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59 views23 pagesChapter 9
Electronic devices important terms physics
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mehakyadav847000Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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/ 23
» Electronic Devices ~*~
IMPORTANT TERMS AND DEFINITIONS
B. Semiconductor Devices
solids
C. Logic Gates
D. Integrated Circuit
ns
line and amorphous: solic rn
j onsial solids, 2, Liquid crystals, 3. Definitions of some important terms,
ie as solids, 5. Semiconductors.
e and Amorphous Solids.
ye Mrdinary temperatures, solids possess a
ite shape and volume. In solid state, atoms
ormolecules of matter are strongly bound with |
tach other. Solids have two forms :
(@) Crystalline and (b) Amorphous.
(a) Crystalline solid. Crystalline solids are |
hose in which there is a regular and
periodic arrangement of atoms or
molecules in three dimensions.
(Amorphous solid. Amorphous solids are
those in which there is no periodic
arrangement of atoms.
, Liquid Crystals.
These are organic crystalline solids. On heating,
though they become fluid like, they retain their
| anisotropic property.
SAefinitions of Some Important ‘Terms.
P'Unit cell, It is the smallest portion of the
crystal formed by minimum number of
atoms or molecules which when repeated
over and again in three-dimensional space
| builds upto whole crystal structure.
®) Crystal Iairice. It is a geometrical
arrangement of points in space such that
by placing atoms or molecules of a solid
accordingly, we obtain an actual crystal
Structure of the solid.
(©) Single crystal, The crystals in
Periodicity of the pattern
throughout the piece of crystal are known
as single crystal.
which the |
extends |
(d) Polycrystal. A polycrystal is the aggregate
of the monocrystals whose well developed
faces are joined together.
i-ineray Bands in Solids.
The electrons in an isolated atom have well
defined energy values/levels. However, in a
very large assembly of atoms, these energy
values get modified and lie in a certain range.
Region D
4.NSates
je—Region B—+-—Region A——"}
eNSates
6 NElectrons
2NSates,
121 Se
8NSates
4 NElectrons
The collection of very closely spaced energy
levels is called an energy band. The band of
filled energy levels is called the valence band
and the band of unfilled energy leyels is called
the conduction band. Generally, the valence
band and the conduction band are separated
by a gap called the forbidden band. It is the
width of the forbidden band which decides
whether a solid is a semiconductor, an insulator
‘or a conductor. In insulators, the forbidden
Electronic Devices | 221 5, Semiconductors. V1 fi
222
|
of 6 eV whereas, in semi-
f the order of 1 eV. In
iden band does nol exist,
nine
@ Semiconductors are el et a
whose electrical conductivil lies betwee
that of metallic conductors and insulators.
Ge and Si are the most common examples
‘of semiconductors.
(b) The conductivity of semi
creases with increase in temperature.
(6) Charge carriers in
electrons and hole: n
ductor, number density of free electrons is
exactly equal to the number density of
band is of the order
conductors, it is of
conductors, the forbid
In a pure semicon-
(q@) The energy gap between conduction band
and valence band is of the order of 1 eV.
At any temperature, the number density of
holes in the valence band is equal to the
number density of electrons in the
conduction band in a pure semiconductor.
For silicon, energy gap = 1.21 eV
For germanium, energy gap= 0.79 eV.
Extrinsic or Doped Semiconductors.
(a) Conductivity of pure (or intrinsic)
semiconductor such as Ge or Si can be
considerably enhanced by doping it with
trivalent or pentavalent ‘impurity’.
(b)
Doping a pure semiconductor with
pentavalent impurity atoms such as
P, As and Sb yields n-type semiconductor.
Each impurity atom contributes one free
electron in addition to electron hole pairs
already existing in the semiconductor. Thus
ina m-ype semiconductor n, > n,. For this
reason, electrons are called majority charge
carriers and holes are called minority
change carriers in a n-type semiconductor
Each impurity atom is called a donor as it
donates one free electron to the semicon.
ductor. For phosphorous or arsenic
silicon, the lowest energy level occupied
by the donor electron is 0.045 eV heise
the bottom of the conduction band. ‘The
gether with? Physics (XI)
iconductors in-
semiconductor are free |
energy is comparable with the room,
perature energy of 0.03 eV and is mp
{inaller in comparison with the energy g
of 1.1 eV as shown below.
Donor Energy
f
Empt
Bea Conduction
Forbidden
(1.1eV) Bang
LTE AE eee
(c) Doping a pure semiconductor with trivalent
impurity atoms such as B, Al, In yields
p-type semiconductor. Each impurity atom
contributes one hole in addition to electron
hole pairs already existing in the semicon.
ductor. Thus, in a p-type semiconductor
n, > 1, For this reason, holes are called
majority charge carriers and electrons are
called minority charge carriers in a p-type
semiconductor.
Empty Conduction
Band
Forbidden Band
(-1.4 eV)
Completely Filled
Valence Band
9000000000000000
WOT:
Acceptor
Energy Level
Ej
ch impurity atom is called an acceptor
ise it accepts an electron in the valence
band to become ionised as negatively
charged. The holes introduced into the
Semiconductor by the acceptor impurity
have energy which is 0.04 eV above the
highest energy level of the valence band
as shown in figure,
ornare in semiconductors. \nwinsi¢
emiconductors conduct due
electrons ; 1e
ms and holes whose number density
same, ie., 1 je rumber density of charge Carriers ig tclaed
P
induction ina semiconducte F
poeta difference V is applied aereen a
semiconductor of area 4 and length 1 ae
euectrons and holes contribute 19 the einen,
its :
TeL+heenay 4 en, Av,
[SEMICONDUCTOR DEVICES
prdunction (Diode), 2. (a) Solar cel},
i Me junction transistor
i.pa Junction (Symbol —ff
Ap junction consists of wafers of
and n-type semiconductors fused toge
gown on each other. At the time of j
formation, the free electrons from type
Semiconductor and holes from p-type.
Semiconductor (i.e., majority charge carriers) |
diffuse into each other and their recombination
‘Geates a depletion region which is of th der
of a few microns in thickness. Creation of
P-type
ther or
junction
7
V,=U3 V. {This barrier potential seis up a
fil Across the junction directed from n-type |
semiconductor to p-type semiconductor. Under
the influence of this field, minority charge
carriers drift across the junction in a direction”
“Opposite to the diffusion current till drift current
mes equal to the diffusion current.
absence of external applied field, the
Re cinent through ‘the junction)
Deptetan region, ee) |
@ tis a Tegion near the p-n junction that is,
depleted of any mobile charge carrier, It)
only consists of uncovered immobile carrier.
© The width of depletion region (w) isa)
Measure of resistance (X) of p-1 junction |
diode.
ie, Re fy
, (b) Photodiode, (c) Zener diode,
The electrical conductivity of the semi-
Conductor is given as
Oe, hen #9]
Here 1, is the mobility of electrons and p, is
the mobility of holes. Mobility 1 of a charge-
Cartier is defined as the drift velocity per unit
Clectric field. Change in conductivity of a
Semiconductor is mainly due to changes in
Carrier concentration,
|
(d) Characteristics of LED,
(©) The depletion width depends on
(i) type of biasing,
(ii) extent of doping.
Forward biasing of a p-n junction.
(@ A p-n junction is said to be forward biased
when p region is maintained at a higher
potential with respect to the n region as
shown below :
(@) When forward biased, majority charge
carriers in both the regions are pushed
through the junction. The depletion region’s
width decreases and the junction offers low
resistance.
x R
3V ov
R
o>} wws-—_§o
ov -5V
Reverse biasing of a p-n junction.
(a) A p-n junction is said to be reverse biased
when its p region is maintained at a lower
potential with respect to its n region as
shown in figure.
R
ov 3v
R
o——_ >} wwe
3v 6v
(b) When reverse biased, majority charge
carriers in both the regions are pushed
away from the junction, The depletion
region’s width increases, The minority
Electronic Devices | 223 charge carriers, however, are pushed |
through the junction thereby causing a little |
current in the reverse biased p-n junction. |
The p-n junction thus offers a very high |
resistance when reverse biased.
g |
=
£
3
: (knee voltage
or onward Bias (V) at ;
jeyerse Bias
7 3
i
Breakdown 2
voltage or §
Zener voltage g
=
tS
Forward characteristics of a p-n junction
diode. It is a graphical relation between the
forward bias applied and the forward current
flowing through the diode. As long as the
forward bias is less than the barrier potential,
no current flows. But when a forward bias is
greater than barrier potential applied, an almost
linear forward current (= a few mA) flows due
to the flow of majority carriers.
Reverse characteristics of a p-n junction
diode. It is a graphical relation between the
reverse bias applied and the reverse current
flowing (if any). In this case, the majority
carriers in p and n-type both move away from
the junction so that no majority current flows.
However minority carriers do cross over the
junction constituting a small current (= a few
HA) which is called minority current or leakage
current. As the magnitude of the reverse bias
is increased (in magnitude), the leakage current
also rises gradually. At a particular reverse bias,
the reverse current increases abruptly (i¢.. i:
becomes very large ee
nes 'ge suddenly). The reverse bias
at which the reverse current rises abruptly, is
called Zener voltage or Breakdown voltage
Pn Junction as a rectifier. Rectification is hg
Process of conversion of AC into DC. A single
P-n junction or two or four P-nj ee
be used for this purpose,
junctions can
“Vegethen with? Physics (XI)
a ectification. A single p-n juncy;
Half ware ania rectification,
conducts only during alternate half-cycle oft,
input AC voltage. Asa result, the voltage ow
ddges not change in polarity. The average of
voltage from a half-wave rectifier is low,
>
Half-wave rectifier
°
Output d.c/volt Input a.c/volt
P ns. It condticts for both halves
of the cycle. The average voltage of a full-wave
rectifier is more than that of a half-wave rectifier,
for the same rm.s. value of AC voltage.
NI
l
ot
Full-wave rectifier
Output d.c/volt Input a.c./volt cell A solar cell is a junction
can convert light energy ‘inte lode
. In such a p-n junction, one oie
jons either p orn is made 50 thin i ji
"ight enerey falling On it is not absorbed
~ much before reaching the junction, The thi
jon in such a junction is calleg emit :
~ 4nd the other region is called the base, _
Photodiode. It is a junction diode from
sitive semiconductor material, ih
“ach adiode, there is a provision to ation
the light of suitable frequency to fall on
the p-? junction.
) Zener diode. A zener diode is a speci
~ designed junction diode which cae
continuously, without being damaged in the
_ segion of reverse breakdown Voltage.
ie Zener diode as a voltage regulator : For
widely different zener currents, the voltage
across the zener diode remains constant.
On account of this fact we can use zener
_ diode as a d.c. voltage regulator. }
| Uneguiatea [| _,'
2 OY Regulated
, voltage (V)
or: e
, For input voltage V, > V., zener diode is in
the breakdown condition. Thus, for wide
Tange of values of load (R,), current
through the zener diode may change but
the voltage across it remains constant.
Characteristics of LED (CBSE
Supplementary Textual Material). Light
_ Emitting Diode is basically a p-n junction |
diode, It has two leads. Longer lead
Corresponds to p side (anode) and shorter
lead is for symbol for LED is)
side (cathode). |
oat fat
s_ pf
Forward
current
(ma)
For LED, I-V characteristics is similar to
that for p-n junction diode.
In case of LED ‘barrier potential’ changes
Slightly with the colour.
pidiai
g
iE
ye
Due to recombination of holes and
electrons energy is released at the junction,
which is emitted as light.
Hence, junction diode is called light
emitted diode.
The colour of light emitted by a given LED,
depends on its band-gap energy.
The photon emitted by an LED is of energy
equal to or slightly less than the band-gap
energy. Forward current conducted by the
junction determines the intensity of light
emitted by the LED.
A low voltage DC supply is required to
operate an LED. Current drawn by LED's
is of the order of milliampere. So, in
practice a resistor of suitable value is joined
in series with the LED to limit the current
up to the safe value required.
3. The Junction Transistor.
A junction tra u
terminals device. The three terminals are called
sistor is a two junctions, three
Electronic Devices |225 [ma6)
dj
base and the collector and
a in and p-type of
the case of @
pe of semi-
iter,
semiconductors respectively in
p-nep transistor or n. p and 1-1
‘conductors in the case of @
shown in figure. Figure also sho\
of a p-n-p and a n-p-n transistors.
c c
P
fl B |
Bot |
|
a '
Te &
p-rp Transistor
c c
A
n
B
Bin
a
Te E
n-p-n Transistor
The two junctions in a transistor are the |
emitter-base junction and the collector-base
junction.
Transistor as an amplifier. An amplifier is a
device which is used for current or voltage
amplification as a result of which we generally
get power amplification. A transistor, whether
p-n-p or n-p-n, can be used as an amplifier in
the following three configurations :
(a) Common Emitter Configuration.
(6) Common Base Configuration.
(c) Common Collector Configuration.
In each configuration, the emitter-base junction
is forward biased and the collector-base junction
is reverse biased as shown below :
roe fl
=n |p on Ie.
|B
Mos a
tHE hs]
“Pigether with? Physics (X11)
show emitter
sven figures also s! current 7
ei iy ‘and the collector current 7;
: pbs
sapectve directions and it canbe Soy
they are related as :
Ip + Fo
While constructing a transistor, it base rg
is lightly doped and is kept thin. As a rey
this, base current J, is only a small fraction o¢
the emitter current /,.
it gain of a transistor.
The arent gain ot is defined as the ratig, o
change in collector current to the change iy
emitter current for constant value 7]
collector voltage in common-base
fe-(Ge) | %
a ranges from 0.9 to 0.99 for junction
transistors.
(b) The current gain B is defined as the ratio of
change in collector current to the change in
base current for constant value of collector
voltage in common-emitter configuration,
ie,
= constant
Ale |
(2)
B ranges from 20 to 200, which means
that a small change in base current will
cause a large change in collector current.
(©) On the basis of definitions of ot and Band
using the relation 1p = Ty + [py it can be
shown that
Common base transistor amplifier.
Principle : In common-base configuration, the
Emitter base resistance (input resistance) is Ve
Small. The collector base resistance (ou!
Torenne®) is very large (of the order °
(ie) dads permits a very large value of! e
a ‘ ich can be connected in the collec! :
oulputem ee? circuit without affecting 1
is that ¢ rent (© @ large extent. The advan
# high output is obtained across Rr E
at ; The input signal (V,) to be i
Fn: A ampl
Gir nected BCTOSS the emitter termi ‘a
* os with the forward bias battery, A hi fh
6" ace R, called load (108 Q) j ted
Characteristics :
0) A large voltage gain
(i) A large current gain
stancs connect ‘in
ies ihe collector-base terminals in ae (iii) A large power gain :
site reverse bias aley. The output i (iv) There is a phase difference of m in between
Mined ar0ss R; _ the output and the input voltage.
Limitations :
(i) The voltage gain of the amplifier can’t be
increased to very large values.
(ii) The voltage gain is constant for a limited
range (amplitude) of the a.c. input signal.
Common emitter amplifier is most efficient :
(®) High current gain.
1. = Bl.
Current gain may range from 20 to 500.
(ii) High voltage and power gain.
(iii) Moderate output to input impedance ratio.
The ratio of output impedance and input
impedance is small (about 50). Hence
| common emitter configuration is used for
| coupling between the various transistor
stages.
Common base characteristics of a transistor.
The characteristics of a transistor, when the
base is kept as the common terminal and
grounded (zero potential), the emitter as the
input terminal and collector as the output
terminal, are called common base
characteristics of the transistor, There are three
types of common base characteristics :
(i) Emitter characteristics (or input
characteristics). This is a graph showing a
relation between emitter voltage and
emitter current at constant collector voltage.
Characteristics :
=(g Low current gain («1 < 1)
(i) High resistance gain
fi) High voltage gain
(i) High power gain |
xR,
() Voltage gain = &
(i The output voltage is in phase with the
applied input voltage.
Common emitter transistor amplifier.
The a.c. input signal to be amplified is |
‘connected in between emitter and base in series
‘with the forward bias battery. The load is
connected in between the collector and the
emitter in series with reverse bias battery. The
ouput is taken across the collector and the
ground.
Tem),
eo),
yb @ @ >
Output |
Emitter current
| 0 02 04 06 08 10
| feo \V)
Emitter base voltago—>
(ii) Collector characteristics (or output charac-
teristics). This is a graph showing a relation
lo
Fora transistor circuit
oJ. FI,
athe output eitcuit
go = TR, + Veg
ce = Veg = IR,
Electronic Devices
227 between collector voltage and collector |
current at constant emitter current. |
|
3 0. 5 1015 20 2
Collector voltage (Ves)—>
Common emitter characteristics : |
The characteristics of a transistor when the |
emitter is kept as a common terminal and
grounded (zero potential), the base as the input
terminal and collector as output terminal, are
called common emitter characteristics. There
are two types of common emitter charac-
teristics :
(i) Input characteristics. This is a graph
showing a relation between V,, and J, at
a constant Vip.
O 04 06 08 1.0 12 14 16
Base to emitter voltage Vee (V)—>
(ii) Oupur characteristics. This is a graph
showing a relation between Vo, and J. at
a’constant J.
Bese cet
—_
&
Collector current /-(mA).
~
a
0 05 1015 20 25 3.0
0 35
Collector to emitter voltage Vee(V)——5,
228} Fagether wirk® Physics (XII)
Transistor as an oscillator.
‘An oscillator is basically a transistor ampli,
in which a portion of the output current, ©
back to the input w1 th the help of an indye
‘There is a tank-circuit which consists pe
inductor L and a capacitor C in paralig
each other. The frequency of the ac, cat
ee 7
from the oscillator is given by f= —1_
2nVic*
The portion which feeds back the curren»
the input from the output is called feed back
portion. It makes good the loss of energy
the tank circuit and hence produces electricg,
oscillations of a constant amplitude,
A simple LC oscillator (collector tuned) cir
diagram is shown in figure.
Mutual
| Induction
Te Output
>
Transistor as a switch
Depending on the values of V, the output voltage
(V,) switches between high and low values.
On applying Kirchhoff’s voltage rule to input
circuit (base-emitter circuit) we get
Vag =
Al)
jico™
Here, once y ;
2 ; sil
transistor) ig te feaches 0.6 V (
, y is
increases very little as Yi further (say upto 6 Vv)
ollector-emitter circuit, We have
Veg =I * Vee
1 Vor = Yeo 7 IR,
. Vee = Vv,
© VE Vee ~ IR, +(2)
equations (1) and (2) we conclude
When V; ie. Vag increases ftom zero
; |
digital circuit that follows certain logical
ship between one or more than one |
ingot and the output. The input and the output |
“ge voltages which are always in two state
oily. Thus, input or output may be low (called
ic 0) or high (called logic 1). Here logic
“may correspond to 0 V whereas logic 1 ma
‘comespond to 5 V.
The most commonly employed logic gates are
“OR, AND, NOT, NOR and NAND. Following
the logic symbol, Boolean expression and
‘Truth Table for each of these gates.
Gate:
1is a
TS ate SY S.
Y=A+B
(read as ‘Yis AOR B)
| Realisation of OR gate. |
0,
Pr ole]
Dz
wl] 4
‘onwards upto 0.6 V (for Si transistor)
transistor is in cut off state ie. I, is zero.
Since I,R, = 0, V, = Vee
(b) When V, becomes greater than 0.6 V the
transistor is in active state i.e. 1, starts
growing in the output circuit. V, decreases
as the term /-R, increases. Once I, attains
its maximum value (I= Ve/R,), transistor
is said to be saturated and V, = 0.
Y=A-B
(read as ‘Yis AAND B)
Realisation of AND gate.
D,
Le
NOT Gate :
"TRUTH TABLE
Yea
(read as 'Yis ANOT)
Realisation of NOT gate.
Electronic Devices | 229 +B
(read as ‘Yis AOR BNOT’)
hie
INTEGRATED CIRCUIT (I.
In a conventional electronic circuit we find
many passive components like R and C, active |
devices like diode and transistor all inter- |
connected by a number of wire connections.
Such circuits are bulky and large in size.
An entire conventional electronic circuit
fabricated on a small single semiconductor chip
is known as Integrated Circuit (IC).
Integrated circuit is light, low cost, more
reliable, small device which requires lesser
Power to operate.
In common emiter transistor configuration,
1. Input resistance
_ (Ape)
i
aly
7 ‘ce
2. Output resistance
_ (AVep)
are)
“Regethex with? Physics (XIN)
(read as ‘Yis AAND B NOT)
Depending on number of circuit components
or logic gates ICs are classified as
(a) Small scale
integration, SST
(b) Medium scale
integration, MSI (logic gates < 100)
(c) Large scale
integration, LSI
@ Very large scale
integration, VLSI (logic gates > 1000)
(logic gates < 10)
(logic gates < 1000)
Important Formulae
3. Current ampliplication factor
p = (Ble)
Bly Vee
1
a, = 4
4. Voltage ,
>
f
xis 4 is increaseq
tly, How will it affect (i) cot,
a sev comen + ‘ctor current
| [Delhi 29,
happens to the width of depletion ine |
ee junction when it is (i) forward Giner
ii) reverse bias 2 Al
give the logic symbol of NOR a A ol
Give the logic symbol of NAND gate, :
2009) \
5 [Al
Give the logic symbol of AND gate.[AT 2009) |
gate the reason, why GaAs is most commonly |
ysed in making of a solar cell, [AT 2008] |
Why should a photodiode be operated at a.
reverse bias ? [AT 2008] |
§, What will be the values of input A and B for.
the Boolean expression (A+B) (4-3) =1?
[Delhi 2007]
4, Which one of the two diodes D, and D, in the
given figures (7) forward biased, (ii) reverse
biased ? [Foreign 2007]
&
& Draw the voltage-current characteristic of a
tener diode.
\Daw the voltage-current characteristic of @
Pn junction diode in forward bias.
lame the logic gate realised using p-n junction
lodes in the given diagram. Give its logic
‘Ymbol.
- tO
= 4
Peer
y SHORT ANSWER Ty pp
—————--- Preyj,
: ous
Fgavanitr, doping level in base Years?
-2V
10. How does the d.c, current gain of a transistor
change, if the width of the base region is
increased ? [Delhi 2006C]
Name the type of biasing of a p-n jun
diode so that the junction offers very high
resistance, [Delhi 2005C]
Name one impurity each, which when added
to pure Si, produces (i) n-type, and (ii) p-type
semiconductor. [Delhi 2005C]
Draw energy band diagram of a p-type semi-
conductor. [Delhi 2005C; AI 2005C]
Why is the conductivity of n-type semi-
conductor greater than that of the p-type semi-
conductor even when both of these have same
level of doping ? [AT 2005C]
Name two factors on which electrical
conductivity of a pure semiconductor at a given
temperature depends. [AI 2005C]
M:
2
13.
14,
Z Additional Questions
19. How does the width of the depletion region of
a p-n junction vary, if the reverse bias applied
to it decreases ?
20. Write the truth table for the following
combination of gates :
Electronic Devices |231 i \
a1. In the given diagram, is the diode D forward |
or reverse biased? |
20 aw Die
22, Write the truth table for the combination o
gates shown here.
a—_ :
ae—
‘25, Draw the output waveform across the resistor.
+V
o---
=v
Input waveform at A
23, What happens to the width of the
— NCERT Question
Se the depletion
region in a p-n junction when itis (i)
biased (ii) reverse biased ? Orwary
24, Why can’t transistor be used as a rectifier
(Hors)
[NCERT Exemplar Problem)
[SHORT ANSWER TYPE QUESTIONS [1] [2 Marks]
Previous Years’ Questions
The current in the forward bias is known to be
more (mA) than the current in the reverse bias
(-HA). What is the reason, then, to operate the
photodiode in reverse bias ? [Delhi 2012]
. Describe briefly with the help of a circuit
diagram, the paths of current carriers in an
n-p-n transistor with emitter-base junction
forward biased and base-collector junction
reverse biased. [AT 2012]
. Write two characteristic features to distinguish
between n-type and p-type semiconductors,
[Foreign 2012}
4, Draw the output waveform at X, using the given
inputs A and B for the logic circuit shown
below. Also, identify the logic operation
Performed by this circuit. {Dethi 2011]
232
rp
»
“Pegether with? Physics (XIN)
' el | H
Pepe | Lt
1 ! 1 1
i ' !
Ce ecto
yop for ra
Toroto to tog 4
ic ' i 1 1 1 1
|| it
14 t tot
Pi}: Li
Se | kind
H it ‘ 1 1 ' 1
Pen A hw fe
t t 4 & ie & t
5, Draw the output waveform at X, using the give"
inputs A and B for the logic circuit show?
below. Also, identify the logic operatio®
Performed by this circuit, (Delhi 201!)
—>—T > ny
1 Name the device ‘D' which is used asa voltage
Tegulator in the given circuit and give its
Symbol. [Delhi 2011C]
o-aity—A
Fluctuat
De, Valage D et!" Conatant
i
1
g
f B
12, i ‘i
: | Identify the logic gates ‘X’ and ‘Y’ in the
Name the semiconductor device that can be | figure. Write down the truth table for output Z
ysed. to regulate an unregulated de | for all possible inputs ‘A’ and ‘B’.[AI 2011C]
| ply. With the help of 7-V characterinee
| soply. 9 ee tistics of Ao]
“this device, explain its working Principle. 5 Zz
(Delhi 2011 a '
j,baw the transfer characteristic curve g pe 13. Draw the circuit diagrams showing how a
i rae ‘ p-n junction diode is (i) forward biased and
transistor in ant
eee tive ei Suton, Esplin (ii) reverse biased. How is the width of
early ho omer ’, Versus depletio i 7
YV,curve in a transistor is used as an amplifier, | miei lager afta in ve
| [AI 2011C]
(Delhi 2011] 14,
} Pe (a) For the digital circuit given below, write
§. Wite the truth table for the logic circuit shown _ the truth table showing outputs Y, and Y>
below and identify the logic operation | for all possible inputs of A and B.
Baeemed by this circuit. (Delhi 2011) (b) Show output waveform for all possible
& (3 inputs of A and B. [AT 2011C]
| A YY;
15. Identify the logic gates marked *X’ and *Y" in
4In the given circuit, a voltmeter “V’ is
the following figure. Obtain the output Z of
connected across lamp ‘L’. What changes
the combination for all possible inputs
Would you observe in the lamp ‘L’ and the ;
Voltmeter, if the value of resistor ‘R’ is : :
Teduced? (Delhi 2011C] |
. i
A and B. [AI 2011C]
16. Draw the logic circuit of a NAND gate and
write its truth table, [Foreign 2011]
17. Draw the logic circuit of AND gate and write
its truth table. (Foreign 2011]
18. Draw the logic circuit of NOT gate and write
‘ its truth table. [Foreign 2011]
"Caton i valence 19, Draw the circuit diagram of an illuminated
lect Bail silleon both aye oe they | photodiode in reverse bias. How is photodiode
ist Bait cach. How [Delhi 2011C] used to measure light intensity 2[Delhi 2010]
ished?
Electronic Devices |233 20. (p Identify the logic gates marked P and Q)
in the given logic circuit. i
X for the inputs |
(ii) Write down the output at or TAT 2010)
A=0,B=0andA=1,B
21. (j) Identify the logic gates marked P and Q
in the given logic circuit.
(ii) Write down the output at X for the inputs
A=0,B=0Oand A =1, B= 1{AI 2010]
22. Write the truth table for the following circuit.
Name the gate that this circuit represents.
[Foreign 2010]
At
Y
23. Draw the output waveform for the following
gate. Also, name the gate. [Foreign 2010)
A
: bok &
24. Write the truth tabl i
le for the following circu
Name the equivalen cir
t gate that this circuit
represents, [Foreign 2010]
(234) gehen wk? Physics (XID)
28,
29.
i. following figure shows the inpuy
25. re (A, B) and the Output _Wavefor
0” of a gate. Identify the gate, write its try
table and draw its logic symbol.[Delhi 2099)
1 toy
Ar i 1
: + —
1
i
Hp oo
ot
1
toa
os
y
ee nels
:
: tod
0 1 2 3 4 5 6 7
26. (i) Sketch the output waveform from an AND
gate for the inputs A and B shown in the
figure.
Y
1
1
t
'
1
1
1
1
'
'
i
'
t
1 '
'
1
'
'
'
1
ron
rot
208 6 5 6
(4) If the output of the above AND gate is
fed to a NOT gate, name the gate of the
{Delhi 2009]
27. Draw the logic symbol of the gate whose truth
table is given below
combination so formed.
Input Output
A B Y
0 0 1
0 1 0
1 0 0
: 1 0
If i logic gate is connected to NOT gate, what
7 I be the output when (i) A= 0, B = 0 and
a A= 1. B= 1? Draw the logic symbol of
te combination, {Foreign 2009]
GyoBic gate is obtained by applying output of
al to a NOT gate. Name the gate 5°
eh ed. Write the symbol and truth table of
Alogie pate j Pe
{i © is obtained by applying output of
AnD g Want NOT gate. Name the gate 5°
this gate," "M Symbol and truth table of
. [Foreign 2009] ish between an intrinsic sem;
semiconductor, Give
a Teas
semiconductor crysta} is electrical
| ar sthough n, >>.
Eyal, although , >> n_-9 hi
gre given inputs-A, B are fed to a el
be NAND gate. Draw the output wave fonn of
tte ee Ate Leste (Delhi 2008)
'
eo
ee
oI LIAL f-
(ret) 1 t '
rp
1 t 1 i t
ft b bh uy & &
p lithe output of a 2 input NOR Gate is fed as
poth inputs A and B to another NOR gate,
ite down a truth table to find the final output
for all combinations of A, B.
[Delhi 2008}
g, Daw a circuit diagram showing the biasing of |
am LED. State the factor which controls _
() Wavelength of light (ii) Intensity of light
emitted by the diode. [Foreign 2008]
4. Draw and explain the output wave forms across
the load resistor R, if the input wave form is
asshown in the given figure. [Delhi 2006]
+5V
R
-5V
§, The output of an unregulated d.c. power supply
isto be regulated. Name the device that can
beused for this purpose and draw the relevant
Gireuit diagram. [Delhi 2006C]
The following figure shows the input wave
forms (4, B) and the output wave form (¥) of
4 gate, Identify the gate and write its truth
table, [AI 2006C]
A
(Input)
B
(Input)
y 1
(Output) t bt & &
It, |
37. The ac, current gain of a transistor is 120.
What is the change in the collector current in
the transistor whose base current changes by
| 100 pA ? [AT 2006C]
| 38: Show the biasing of a photodiode with the
| help of a circuit diagram, Draw graphs to show
| Variations in reverse bias currents for different
illumination intensities. [Delhi 2005C]
39. The signals A and B are used as two inputs of
NOR gate. Sketch the output wave form. Draw
| its logic symbol. [Delhi 2005C]
40. Which of the diodes is (i) forward biased, and
(ii) reverse biased in the following circuits ?
Justify your answer. [Delhi 2005C}
+3V
oo)
Vv +8V
0 (*)
41. Two signals A and B shown in the given figure
are used as two inputs of a NAND gate, Draw
its output wave form. Give the logic symbol
of NAND gate. [AT 2005C]
Draw the circuit diagram showing use of a
transistor as an oscillator. [AT 2005C]
-10V
— pew,
-8V
42.
Electronic Devices [235 a
aoe ee Additional
‘ow the biasing |
its two
diagram, show ©
diode (LED). Give |
‘ntional incandescent |
|
43. With the help of a
of a light emitting
advantages over conve!
lamps.
Draw a circuit diagram to
photo-diode is biased. Draw its
curves for two different
intensities. .
The V-I characteristic of a silicon diode is |
given in the figure. Calculate the diode resis- _
tance in forward bias at V= + 2 V. |
ram to show how @
characteristic
illumination |
45.
80 i-diode
3 Sidi
§ 60)
50)
§ a9
= 30)
= 20}
10}
1-2
va
V (Forward bias)
Cutin voltage
. A change of 0.2 mA in the base current causes
a change of 5 mA in the collector current for
a common emitter amplifier.
(i) Find the ac. current gain of the transistor.
(ii) If the input resistance is 2 kQ, and its
voltage gain is 75, calculate the load
resistor used in the circuit.
47. The potential difference across the collector
of a transistor, used in common emitter mode
is 1.5 V, with the collector resistance of 3 kQ.
Find
(i) the emitter and
(i) the base current, if the dc. gain of the
transistor is 50,
Determine the Currents through the resistance
R’ of the circuits (i) and (ii), when similg
diodes D, and D, a ut
1 ), are connected as shown.
48.
209
ave
236
Tegether with? Physics (XI)
Questions
49.
50.
av 209
the logic gates marked X, Y jy
Identify a
following figure. Write down the output az
when A= 1, B= land A=0,B=1,
a—
B
given below the
In only one of the circui
it ? Give reason
lamp L lights. Which circuit is
for your answer.
(a) (b)
When is a transistor said to be in a state of
saturation ? Explain
. When is a transistor said to be in a state of
cut-off ? Explain,
- A p-n junction, when forward biased, has #
Potential drop of 0.7 V which is assumed !
be independent of current. If a 5 V batten
(2,be used to forward bias it, calculate the
value of the resistance which should be ¥5
10 series with it, for 1 mA current to flow it
7a [Ans, 43 x 10°91
a circuit shown in figure uses 2 2!
7 o 2 V as a voltage regulator 10
What sre tate fecorder on 110 V de. lie
Should be the value of the resistor Ry’,
Ie dic, supply varies from 102 V -I! :
juots!
di
2
\ Oe i5V
void Tape
recorder
- n-p transistor circuit, the collector
|
wes 10 mA. If 90% of the holes reach |
4 collector, find emitter and base current, |
(HOTS)
jie the truth table for the combination of
aes shown (HOTS) |
SO
pla |
% |
|G Siand Ge have same lattice structure. Why |
isC insulator while Si and Ge intrinsic semi- |
‘conductors ? |
|The current in the forward bias is known to be |
‘more (~mA) than the current in the reverse
bias (~ pA). What is the reason, then, to
‘operate the photodiodes in reverse bias ?
L From the output characteristics shown in |
igure, calculate the values of B,, and B,, |
ofthetransistor when V_,,is 10V and/,=4.0mA.
Base current (/,)
pos
Collector current (MA).
2 ©
0 2 4 6 B 10 12 14 16
Collector to emitter voltage Vee (V) >
BA pon Photodiode is fabricated from
qitmiconductor with band gap of 2.8
it detect a wavelength of 6000 nm *
[NCERT Exemplar Problem] |
57,
58,
NCERT Questions —
f 2.8 eV.
Draw the output wave forms for the gates
shown below :
JUL
0 a Y
Alea —
Find out the readings of the ammeters A, and
A, shown in the figure. Neglect the resistance
of the meters, {HOTS}
A 102
O-K
oO->
63. s 4
= T
(HA)
Pp
—— Vv
a
(a) (0)
(i) Name the type of a diode whose
characteristics are shown in the figure (a)
and figure (b).
(ii) What does the point P in Figure (a)
represent?
(iii) What does the points P and Q in Figure
(b) represent?
[NCERT Exemplar Problem]
64, Three photo diodes D1, D2 and D3 are made of
semiconductors having band gaps of 2.5 eV, 2eV
and 3eV, respectively. Which ones will be able
to detect light of wavelength 6000 A?
(NCERT Exemplar Problem]
65. If the resistance R, is increased figure, how will
the readings of the ammeter and voltmeter
change? [NCERT Exemplar Problem]
Electronic Devices | 237 either of the garages or cars enter both, Dey;
ecireuit that resembles this situation y5i®
diodes for this situation. ing
[NCERT Exemplar Pro},
are elemental dopants for Silicon
Germanium usually chosen from group m
or group XV? [NCERT Exemplar Probie
Can the potential barrier across ap-n unctionty
'd by simply connecting a voltmere,
?0NCERT Exemplar Problem
67. Why
68.
measure
across the junction
common gate which |
66. Two car garages have a
ly when a car enters |
needs to open automaticall
stions ——
For what purpose is a photodiode used ?
1—V characteristics for different
s of illumination. [AT 2011C]
— _. Previous Years’ Que
1. Draw a labelled diagram of a full wave rectifier | (b)
circuit. State its working principle. Show the
input-output waveforms. [AT 2011] |
2. You are given a circuit below. Write its truth 6. (a) Describe the working of light emitting
table. Hence, identify the logic operation diodes (LED's).
carried out by this circuit. Draw the logic (b) Which semiconductors are preferred to
make LED's and why ?
symbol of the gate it corresponds to. [AI 2011] |
| (c) Give two advantages of using LED’s over
conventional incandescent low power
a—_of>»—*
Bl lamps. [AI 2011C]
[ 7, Name the important processes that occur during
ae—_{ »— y the formation of a p-n junction. Explain briefly,
with the help of a suitable diagram, how a p-
n junction is formed. Define the term ‘barrier
potential’. [Foreign 2011]
8. Draw transfer characteristics of a common
emitter n-p-n transistor. Point out the region in
which the transistor operates as an amplifier.
insistor
-—t) > x Define the following terms used in tral
amplifiers : (Foreign 2011]
Zz (i Input resistance
—) »— (i) Output resistance
(ii) Current amplification factor
9. Give a circuit diagram of a common emitter
3. You are given a circuit below. Write its truth
table. Hence, identify the logic operation
carried out by this circuit. Draw the logic |
symbol of the gate it corresponds to. [AI 2011]
4, Explain briefly with the help of a circuit
diagram, the working principle of a transistor amplifier using an n-p-n transistor. Draw th
amplifier as an oscillator. [AI 2011C] | input and output waveforms of the signal. write
5. (a) Why is a photodiode operated in reverse the expression for its voltage gain.[Al 2
bias mode?
238 | “Jogether with® Physics (XII) with the help of circuit q
stinguish between forward bj
reverse biasing Of @ p-n junct;
yw V-I characteristics of a
UM giode in (a) forward bias,
bias-
iagrams, |
lasing and |
ion diode, |
P-n junction
() reverse
i : ' [AT 2009]
I Pin with the pe of a circuit diagram how |
eet diode works as a DC Voltage Tegulator,
its 1- V characteristics. [AT 2099) |
w the circuit diagram of a ful) wave |
ifier. Explain briefly its working Principle. |
plot the graphs of the input and output
avefOrms. (Foreign 2008] |
the circuit diagram of a common emitter
amplifier using n-p-n transistor. What is the
difference between the input signal and
output voltage ? State two reasons why a
common emitter amplifier is preferred to a
Additional
| Explain briefly why the output and input signals
ofacommon- emitter amplifier differ in phase
by 180°.
{ Draw the circuit diagram of a common-emitter
amplifier using an n-p-n transistor. Draw the
input and output wave forms of the signal.
Write the expression for its voltage gain.
| (@)In the working of a transistor, emitter-
base (EB) junction is forward biased while
collector-base (CB) junctions reverse
biased. Why ?
(6)The input resistance, in the common
rcuit of a given
a value of 1.5 kQ The
Output of this circuit, is obtained across a
collector resistance of 7.5 k2. What would
be the output voltage, corresponding to
an input voltage of 5 mV, if the current
amplification factor, of this transistor has
value of 60 2
| =
emitter ampl
transistor, ha
Questions
{AI 2007]
Hi is an intrinsic semiconductor ? How can
Ns Material be converted into (i) p-type
(i) n-type extrinsic semiconductor ? Explain
with the help of energy band diagrams.
[Delhi 2006}
For a common emitter transistor amplifier, the
audio signal voltage across the collector
Tesistance of 2 kQ is 2 V. If the current
amplification factor of the transistor is 100,
calculate (i) input signal voltage, (ii) base
Current, and (iii) power gain. Given that the
Value of the base resistance is 1 kQ.
[Delhi 2005C]
On the basis of the energy band diagrams
distinguish between metals, insulators and
semiconductors. [AI 2005]
Common base amplifier.
14,
15.
16.
20. With the help of a labelled circuit diagram,
explain how an n-p-n transistor is used to
produce self-sustained oscillations in an
oscillator.
21. With the help of energy band diagrams, distin-
guish between conductors, semiconductors and
insulators.
Draw the circuit diagram of a common-emitter
amplifier, with appropriate baising. What is
the phase difference between the input and
output signals ? State two reasons why a
common-emitter amplifier is preferred to a
common-base amplifier.
. Draw a circuit diagram to obtain the
characteristics of a n-p-n transistor in emitter
configuration. Describe how you will obtain
input and output characteristics. Give shape of
the curves.
|. With a circuit diagram, briefly explain how
a zener diode can be used as a voltage
22.
239
Electronic Devices 25.
26.
_____NCERT Q
cr, the audio signal
stance of 2 k
ification factor
For a CE-transistor amplifi
voltage across the collected res
is 2 V, Suppose the current amp!
of the transistor is 100, find the input signal
Voltage and base current, if the base re stance
is 1kQ. [V,= 0.01 Vs J, = 108A]
Two amplifiers are connected one after the
other in series (cascaded). The first amplifier
has a voltage gain of 10 and the second has a
voltage gain of 20. If the input signal is 0.01
volt, calculate the output a.c. signal.
[Ans. 2 V]
Write the truth table for circuit given in figure
below consisting of NOR gates and identify
the logic operation (OR, AND, NOT) which
this circuit is performing.
(Hint : A = 0, B= 1 then A and B input of
second NOR gate will be Q and hence Y = 1.
Similarly work out the values of Y for other
combinations of A and B.
Compare with the ©
truth table of OR, AND, NOT gates and find |
the correct one.)
>>
Write the truth table for the circuits given in |
figure below consisting of NOR gates only. —
Identify the logic operations (OR, AND, NOT)
performed by the two circuits.
ane
=D.
i’)
yo
240
“Tegether with® Physics (XIN)
uestions ——
29.
30.
31.
32.
33.
.n two circuits as shown in fj 7
t of NAND gates. Ieentify 4
carried out by the two ies
‘You are give!
which consis
fogic operation
ae
é
aoa! (a)
tL]
off,
®
0 circuits as shown in figuy
re,
(a) acts as OR gate while
AND gate.
Y
You are given tw‘
Show that circuits
the circuit (b) acts as
> >
:
@
®
Write the truth table for NAND gate connected
as given in figure. Hence identify the exact
logic operation carried out by this circuit.
A
ee
Y
How would you set up a circuit to obtain NOT
gate using a transistor?
[NCERT Exemplar Problem]
A Zener of power rating 1 W is to be used &
a voltage regulator. If zener has a breakdow?
_ 5V and it has to regulate voltage which
luctuated between 3V and 7V, what should be
the value of Rs for safe operation ?
[NCERT Exemplar Pro!
lem
Rs
Unregulated
voltage Regulated
voltage ws simple circuit of a CE transi
;fier. Explain its working, ee
ge gains Ay» Of the amplifier is given by
i
load resistance and r, is i
ance of the transistor. te
significance of the negative sign in the
expression for the voltage gain ? |
: (Delhi 2012) |
Draw the circuit diagram of a full wave |
: rectifier using p-n junction diode. Explain |
| ts working and show the output, input |
| waveforms. {
| (p Show the output waveforms (Y) for the
following inputs A and B of i
(i) OR gate (ii) NAND gate [Delhi 2012] |
! |
mB. , |
E ee , where B. is the current atin, R, |
ett th tte t Dp
as i |
i a
= |
Beeb i iid ' '
! wot rie 1 1
Beeit oui ! !
i '
meif iu 1
a i '
mii ty ' '
todt i
4 (@) Draw the circuit for studying ¢
he: and output characteristics of an D-P-P
i transistor in CE configuratio
; the input
how, from the output characteristic
; amplification factor (B,) can be obtained
yn. Show,
s, the
information about the current
(6) Draw a plot of the transfer characteristic
(V, versus V,) for a base-biased transistor |
| in CE configuration. Show for whic!
Tegions in the plot, the transistor can
[Foreign 201
Operate as a switch. ial |
Why is a zener diode considered as @ SPC"
Purpose semiconductor diode ?
ONG ANSWER Ty,
TYPE QUESTIONS [5 Marks]
. Previous Years?
Questions
Draw the I - V characteristics of a zener diode
and explain briefly how reverse current
Suddenly increases at the breakdown voltage.
Describe briefly with the help of a circuit
diagram how a zener diode works to obtain a
constant de voltage from the unregulated de
output of a rectifier. [Foreign 2012]
5. (a) Describe briefly, with the help of a
diagram, the role of the two important
processes involved in the formation of a
p-n junction.
(b) Name the device which is used as a
voltage regulator. Draw the necessary
circuit diagram and explain its working.
[AI 2012]
Explain briefly the principle on which a
trnasistor-amplifier works as an
oscillator. Draw the necessary circuit
diagram and explain its working.
(b) Identify the equivalent gate for the
following circuit and write its truth table.
ato—_ a
=
aef>o——_
[AI 2012]
7. Ana.. signal is fed into two circuits “X’ and
+y’ and the corresponding output in the two
cases have the waveforms as shown.
Oo Po ++
aA Po?
s °X’ and *Y’. Draw
t diagrams.
(a) Identify the circuil
their labelled ¢
(b) Briefly explain the working of circuit Y.
(c) How does the output waveform from
cuit Y get modified when a capacitor
is connected across the output terminals
arallel to the load resistor?
a [Delhi 2011C]
Electronic Devices |241 SX @) Explain the formation of depletion layer |
and potential barrier in a p-n junction.
(®) In the figure given below the
input waveform is converted into
the output waveform by a device ‘X’
Name the deyice and draw its circuit
diagram.
|
Output
Input
(c) Identify the logic gate represented by the |
circuit as shown and write its truth table. |
[Delhi 2010] |
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