Semiconductor Devices

carrier. Thus, at room temperature, apure semiconductor n

Energy Band Structure For will have electrons and holes wandering in random F
D if fe re nt M at er ia ls directions. These electrons and holes are called the charge
carriers in asemiconductor. In a pured besem iconductor, the
Mans. etals are good conductors of electricity, insulators
not conduct electricity,
semiconductors have conductivity in between
while number of electrons and holes woul same and such a
semiconductor is called intrinsic semiconductor.
V

those of metals and insulators. Let us make A number fo free

s hte crystal si electrically neutral,r ofhteholes.
dis tin cti on between conductors, insulators and electrons will be equal ot the numbe fI we apply
semiconductors on the basis of band theory of solids. potential difference across the semiconductor, the
electrons will move towards positive terminal and the
Asolid si alarge collection of atoms. The energy levels of holes towards the negative terminal of the battery. It may
an atom get modified due to the presence of other be clearly understood that electrons and holes are not
surrounding atoms and the energy levels in the outermost current in themselves but act only as the negative and
shells of all the atoms form valence band and the
conduction band separated by a forbidden energy gap. positive carriers of the current respectively.
The energy band formed by a series of energy levels nI na intrinsic semiconductor, fi n°!) denotes hte electrons
containing valence electrons is called valence band. At number density ni conduction band, n, the hoels number
OK, the fermi levels as well as the lower energy levels are
completely occupied by the electrons. As the temperature density ni valence band and n, hte number deusity or
rises, the electrons absorb energy and get excited. These
concentration of intrinsic carriers, then no) =g)n( =n1,
excited electrons jump to the higher energy levels. These The conductivity of the semiconductor increases with rise
electrons in the higher energy levels are comparatively at in temperature of the semiconductor. Further, when an
larger distances from the nucleus and are more free as electron is raised from the valence band to the conduction
compared to the electrons ni the lower energy levels band, a vacancy is created in the valence band. This
Energy band formed higher energy levels where electrons vacancy created in the valence band (where electron was
present before moving ot conduction band) acts sa the hole,
ujmps no suppyl osme chegrysi etrmed sa conducoitn
n-Type and pT
-ype
Depending upon the energy gap between valence band
and the conduction band, the solids behave as conductors, Semiconductors
insulators and semiconductors. It follows that a pure semiconductor at room temperature
possesses free electrons and holes but their concentration
C u r r e n t C a r r i e r s in si so small that conductivity offered by the pure
Semiconductors semiconductor cannot be made of an practical use.
By the addition of certain impurities ot the pure
In apure semiconductor, each atom behaves as fi there are
8 electrons n
i its valence shell (due to formation of covalent
semiconductor n
i a very small ratio_ (1:10"), the
bonds) and therefore the entire material behaves as an conductivity of a Si-crystal (or Ge-crystal) can be
insulator at low temperature. A semiconductor atom remarkably improved. The process of adding impurity toa
needs energy of the order of 11. eV to shake off the valençe pure semiconductor so as ot improve its conductivity, si
electron. This energy becomes available t o the called doping.
semiconductor even at room temperature. Due ot therma! The impurity atoms aer of two types
agitation and breaking of the bond vacancy is created (i) Pentavalent impurity atoms i.e., atoms having
there. The vacancy in the covalent bond (where there
should have been an electron) is called a hole.
5valenceSuch
electrons such as antimony (Sb) or arsenie
atoms, when added to a pure
This hole can be filled by some semiconductor, produce excess of free electrons i.e.,
donate electrons to the semiconductor. For this
reason, pentavalent impurity atoms are called donor
covalent bond moves to fill the impurity atoms. The semiconductor os produced si
hole, the hole is created n
i the called n-type extrinsic semiconductor.
covalent bond from which the (ii) Trivalent impurity atoms i.e., atoms having 3 valence
electron has moved. In other
Hole
electrons such as indium (In) or gallium (Ga). Suc
h,
words, one can say that the. atoms on being added ot a pure semiconductor,
hole shifts from one covalent Free electron
instead of producing free electrons, accept electrons
bond to another in a similar way as an electron does an from hte semiconductor. For this reason, trivalent
attempt to fill the hole. Since, the direction of movement impurity atoms are called acceptor impurity atoms.
of hte hole si opposite ot that of the negative electron, w
e The semiconductor os produced is called p-ty
extrinsic semiconductor. pe
can consider that a hole behaves as a positive charge

IIT March 2013 8

 ber density of
ictor
n-Type (Extririsic) Semiconductor nI n-type (extrinsic) semiconductor, the num tely equal to
dom electrons in conduction band is approxima
e as compared to the
arge Figure shows the effect of that of donor atoms and very larg
, the adding pentavalent impurity number density of holes in valence band. Thus,
ch a arsenic to silicon crystal.
When the arsenic impurity
free
atoms are added to the
where N, represents the number density of donor atoms
silicon crystal in a small Free
ply
р-Type (Extrinsic) Semiconductor
electron
ratio (1:10°), its atoms
the
the replace the silicon atoms Figure shows the effect of
here and there. The four electrons out of the five valence adding trivalent impurity
nay
not electrons of As atom take part in covalent bonding with indium to silicon crystal.
and four silicon atoms surrounding it. The fifth electron is set The four silicon atoms
free. Obviously, the extra free electrons created in the surrounding the In-atom,
o n s crystal wil be sa many as the number of the pentavalent can share one electron each
- Hole

impurity atoms added. As, the pentavalent impurity with the In-atom which has
ber increases the number of free electrons, it is called donor got three valence electrons.
or impurity. The silicon crystal so obtained is termed as In an attempt to have 8 electrons in valence shell, the
silicon
n-type Si-crystal. The electrons so set free in the
crystal are called extrinsic carriers and the n-type
In-atom robs one of the nearby covalent bonds of one
ise
Si-crystal is called. n-type extrinsic semiconductor or
electron. Thus, the valence shell of the In-atom possesses
8 electrons but a hole is created in the covalent bond from
simply n-type semiconductor.
an
ion
As said earlier, due to thermal agitation, even the pure
which electron has been robbed. Thus, for every trivalent
his
impurity atoms added, na extra hole wil be screa ted. As
Si-crystal possesses a few electrons and holes. Therefore, from the
vas
n-type Si-crystal will have a large number of free electrons the trivalent impurity atoms accept electron
e.
(majority carriers) and a small number of holes (minority silicon crystal, it is called acceptor impurity. The
carriers) and as concentration of charge carriers increases Si-crystal so obtained is called p-type as it contains free
the conductivity of semiconductor increases. holes. Each hole si equivalent ot positive charge. The
In n-type semiconductor, holes so created are extrinsic carriers and the p-type
tre
the fifth electron of the As
Conduction Band Si-crystal so obtained is called p-type extrinsic
o n atom revolves around the 0.045 eV semiconductor.
Ire
donor atom inside the • Donor energy level Again, as the pure Si-crystal also possesses afew electrons
Si-crystal. As dielectric and holes, therefore, the p-type Si-crystal will have a large
number of holes (majority carriers) and asmall number of
Valence Band
re constant of silicon is very
he high, it is bound to the electrons (minority carriers).
be donor atom with a very small amount of energy, which is In the extrinsic p-type Conduction Band
of the order of 0.045 eV. In terms of valence and Si-crystal, the
hole
IS conduction band, one can think that ali such electrons produced revolves round 7. Acceptor energy evle
(extrinsic carriers) create a donor energy level just below the n u c l e u s o f t h e In-atom. 0.04Ve IS

(0.045 eV) hte conduction band asener s the

shown ni figure. A As the hole may be treated
18 energy gap between donor gy level and the as a particle of same mass as
Valence Band

ic conduction band is very small, the electrons can easily electron but having an
re raise themselves to conduction band even at room equal positive charge, it possesses asmall positive energy
?., ductivity of n-type extrinsic
temperature. Hence, the conincr of the order of 0.04 eV. Such holes create an acceptor
is semiconductor is markedly eased
energy level just above=( 0.04 V e ) the top of the valence
or
In a doped or extrinsic semiconductor, the number density band (figure]. The electron from valence band can raise
is of electrons ni the conduction band (n.) and hte number themselves to the acceptor energy level by absorbing
density of holes ni hte valence band (n,) differ from that in thermal energy at room temperature and in turn create
a pure semiconductor. If n is the number density of the holes in the valence band.
h
conduction band (n.) and the number density of holes ni The number density of holes (n,) in valence band ni a
г, the valence band (n) differ from that in a pure
semiconductor. If n is the number density of electrons in p-type semiconductor is approximately equal to that of
conduction band or the number density of holes in valence the acceptor atoms (Na) and si very large as compared to
band in a pure semiconductor, then it can be proved that the number density of electrons (ne) in conduction band
e
Thus,

I I T билиолі March 2013 9

er density of le connected ot p-region
the junction with its negativetopon-r
ely equal ot ctrons
l elem.
Example 1 Puer iSat 30 Khas5 e×qua10' (ne) and
. Doping yb and positive pole connected
egion.
hoels (n,) concentration of1. ss the junction
pared to the ndii increases n, ot 45. ×102 m.' Calculate ,n ni the
um The potential difference developedrgeacro carriers si called
US, doped silicon. due to migration of majority cha ther diffusion fo
Solution. Here, n, 5=1. ×106' m*) potential barrier. It opposes thofe thefurpotential barrier is
n, = 4.5 ×102 m*'
charge carriers. The magnitude t 0.7 Y
lonor atoms. about 03. Vfor germanium junction diode and ofabou ential
n. ก, = n for silicon junction diode. However, the value pot
Now,

uctor Therefore, 70- 位 (14.5

.5 × 1g03)'
× 1022
barrier depends on the magnitude of doping fo hte
semiconductor crystal.
wn ot have an electrons junction, a veyr large
It may be pointed out that across thote pote
Example 2 Asemiconductor si knoand electric field is set up due ntial difference
concentration of 8 ×103' cm-3
aholes concentration
n layer fo
fo5 × 10?' cm3 developed across it. Taking width of the depletio
a)( sI the semiconductor n-type or p-type? asilicon junction diode'as 10°m and a potential
H
eol
b() What si het resistivity of hte sample, fi the electron difference of 07. Vset up across it, the strength foelectric
mobility si 23000 cm' V*'s and hole mobility si field is of the order of E= - -=7×10' Vm*'
100 cm? V-' s*'? (Take charge on electron, %
10-
.
e= 1.6 × 10-19 )C
Thus, the formation of p-njunction results ni a very strong
nce shell, the Solution. Her, n, 8= x310' cm' =8×109' 3m
* electric field o(r potential gradient) across het junction.
bonds of one 17, = 5x 102' mc 3= 5 x 108'm*
tom possesses , =23000 cm' V
M ' V' 5)
'- 5' =23. m Forward and Revser Biasing
ent bond from H,=100 cm'V'5'= 00.1 m'V"'s' Mode o f a np- Junction
very trivalent a)( Snice hte semiconductor has greater electron concentration, ti si
se created. As n-type semiconductor. Ajunction diode can be biased in the two ways
ons from the b() Now = e(noH, + H
gnA
) 1 . Forward Bias :
mpurity. The =1.6 ×10-19 8( × 10'' × 23. + 5× 108' ×0.01) C source is connected to the diode in
When an external D
contains free
e charge. The =16. × 101-9 (1.84 × 1020 + 5 ×106') - 29.44 such a way that p-side is having more potential sa
compared to n-side then the junction diode is said ot eb
dn the p-type or p= = 3.397 ×102- 2 m forward biased as shown ni figure.
29.44
pe extrinsic
Action of p-n Junction
afew electrons
Formation o f p-n Junction When the p-n junction is
il have a large When a p-type crystal is placed in Fictitious--|--
battery Junction
forward biased, the positive
contact with n-type crystal so as to holes in the p-section are
nal number of repelled by positive pole of the
form one piece, the assembly so
obtained is called p-n junction or battery towards the p-n
Band junction diode or crystal diode. The junction. Simultaneously, the
surface of contact of p and n-type. negative electrons in the
n-section are repelled by
r engeyr level
a s
crystals is called junction. In the Depletion Fow
rdar bias
0.4 Ve p-section, holes are the majority layer
negative pole fo the battery
carriers; while in n-section, the majority carriers are towards the junction. However, the movement fo
and electrons. electrons and holes across the junction is opposed by the
Dueot the high concentration of different types of charge potential barrier developed across the junction. Just near
the pen junction, electrons and holes combine and cease ot
ositive energy carriers ni the two sections, holes from p-region diffuse exist as mobile charge carriers after potential barrier si
te an acceptor into i-region and electrons from n-region diffuse into
of the valence p-region. nI both cases, when an electron meets ahole, the overcome by the applied potential.
w
to cancel the effect of each other and sa aresult, athin For each electron-hole combination that takes place near
band can raise
layer at the junction becomes depleted of charge carriers. the junction, a covalent bond breaks ni the p-section near
by absorbing the positive pole of the battery. Of the electron and the
i n t u r n create
Thsi region si called depletion layer.
The thickness of the depletion layer is of the order of hole produced, the electron is captured by the positive
10 m. Due ot the diffusion of holes and electrons, the terminal, while the hole moves towards the junction. nO
ence band in a the other hand, as soon as the hole is created in the
qual ot that of w
t o sections of the junction diode no longer remain p-section due to the breaking of a covalent bond, an
neutral. The p-section of the junction diode becomes electron is released from the negative terminal of hte
sa compared to slightly negative, while the i section becomes positive. It
battery into the i-section to replace e election lost yb
the combination with ahole at the junthctio
duction band. appears as if some fictitious battery is connected across
n.
IIT March 2013 10
 ms and
density of impurity atovolt.
These electrons move towards the junction, where they Its value depends uponorthe2 V
1on

again get neutralised on meeting the new holes coming may have a value of 1 to several hundred
bias, the
on from left. As a consequence, a relatively large current, It may be pointed out that during the reverse
led called forward current flows through the junction. The s battery developed
applied DC voltage aids the fictitioupote
of current in the external circuit is due to the electrons and is
across the junction. Due to this, the ntial drop across
the diffusion of
•is from negative terminal of battery to positive terminal the junction increases and as a result, of
٠V through the junction diode. holes and electrons across hte junction decreases. tI makes
ial A
s discussed above, during die forward bias, the applied the depletion layer thick and the junc tion diode offers
he C voltage opposes the fictitious battery developed
D high resistance during reverse bias. It may be noted that
across the p-n junction. Due to this, the potential drop the potential barrier opposes the forward current, while ti
ge across the junction decreases and as a result, the diffusion aids the reverse current.
cel of holes and electrons across the junction increases. It
of makes the depletion layer thin and as such, the junction Characteristics of a p-n Junction
al diode offers olw resistance during forward bias. Forward Bias Characteristic The forward bias
connections of a p-n junction are as shown ni Fig. .a)( The
2. Reverse Bias : positive pole of hte battery si connected ot the p-section
When a battery is connected to junction diode with and the negative pole ot the n-section. nI the beginning
gi p-section connected to negative pole and i-section when the applied forward bias si low, practically no
connected ot the positive pole, the junction diode si said to current flows through the junction diode. tIsi because, the
be reverse biased as shown in figure. potential barrier (which si about 3.0 Vni esac of eG p-n
=
-1*|- junction and 07. Vni case of Si p-n junction) opposes the
applied voltage.
Therefore, a small forward current flows, till hte applied
forward bias does not become greater than the potential
A fo the
barrier voltage. It si represented by portion O
graph between forward bias and the forward current F [gi.
(b)]. As soon sa the forward bias becomes greater than the
Reverse bias potential barrier, the forward current increases almost
linearly. The point A in the forward characteristic
Action of p-n Junction When the p-n junction is corresponding ot the potential barrier appears keil aknee
reverse biased, the holes (majority carriers) in the and the forward voltage corresponding ot knee point Asi
p-section get attracted towards the negative terminal of called knee voltage.
battery and therefore, the holes move away from the 6
junction. At the same time, the electrons (majority
( A)
current m

5
carriers) in the n-section get attracted towards the positive
terminal and move away from the junction. As a very
small number of holes and electrons (minority carriers) are
Forward

left ni the vicinity of the junction, practically no flow of

current takes place. - K
een agvteol

However, due ot thermally generated electron-hole pairs 0.2 40. 0 . 6 0 . 8 1.0

Forward bias V )( - -
within p-region as well as n-region, a small current (a) (b)
=( a efw microamperes) still flows. Some covalent bonds
always break because of the normal heat energy of the Thus, when a p-n junction is forward biased, the current
crystal molecules. Electrons liberated by this process ni increases linearly and rapidly above the knee voltage and
the p-region move to the left across the junction, while the variation in current with increase of forward bias is
holes generated in the n-region move to the right under extremely slow below the knee voltage.
the electric field produced by the battery. Thus, a small Further, when the p-n junction is forward biased, the
electron-hole combination current, called reverse current
depletion layer becomes thin. It si because, the polarity of
si maintained by the minority carriers. the external DC source opposes the fictitious battery
If the reverse bias is made very high, all the covalent developed across the junction. A
s aresult, the potential
bonds near the junction break and a large number of drop across the junction decreases making the depletion
electron-hole pairs are liberated and the reverse current layer thin. It leads ot the low resistance of the junction
increases abruptly to a relatively high value. The
diode during forward bias.
maximum reverse potential difference, which a diode can Reverse Bias Characteristic If the p-section is
tolerate without breakdown is called reverse breakdown
voltage or zener voltage. It si greater for Si than for the Ge. connected to negative terminal and asection ot the

I I T Bazzword March 2013 11

 positive terminal fohte battery, hte junction doide si snid half cycles of alternating input emf provide opposite kinds
ot eb reverse biased (g o). When hte pen junction si of bias ot the junction diode.
reverse biased, the current flows due to minority charge If the junction diode gets forward biased during first half
carriers and hence a microammeter si used to measure the
smal current that flows during reverse bias. The reverse cycle, it will get reverse biased during the second half
cycle and vice-versa. In other words, when an alternating
bias opposes the majority carriers but makes the
minority carriers to cross h
te pen junction. Therefore, a emf signal si applied across a junction diode, it wil
A si obtained, on applying reverse
current of about 1-2 M conduct only during those alternate half cycles, which
bias it in forward direction.
bias ot the p-o junction. tI remains almost constant till het
zener voltage si reached and then it increases suddenly as
shown ni Fxi. (b).
Half-Wave Rectifier:
When the pon junction si reverse biased, hte depletion Arectifier, which rectifies only one half of each CA
layer becomes thick. It si because, the external D
C source supply cycle, si called a half-wave rectifier.
ni this case aids the fictitious battery, tI results ni the Principle It si based on the principle that junction
increase of potential drop across the junction and the diode offers olw resistance path, when forward biased;
depletion layer appears thick. Becuuse of hte incrensed and high resistance, when reverse biased. When AC input
thickness of the depletion layer, hte p-n junction offers is applied to a junction diode, it gest forward biased
high resistance during reverse bias. during one-half cycle and reverse biased during the next
Zonr
Revorso
agovtl
bias H
A
)(
opposite half cycle. Thus, output si obtained during
HA 01- - 8 6- alternate half cycles of the CA input.
A
esver curre nt w
()

Inpul : O
put
CA DC
agvtol voltage

6
R

8 The circuit diagram for diode to act as a half-wave

a)( ( b) C supply is fed
rectifier si shown in figure The input A
across the primary coil Pof a step-down transformer. The
Dynamic Resistance Both the forward bias and secondary coilS of the transformer is connected ot the
reverse bias characteristics of the pen junction are
junction diode and aload resistance R
, sa shown ni figure
non-linear and hence Ohms' alw si not obeyed. Therefore, The output D
C voltage is obtained across the load
the resistance offered yb the junction diode depends upon resistance.
the applied voltage. The dynamic resistance of a function
diode si defined as the ratio of small change in voltage to Suppose that during the first half of the input cycle, the
the small change in current produced. It si also called AC junction diode gets forward biased. The conventional
resistance of hte junction diode and si denoted yb ra. current will flow ni the direction of the arrow-heads. The
upper end of R, will be at positive potential wit the lower
Thus, a' = end. The magnitude of output across R
, during first half
The region of the characteristic curve, where dynamic cycle at any time will be proportional ot the magnitude of
resistance si almost independent of the applied voltage, si current through R , ie., proportional to the number of
called h
te linear region of junction diode. majority carriers crossing, the junction, which ni turn will
be proportional ot the magnitude of forward bias and
The junction diode si represented by the P.. n
which ultimately depends upon the value of CA input at
symbol sa shown in figure. The arrow-head that time. Hence, during the first half of the input cycle,
represents h
te i-section of the junction diode and points when junction diode conducts, output across R, wil vary
in the direction in which the hole current or conventional in accordance with C
A input.
current will flow, when junction diode si forward biased
During the second half cycle, junction diode will get
J u n c t i o n Diod e a s Rectifier reverse biased and hence no output wil eb obtained
across Ry. Critically, a small current wil flow due ot
n electronic device which converts A
A C power into C D minority carriers and a negligible output wil be obtained
power si called a rectifier. The study of the junction diode during this half cycle also. During the next half cycle,
characteristics reveals that the junction diode offers a
low output is again obtained sa the junction diode gets
resistance path, when forward biased; and a high forward biased. Thus, a half wave rectifier
resistance path, when reverse biased This feature of the gives
junction diode enables ti ot be used as a rectifier. The two discontinuous and pulsating C
D output across the load
resistance.

March 2013 12
Is
FuW
-l ave Recefitr: .C

If
A rectifier which rectifies both halves of the A
C input si
f(
called afull-wave rectifier. T
o make use of both the halves
of input cycle, two junction diodes are used.
The three sections of the transistor are called emitter (E),
Principle It also works on D,
base (B) and collector (C). The base of atransistor si made
]!
the principle that a junction Input thin and sa ti si comparatively lightly doped, eht
diode offers low resistance CA
concentration of majority carriers in the base si always
during forward bias and high voltage
lesser as compared to that in emitter or collector. The
resistance, when r e v e r s e emitter supplies majority carriers for current flow and the
D
,
biased. Here, two diodes are collector collects them. The base provides the junctions
connected in such a manner that fi one diode gets forward for proper interaction between the emitter and the
biased during first half cycle of AC input, the other gets collector.
reverse biased but when the next opposite half cycle
In the symbol for a transistor,
comes, the first diode gets reverse biased and the second the arrow points hole current
forward biased. Thus, outpat si obtained during both the i.e., conventional c u r r e n t .
half cycles of the AC input. Therefore, the emitter in
Arrangement The AC supply is fed across the primary n1- p - n transistor iS n-p-n D.n.p
coil P of a step-down transformer. The two ends of the represented by an arrow
（a ） (b)
secondary coilS of the transformer are connected to the pointing away from the base, while the emitter ni p-n-p
p-sections of the junction diodes D
, and D2. A load transistor is represented by an arrow pointing towards
resistance R, si connected across the n-sections of the two the base. The symbols for n-p-n and p-n-p transistors are
diodes and the central tapping of the secondary coil as
respectively shown ni Fig. (a) and Fig. (b).
shown in figure. The D
C output will be obtained across A transistor can operate in three regions, saturation
the load resistance RI region, active region and cut-off region.
Theory Suppose that during first half of the input cycle, →In saturaton region, both the junctions of the transistor
upper end of S coil is at positive potential and the lower are forward biased and resistance of the circuit is very olw
end is at negative potential. The junction diode D, will get while in cut-off region both the
junctions are reverse
forward biased, while the diode D, reverse biased. The biased and the transistors offer infinite resistance. While
conventional current due ot hte diode D, wil folw along → in active region, the emitter base junction is forward
the path of full arrows.
biased and collector base junction si reverse biased. In this
When the second half of the input cycle comes, the situation the transistor can be used to work as an
situation will be exactly reverse, Now, the junction diode amplifier. The transistor can be used as a switch or
D, will conduct and the conventional current will flow oscillator also fi it is made to switch its operation from
along hte path of hte dotted arrows. Since, current during cut-off to saturation region and vice-versa.
both hte half cycles flows from right ot left through hte As the transistor is a three terminals device, it would be
load resistance R,, the output during both the half cycles used ni the circuit in such a way that output is taken
will be of the same nature. The right end of the load
resistance R, will be at positive potential wrt the left end. across its two terminals. When input si applied across its
As discussed in case of half-wave rectifier, the magnitude
other two terminals making one terminal common.
Hence, transistor can be used in three configurations,
of output across R, at any time will vary ni accordance .1 Common emitter
with the AC input.
2. Common base
Tr a ns is t o r s : 3. Common collector
Among these common emitter is the most common one.
Ajunction diode cannot be used for amplifying a signal.
For amplification, another type of semiconductor device
A c t i o n of a T r a n s i s t o r
called transistor is used. It is fabricated by sandwitching a
n-type semiconductor between p-type The action of both type of transistors i.e., v-p-nand p-n-p si
semiconductors or It is a three-leg
vice-versa. similar, except that the majority and minority carriers ni
semiconductor device. A transistor can be n-p-n or p-n-p the two cases are of opposite nature. Here we are
type. discussing the action of a transistor by using n-p-n
t r a n s i s t o r.
Inan n-p-n transistor, the p-section is sandwitched
between two n-sections, while in a p-irp transistor, the Figure shows the proper biasing of ann-pentransistor. The
n-section is sandwitched between two p-sections. n-type emitter si forward biased by connecting ti to

IIT March 2013 13