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Transcript of Mayank's Presentation of Transistor as a Switch 1
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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'
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ACKNOWLEGEMENT
) %nuj Pandey of standard +)) of $cience $trea, $t" !ary(s
)nter *ollege #tawah is genuinely thankful to ,y teacher !r"
#dwin for their help and able guidance which ) received for the
co,pletion of this project"
./1)3 T%$)$T %$ $/)T*6
) a, also thankful to the principal ,y co-student and parents
who have always given ,e their support which was of i,,ense
help towards the co,pletion of this project"
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DECLARATION
) here by declare that the project entitled /1)3
T%$)$T %$ % $/)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
$ignature of the supervisor
7"
2"
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INDEX
CONTENTS PAGE NO.
1) )TD8*T) 7
2) %)! 7
3) *!P#T$ 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 $e,iconductors 70
4"a"? 3eneral )nfor,ation 72
4"a"70 Distinction between )ntrinsic se,iconductors 74
and #=trinsic se,iconductors
4"a"77 Distinction between P-type se,iconductors 75
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and -type se,iconductors
4"b Transistors
4"b"7 Principal and working of junction Transistor
4"b"2 !odes of study of junction Transistors
4"b": %dvantages of Transistor
4"b"4 Drawbacks of Transistor
4"c *arbon esistor @70kA - 0"5kAB
4"d &ariable esistor @70kA 700kA - 0"5wB
5" P)*)P9# C /1)3 T%$)$T %$ % $/)T*
;" **98$)
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INTRODUCTION
$witch has its own i,portance in the electricity because it is
the factor which decides the passage of current through circuit" )f
on than current passes through otherwise not" There are different
types of switches such as ,echanical electrical electronic etc"""
ut of these electronic switches best because of their high speed of
operation and absence of sparking" % transistor can be used as
switch by turning it back and forth between on and off stage"
AIM
To ,ake a circuit showing the use of a transistor as a switch
and to study the working of a transistor as a switch"
@7B y using an n-p-n transistor and varying the current in base
circuit"
@2B y using an n-p-n transistor and varying the potential in base
e,itter circuit"
COMPONENTS
1. n-p-n transistor @* 70> or 2T+ :00B
2. carbon resistor @701A - 0"5wB
3. variable resistor @701A 700kAB- 0"5w
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4.battery @;&B
5.bulb @;& F 0"0;%B
SEMICONDUCTORS
The energy band diagra, of se,iconductors is shown in the
figure given below"
ere also the valence band is totally filled and conduction
band is e,pty but the forbidden gap between conduction bands is
Guite s,all" )t is about 7 e&" or e=" The forbidden gap for
ger,aniu, is of 0"
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Formation of hol! in !mi"on#$"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" $ince the absence of a
negatively charged electron is eGuivalent to the presence of an
eGuivalent a,ount of positive charge therefore a hole is
considered as a seat of positive charge therefore a hole is
considered as a seat of positive charge having a charge eGual to
that of an electron" The hole is considered as an active particle in
the valence band as an electron in the conduction band" The ,otion
of the electrons in the conduction band and also the ,otion the
electrons of holes in the valence band are responsible for the
electrical conduction in se,iconductors"
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). From th *aln" (on# !t$#' of th !mi"on#$"tor
*onsider a se,iconductor crystal of silicon or
ger,aniu, under study" /e know that the $i or 3e have four
valence electrons"
#very ato, of 3e tends to share one of its four valence
electrons with each of its four nearest neighboring ato,s and also
to take share of one covalent bond shares two electrons one for,
each ato," y for,ing such covalent bonds each of 3e ato,s in
the crystal behaves as if the outer,ost orbit of each ato, is
co,plete with eight electrons in the 3e structure"
%t te,perature 01 for 3e structure the valence bond is full"
The energy gap is 0"
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crossing over to the conduction band leaving behind eGual nu,ber
of holes in the valence band" )n intrinsic se,iconductor at roo,
te,perature er,i level is about half the way in the forbidden gap"
/hen an electron break away fro, a covalent bond the
e,pty place or vacancy left in the bond is called hole shown by
hollow circle in the ig" /hen an e=ternal electric field is applied
these free electron and hole s ,ove in opposite direction and
constitute a current flow through 3er,aniu, crystal" The nu,ber
of free electron @in conduction bandB and holes @in valence bandB is
e=actly eGual in an intrinsic se,iconductor"
Thus in intrinsic se,iconductor
neI nh I ni
where ne nh are nu,ber density of electrons in band and
nu,ber of density of holes in valence band n i is the nu,ber
density of intrinsic carriers @electron or holesB in a pure
se,iconductor"
The nu,ber of electrons or holes in a pure se,iconductor at a
given roo, te,perature T1 is given byneI nhI %T
:J2eF #gJ2 1T
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/here k is a oltH,ann constant and #g is the energy gap
between conduction band and valence band" )t ,eans on increasing
te,perature the nu,ber of current carriers increases"
)t is very difficult to ,ake an intrinsic se,iconductor because of
the difficulty in preparing e=tre,ely pure ,aterial"
El"tri"al "on#$"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 $e,i- conductor"
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INTRINSIC SEMICONDUCTOR
% se,iconductor free fro, all types of i,purities is called a
intrin!i" !mi"on#$"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
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%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,
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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"
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#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,
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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
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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
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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
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)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"
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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"
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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"
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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"
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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
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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" $o the voltage of &**@i"e &*B is
Guite large @K45B"
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#lectrons which are ,ajority carriers in e,itter @n-type
se,iconductorB are repelled towards base by negative
potential of &on e,itter resulting e,itter current )e" The
base being thin and lightly doped @p-type se,iconductorB
has low nu,ber density of holes" /hen electrons enter the
base region then only a few holes @say 5LB get neutrilsed
by the electron F hole co,bination resulting base current )b
@ I 5L )e I 0"05 )eB" The re,aining ?5L electrons pass
over to the collector on account of high potential of
collector due to battery &** resulting collector current )c
@ I ?5L )eI0"?5 )eB"
%s one electron reaches the collector it flows to the
positive ter,inal of battery &**through connecting wire" %t thesa,e ti,e one electron flows fro, negative ter,inal of &** to
positive ter,inal of &to e,itter" /hen the electron co,ing
fro, the e,itter co,bines with the hole on base the deficiency
of hole in base is co,pensated by the breakage of covalent bond
there" The electrons so released flows to positive ter,inal of
battery & through connecting wire" Thus n-p-n transistor the
current is carried inside the transistor as well as in the e=ternal
circuit by the electrons" The direction of conventional current in
the various ar,s of the circuit has been shown by arrow heads in
the figure" Thus in this case
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)eI )b Q )c
)n the base )eand )cflow in the opposite directions"
Mo#! of St$#' 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- $ince no heating is reGuired transistors are set into operation
as soon as the circuit is switched on"
:- During operation transistor does not produce any hu,,ingnoise"
4- Transistors reGuire low voltage for their operation as
co,pared to vaccu, tubes"
5- wing to their s,all siHes the circuits involving transistors
are very co,pact"
;- Transistors are shock proof"
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701& variable resistor to ,ake sure that the transistor works as
a switch"
)n the adjust,ent ,ade the biases are so adjusted that
current does not flow the arrange,ent works as an open switch"
/hen the adjust,ent is disturbed the biases change and
current flows" The arrange,ent works as closed switch"
The cutoff region finds both junctions in reverse bias and only
the reverse saturation current is present" )n the saturation
condition both junction are in forward bias and the values of &**
saturation and *# saturation should be s,all in switching for,
saturation to cut off large single operation of the transistor
occur"
Prior to t I 0 in fig the p-n-p transistor has been cut off" %tt I 0 a negative base voltage supplies the base current" There is
delay td during which charge is being supplied to build up a
hole concentration in the base and to charge the capacitance
associated with the e,itter base and the base connector
junction"
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&%)%9# #$)$T%*#
&ariable esistance is a kind of resistance used for
the purpose of getting variable current" )t is of great use in
!odern electronics" )t consists of ,etal and a thin strip of ,etal
surrounded by wires" )ts ,ain purpose is adjust the flow of
current" )t has three legs one in the centre and two in the
outer,ost part" )t is nor,ally s,all in siHe"
)n the project .T%$)$T %$ $/)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,"