of battery & to positive ter,inal of

battery &**" /hen the hole co,ing fro, e,itter co,bines

with the electron in the base the deficiency of electron in


26/33

base is co,pensated by the flow of electron fro, negative

ter,inal of the battery & to base through connecting

wire" Thus the current in p-n-p transistor is carried by holes

and at the sa,e ti,e their concentration is ,aintained" ut

in the e=ternal circuit the current is due to flow of

electrons" The direction of conventional current @of holes

currentsB in the various ar,s of the circuit has been shown

by arrow heads in the figure" Thus in this case

)eI )bQ )c

)n the base )e and )e flow in opposite directions"

@bB n/n Tran!i!tor" )n this case also the e,itter base

junction is forward biased i"e" the positive pole of e,itter

base battery &is connected to the base and its negative

pole to e,itter"

The resistance of the e,itter base junction is very low"

$o the voltage of &@i"e B is Guite s,all @K7"5&B"

The collector base junction is reversed biased i"e the

positive pole of the collector base battery &**is connected

to the collector and negative pole to base" The resistance of

this junction is very high" $o the voltage of &**@i"e &*B is

Guite large @K45B"


27/33

#lectrons which are ,ajority carriers in e,itter @n-type

se,iconductorB are repelled towards base by negative

potential of &on e,itter resulting e,itter current )e" The

base being thin and lightly doped @p-type se,iconductorB

has low nu,ber density of holes" /hen electrons enter the

base region then only a few holes @say 5LB get neutrilsed

by the electron F hole co,bination resulting base current )b

@ I 5L )e I 0"05 )eB" The re,aining ?5L electrons pass

over to the collector on account of high potential of

collector due to battery &** resulting collector current )c

@ I ?5L )eI0"?5 )eB"

%s one electron reaches the collector it flows to the

positive ter,inal of battery &**through connecting wire" %t thesa,e ti,e one electron flows fro, negative ter,inal of &** to

positive ter,inal of &to e,itter" /hen the electron co,ing

fro, the e,itter co,bines with the hole on base the deficiency

of hole in base is co,pensated by the breakage of covalent bond

there" The electrons so released flows to positive ter,inal of

battery & through connecting wire" Thus n-p-n transistor the

current is carried inside the transistor as well as in the e=ternal

circuit by the electrons" The direction of conventional current in

the various ar,s of the circuit has been shown by arrow heads in

the figure" Thus in this case


28/33

)eI )b Q )c

)n the base )eand )cflow in the opposite directions"

Mo#! of St$#' of 0$n"tion Tran!i!tor!

% transistor can be studied with any one of its three

ter,inals grounded which would serve as a link for both the

input and output voltages" Thus there are three e=ternal circuit

connections for transistors as shown in the figures"

A#*anta&! of Tran!i!tor

Transistor because of their ,any ,erits over vaccu,

tubes have practically co,pletely replaced the, in all the fields

of electronics" $o,e of the advantages of the transistor over the

vaccu, tubes are as given below-

7- %s transistors have no fila,ents hence no power is needed

to heat the, cause the e,ission of electrons"

2- $ince no heating is reGuired transistors are set into operation

as soon as the circuit is switched on"

:- During operation transistor does not produce any hu,,ingnoise"

4- Transistors reGuire low voltage for their operation as

co,pared to vaccu, tubes"

5- wing to their s,all siHes the circuits involving transistors

are very co,pact"

;- Transistors are shock proof"


29/33


30/33


31/33

701& variable resistor to ,ake sure that the transistor works as

a switch"

)n the adjust,ent ,ade the biases are so adjusted that

current does not flow the arrange,ent works as an open switch"

/hen the adjust,ent is disturbed the biases change and

current flows" The arrange,ent works as closed switch"

The cutoff region finds both junctions in reverse bias and only

the reverse saturation current is present" )n the saturation

condition both junction are in forward bias and the values of &**

saturation and *# saturation should be s,all in switching for,

saturation to cut off large single operation of the transistor

occur"

Prior to t I 0 in fig the p-n-p transistor has been cut off" %tt I 0 a negative base voltage supplies the base current" There is

delay td during which charge is being supplied to build up a

hole concentration in the base and to charge the capacitance

associated with the e,itter base and the base connector

junction"


32/33


33/33

&%)%9# #$)$T%*#

&ariable esistance is a kind of resistance used for

the purpose of getting variable current" )t is of great use in

!odern electronics" )t consists of ,etal and a thin strip of ,etal

surrounded by wires" )ts ,ain purpose is adjust the flow of

current" )t has three legs one in the centre and two in the

outer,ost part" )t is nor,ally s,all in siHe"

)n the project .T%$)$T %$ $/)T*6

variable resistance is of great use as in the given project

also it is used to adjust the flow of current" The connection

of variable resistance in the given project is shown in the

circuit diagra,"

mayank's presentation of transistor as a switch 1

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