Semiconductor Physics, Electronic Devices and Circuits

8/10/2019 Semiconductor Physics, Electronic Devices and Circuits

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Biyani's Think TankConcept based notes

Semiconductor Physics,

Electronic Devices and Circuits(BCA Part-I)

Ashish SharmaM.Sc. (Physics)

Anupama Upadhyay

Revised By: Mr VijayM.Sc. (Physics)

LecturerDeptt. of Information Technology

Biyani Girls College, Jaipur


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2

Published by :

Think TanksBiyani Group of Colleges

Concept & Copyright :

Biyani Shikshan SamitiSector-3, Vidhyadhar Nagar,

Jaipur-302 023 (Rajasthan)

Ph : 0141-2338371, 2338591-95 Fax : 0141-2338007E-mail : [email protected] :www.gurukpo.com; www.biyanicolleges.org

ISBN: 978-93-81254-35-9

Edition : 2011Price :

Leaser Type Setted by :Biyani College Printing Department

While every effort is taken to avoid errors or omissions in this Publication, anymistake or omission that may have crept in is not intentional. It may be taken note of

that neither the publisher nor the author will be responsible for any damage or loss of

any kind arising to anyone in any manner on account of such errors and omissions.


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Semiconductor Physics, Electronic Devices and Circuits 3

Preface

I am glad to present this book, especially designed to serve the needs of thestudents. The book has been written keeping in mind the general weakness inunderstanding the fundamental concepts of the topics. The book is self-explanatory andadopts the Teach Yourself style. It is based on question-answer pattern. The languageof book is quite easy and understandable based on scientific approach.

Any further improvement in the contents of the book by making corrections,

omission and inclusion is keen to be achieved based on suggestions from the readersfor which the author shall be obliged.

I acknowledge special thanks to Mr. Rajeev Biyani, Chairman& Dr. Sanjay Biyani,Director (Acad.) Biyani Group of Colleges, who are the backbones and main conceptprovider and also have been constant source of motivation throughout this Endeavour.They played an active role in coordinating the various stages of this Endeavour andspearheaded the publishing work.

I look forward to receiving valuable suggestions from professors of variouseducational institutions, other faculty members and students for improvement of thequality of the book. The reader may feel free to send in their comments and suggestions

to the under mentioned address.Author


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SyllabusB.C.A. Part-ISEMICONDUCTOR PHYSICS,

ELECTRONIC DEVICES AND CIRCUITS[This course is of introductory nature as such rigorous mathematical analysis should be avoidedand emphasis should be on concepts and contents of mathematical expressions]

1. Structure of Matter (Molecule, Atom), Atomic Structure (Energy Levels and ElectronicConfiguration), Intermolecular Forces, Phases of Matter, Types of Solids, CrystalStructure of Solids, Atomic Bonding (Ionic Covalent and Metallic Bondings), Energy

Band Theory of Crystals, Energy Band Structure of Insulators, Semiconductors andMetals.

2. Mobility and Conductivity, Electrons and Holes in Intrinsic, Semiconductor ElementaryProperties of Germanium and Silicon, Donor and Acceptor impurities, ExtrinsicSemiconductors, Generation and Recombination of Charges, Diffusion.

3. Energy Band Structure of Open Circuit P-N Junction, Depletion Region, P-N Junctions asa Rectifier, Current Components of a P-N Diode, Ideal Voltage, Ampere Characteristics,Temperature Dependence of the V-I Characteristics, Diode Resistance Varactor Diodes,

Junction Diode Switching Times, Breakdown Diodes, Tunnel Diode, SemiconductorPhotodiode, Photovoltaic Effect, Light Emitting Diodes.

4. Half-wave and Full-wave Rectifiers, Ripple Factor, Efficiency, Voltage Regulation,

Inductor Filters, Capacitor Filters, L and Section Filters, Regulated Power Supplies,Information about SMPS Supply.

5. Bipolar Junction Transistors, Bipolar Transistor Action, Basic Principle of OperationOpen Circuited Transistor, Transistor Biased in Active Region, Current Components in aTransistor, Characteristic Curves in Common Emitter, Common Base and CommonCollector Configurations, Expression for , and , Hybrid Parameters of a Transistor.

6. Transistor as an Amplifier, Frequency Response of an Amplifier, RC Coupled Amplifier,Feedback Concepts and Oscillator.

7. Elementary Information about Field Effect Transistors, Thyristors, OptoelectronicDevices and Display Devices.


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Content

S. No. Name of Topic

1. Structure of Solid

1.1 Types of Solids

1.2 Crystal Structure of Solids

1.3 Atomic Bonding

1.4 Energy Band Structure

2. Transport Phenomena in Semiconductor

2.1 Mobility and Conductivity

2.2 Semiconductor Ge and Si (IntrinsicSemiconductors)

2.3 Extrinsic Semiconductors

2.4 Diffusion

3. Semiconductor Diodes

3.1 Open Circuit P-N Junction

3.2 Temperature Dependence of the V-I Curve

3.3 Types of Diodes

4. Rectification & Power Supply

4.1 Half Wave and Full Wave Rectifier

4.2 Filters

4.3 SMPS Supply


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Chapter-1

Structure of Solid

Q.1 What do you understand by Interatomic and Intermolecular Forces? How arethey different from each other?

Ans.: The Net force of attraction between atoms is called Interatomic Forcewhereasthe net force of attraction between molecules is calledIntermolecular Force.

Interatomic Forces are mainly of four types :

(i) Ionic Bond (ii) Covalent Bond

(iii) Metallic Bond (iv) Vander Waals Bond

Types of Intermolecular Forces :

(i) Dipole Dipole Interactions

(ii) Instantaneous Dipole Induced Dipole Interactions

(iii) Hydrogen Bond

Differences between Interatomic an Intermolecular Forces :

(i) Intermolecular Forcesare generally weaker than theInteratomic Forces.

(ii) The equilibrium separation of two molecules is large compared to theequilibrium separation of two atoms.

(iii) Intermolecular Forcesare generally dependent on the relative orientationof two molecules in addition to the dependence on the distance betweenthen but forInteratomic Forcesit s not so.

Q.2 What do you understand by Space Lattice? List the name of FundamentalCrystal System.


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Ans.: Lattice : The Lattice is defined as an array of points in space such that the

environment about each point is same with the environment about any otherpoint.

Linear Lattice : It is are array of infinite points is one dimension.

Two Dimensional Lattice :It is a repetition of linear lattice in another direction

by a regular interval of distance.

Space Lattice or Three Dimensional Lattice : If the points of two dimensional

lattice are repeated in another non-coplanar direction by a regular interval of

distance, then they form three dimensional lattice called Space Lattice.

Fundamental Types of Lattice :(i) Cubic

(ii) Tetragonal

(iii) Orthorhombic

(iv) Rhombohedral

(v) Hexagonal

(vi) Monoclinic

(vii) Triclinic

Q.3 What do you mean by Crystalline and Amorphous Solids? Point out some

important differences between them.

Ans.: The solids which have regular and periodic arrangement of atoms or molecules

in a definite and long range order are called Crystalline Solids.

Amorphous or Glassy Solids :Solids which do not have any permanent shape

or do not have any regular and periodic arrangement of atoms or molecules are

said to beAmorphous Solids.

Difference between Crystalline and Amorphous Solids


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S.No. Amorphous Solids Crystalline Solids

These are isotropic in nature. These are anisotropic.

All the bonds in the

Amorphous Solids are not

equally strong.

All bonds have same bond

strength.

These are not bounded by flat

surfaces.

The Crystalline Solids are

bounded by the flat surfaces.

They lack a long-range order

of bounding in their structure.

There is definite and long range

order of arrangement of atoms

or molecules in a Crystalline

Solid.

Q.4 What are Conduction Band, Valance Band and Forbidden Gap. Explain these

bands by proper diagrams.

Ans.: Valance Band :The energy band in which the valance electrons are situated is

called Valance Band.

Conduction Band : The allowed energy band above the Valance Band (i.e.

having higher energy state) which is almost empty in the unexcited state is called

Conduction Band.

Forbidden Energy Gap :In between the Conduction and Valance Bandsthere is

an energy gap in which no allowed energy level is situated. This energy gap is

calledForbidden Energy Gap.


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As shown in above figure PQ represents Conduction Band, RS is Valance Bandand QR isForbidden Energy Gap.

Q.5 Compare the resistivities of Conductors, Semiconductors and Insulators. Whatthe order of Forbidden Energy Gap in there substances?

Ans.: Conductors : In conductors, forbidden energy gap is not present i.e. Eg= 0. At

normal temperatures their electrical conductivity is very high of the order of 106

to 108 mho/m and resistivity is very low of the order of 10-8to 10-6Ohmm.

Insulators : The forbidden energy gap Eg in insulators is very large. Their

conductivity is very small of the order of 10 -12to 10-18mho/m. The resistivity of

such materials is very high of the order of Eg> 5 ev.

Semiconductors : In these materials there is a finite forbidden energy gap

between their conduction and valance bands but it is much less their the

forbidden energy gap of insulators, It is of the order of 1 ev.


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The conductivity of semiconductors is of the order of 10 -7 to 10 mho/m and

resistivity of the order of 10-1

to 107

ohm-m.

Q. 6 Write down the Postulates of Bohrs Atomic Model.

Ans.: Postulates of Bohrs Atomic Model : According to Bohrs Model, an atom iscomposed of electrons moving in various fixed circular or elliptical orbits arounda heavy nucleus made up of protons and neutrons.

Diagram : An Atomic Structure

Electrons :Electrons are negatively charged elementary particles moving aroundthe positively charged nucleus in different but fixed orbits. Mass of an electronicis 9.1 x 10-31kg and charge on electron is -1.6 x 10-19 coulomb.

Nucleus :It is a central hard care of an atom and contains protons and neutrons.Neutrons are electrically neutral while protons carry positive charge equal to themagnitude of electrons charge. Mass of Both protons and electron is about 1840times of electrons mass.

Atomic Number (Z) :The number of protons on atom has is called its AtomicNumber (Z).

Atomic Mass Number (A) :The total number of protons (Z) and neutrons (N)contained in the nucleus of an atom is called itsAtomic Mass Number (A).

Multiple Choice Questions:1. According to Daltons atomic theory, an atom can:


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(a) be destroyes

(b) be created(c) either be destroyed ar created

(d) neither be destroyed nor created ( )

2. A matter has neither definite volume nor fixed shape is:

(a) solid (b) liquid

(c) gas (d) None of the above ( )

3. The maximum number of electrons possible in the shell of principal quantum number

1n is

(a) 2n (b) 22n

(c) 2n (d) n(n+1) ( )

4. The strongest bond is:

(a) Ionic bond (b) Covalent bond

(c) Molecular bond (d) Metallic bond ( )

5 The reason for inter-molecular forces is:

(a) Only dipole-dipole interactions

(b) Only induced dipole interactions

(c) Dispersive forces(d) All of the above ( )

6 The strongest bond is:(a) lonic (b) Covalent

(c) vander Waal (d) Metallic ( )

7 In the metals, the most common interaction is:

(a) ion-ion (b) ion-electron

(c) election-electron (d) repulsive ( )

8 Election-volt is the unit of:

(a) Momentum (b) Velocity

(c) Energy (d) Potential ( )

9 The electron orbiting the nucleus possesses:

(a) Potential energy only

(b) Kinetic energy only


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(c) Partially potential and partially kinetic energy(d) None of the above ( )

10 The cause of conductivity in metal is:

(a) Proton (b) Free electron(c) Bound electron (d) Ions ( )

11 The valence bond at OK is:

(a) Completely filled (b) completely empty(c) Partially (d) Nothing can be said ( )

12 Semi-conductors such as Germanium and silicon exhibit.(a) Covalent bonding (b) Metallic bonding

(c) Ionic bonding (d) van der walls bonding ( )

13 Holes are present in:

(a) Valence band

(b) Conduction band

(c) Conduction and valence band(d) None of the above ( )

14 The maximum intertomic distances exists in:

(a) Solids (b) Liquids

(c) Gases (d) Both in liquids and solids ( )

15. Which of the following elements is a covalently bonded crystal?

(a) Ar (b) NaCL

(c) Ge (d) Na ( )

16 The maximum intertomic distances exists in :

(a) Solids (b) Liquids

(c) Gases (d) Both in liquids and solids ( )

17 Each energy band of a small piece of solid containing 100 atoms will have closely

spaced energy levels equal to:(a) 50 (b) 100(c) 200 (d) 500 ( )

18 Which of the following elements is a covalently bonded crystal?


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(a) Ar (b) NaCL

(c) Ge (d) Na ( )

19 The maximum number of electrons in nth

shell of an atom is:

(a) n2 (b) 2n

(c) n (n + 1) (d) 2n2

( )

20. The range of intermolecular force is:

(a) infinite (b) 13 A

(c) 10 A (d) 0

( )

21. The bonds in silicon crystals are:

(a) Covalent (b) ionic

(c) metallic (d) None of the above

( )

22 Example of a semiconductor is:

(a) Al (b) Cu

(c) Fe (d) Si

( )

23 The unit cell with crystallographic dimension, a = b c and0

is:

(a) cubic (b) hexagonal(c) monoclinic (d) tetragonal ( )

24 The number of fundamental crystal structures are:(a) 7 (b) 14

(c) 21 (d) 28

( )

25. On heating resistance of conductor:

(a) increase (b) decreases

(c) does not change (d) becomes zero

(

26 Bonds in semiconductor are:(a) Covalent (b) Ionic

(c) Metallic (d) neutral ( )

27. Band gap in Ge and Si lies in range:

(a) 0.51.5 eV (b) 1.52.5 eV

(c) 2.53.5 eV (d) 3.54.5 eV

( )


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28. In intrinsic semiconductors:

(a) nh < ne (b) nh > ne(c) nh = ne (d) nh = ne = 0

( )

29 The unit of resistance is:

(a) Mho (b) Ohm

(c) Farad (d) Ampere

S.No. Q:01 02 03 04 05 06 07 08 09

Ans D B B A D B A C C

S.No. Q:10 11 12 13 14 15 16 17 18

Ans B C A A C B C B B

S.No. Q:19 20 21 22 23 24 25 26 27

Ans D B A D B A C A B


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Chapter-2

Transport Phenomena in Semiconductor

Q.1 Write down the relation between Drift Velocity and Current Density.

Ans.: Current Density: The electric current passing normally through a units area ofthe metal wire is called Current Density (J).

IJ

A

As shown in figure, let a potentialdifference V is applied between theends of the wire of length L andcross section area A.

If number of free electrons per unit volume in wire are n then total number of

electrons passing through unit area of the wire in time t seconds will be (nAVdt)and flow of charge will be dQ qnAV t

Where q Charge of an electron

dqnAV tQIt t

dI qnAV

Current DensityI

JA

dJ qnV

Q.2 What is Intrinsic Semiconductor? Give examples of Intrinsic Semiconductors?What are the charge carriers in them?


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Ans.: Intrinsic Semiconductor : When a semi conducting material is pure and in itsnatural from it is called Intrinsic Semiconductor. The examples of IntrinsicSemiconductorsare Germanium (Ge) andSilicon (Si).

Crystal Structure : Both Silicon andGermanium has four valanceelectrons. For stability every atomshares one electron from theneighboring four atoms to complete thevalance orbit. Thus the valanceelectrons of each atom form covalent band

with one electron of each of its four neighbors. The crystal structure of bothSilicon and Germanium is diamond structure in which the bands form atetrahedron.

Charge Carriers : At absolute zero temperature, all semiconductor behaves asinsulator and no charge carrier exist. As the temperature increases, some electronfrom valance band, reach the conduction band after crossing the forbiddenenergy gap. Due to this a vacant site is created is valance band is called a Hole.Thus free electrons and holes are created in pairs simultaneously. Therefore inintrinsic semiconductors both electrons and holes are the charge carriers.

Q.3 What are Extrinsic Semiconductor? Explain the construction of N and P-TypesSemiconductors with necessary diagrams.

Ans. Extrinsic Semiconductor : The conducing of Intrinsic Semiconductor is low.When semi conducting material Germanium and Silicon are doped with acontrolled amount of an impurity element, the conductivity increases. Thesemiconductors so obtained are calledExtrinsic Semiconductors.

Extrinsic Semiconductorsare of two types :

(i) N-Type Semiconductor

(ii) P-Type SemiconductorN-Type Semiconductors : If an element of fifth group as arsenic or antimony isadded to Germanium or Silicon whose valancy is four, the semiconductorobtained is calledN-Type Semiconductor.


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As shown in fig.(1), when a pentavalent

impurity atom replaces a tetravalent atom ofGermanium or Silicon, four valanceelectrons of the impurity atom formcovalent band respectively with oneelectron of each of the neighboring atoms, whilethe fifth electron remains almost free. Thus everyatom of the impurity element of fifth groupcontributes one free electron as a charge carrierand due to this property fifth groupimpurity is calledDonor Impurity.

The energy level of the valance electron of thepentavalent element lies in forbidden energy gap ofthe intrinsic semiconductors just below theconduction band. This energy level is called DonorLevel.

InN-Type Semiconductors, electrons are majority charge carries.

P-Type Semiconductor : This type ofsemiconductor is formed when third groupelement like boron is added as impurity in Geor Si material. Due to adding of trivalentimpurity, three valance electrons ofimpurity atom form covalent bandsrespectively with one electron of each of thethree neighboring atoms but the band of the

fourth atom remains incomplete and adeficiency of electron exists there. Thisvacancy is calledHole. Hole has ability to capture available electron there on theimpurity of third group is calledAcceptor Impurity.


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In P-Type Semiconductor, acceptor level lies slightly above the valance band ofsemiconductor. Holes are the majority charge carries inP-Type Semiconductor.

Q.4 Derive the expression of conductivity of an Intrinsic Semiconductor in termsof mobility.

Ans.: In Intrinsic Semiconductors, both electrons and holes are the energy chargecarries.

When a battery is connected between the two ends of a semi-conductingmaterial, both electrons and holes move indefinite direction.

The drift velocity of both electrons and holes,is proportional to intensity of appliedelectric field. i.e.

nV E n nV E

And pV E p pV E

Here &n p are mobilities of electrons and holes respectively.

If current density of electrons is Jnand of holes is Jpthen total current density isan intrinsic semiconductor is

J = Jn+ JP

But we know that

Current Density = Number Density of Charge Carriers X Charge X Drift Velocity

Jn= nqVn= nnqE

And Jp= nqVp = ppqEJ = Jn+ Jp

OR J = qE (nn+ pp) amp/m2

The electrical conductivity of the semiconductor


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J

Eq (nn+ pp) mho/m.

Q.5 Derive of the expression of conductivity on N and P Types of Semiconductors.

Ans.: We have derived the expression for current density in a semiconductor in lastquestion and got

J = q (nn+ pp) E

And therefore conductivity

J

E

q (nn+ pp) mho/m

For N-Type Semiconductors

n = ND

Where ND is the number density of donor impurities

Further, n >> p

Therefore, = q NDn

For P-Type Semiconductors

p NA

Where NAis number density of acceptor impuritiesFurther, P>>n

Therefore, q Nap

Thus is Extrinsic Semiconductors the conductivity depends on the numberdensity of donor or acceptor impurity atoms.

Q.6 Explain of formation of depletion larger in P-N Junction Semiconductor.

Ans.: In a P-N Junction, P-type

semiconductor material is joined to N-type material in atomic sense. The P-material has a large hole densitywhile the N- material has a large freeelectron density. In this way at the


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junction a gradient of density of charge carriers developed. Due to this, the holesdiffuse from P-side to N-side while electrons diffuse from N-side to P-side.

This process of diffusion affects only a narrow region near the junction betweenX1and X2.

Due to the diffusion of electrons from N to P-side and holes from P to N-side, ina thin layer at the junction the holes capture the electrons and the charge carriersno longer remain free. This region of the junction thus depleted of free charge acarriers and the larger so formed is calledDepletion Layer.

Multiple Choice Questions:

1 For producing a p-type semiconductor which of these would used:

(a) Acceptor atoms (b) Donar atoms

(c) Pentavalent impurity (d) Arsenic ( )

2 The merging of a free electron with a hole is called:

(a) decomposition (b) recombination

(c) neutralization (d) covalent bonding ( )

3 Free electron exits in:(a) First band (b) Forbidden band(c) Valence band (d) Conduction band ( )

4 Depletion layer has:(a) Both free electrons and holes (b) None of the free charge carriers

(c) Only free electrons (d) Only free holes ( )

5 A P-N junction in reverse bias acts as a:

(a) Very high resistance (b) Very low resistance(c) Zero resistance (d) None of the above ( )

6 Current in a semi-conductor diode is due to the drift of:(a) Free electrons (b) Free electrons and holes(c) Positive and negative ions (d) Protons ( )

7. In n-type semiconductor the fermi energy level is displaced:(a) towards the valence band


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(b) towards the conduction band

(c) not displace d(d) and depend on quantity of impurity ( )

8 The electronic configuration of Si is:

(a) 1s2, 2s

2, 2p

6, 3s

2, 3p

4 (b) 1s

2, 2s

2, 2p

6, 3s

1, 3p

3

(c) 1s2, 2s

2, 2p

6, 3S

2, 3p

2 (d) 1s

2, 2s

2, 2p

6, 3s

0, 3p

4 ( )

9 The depletion region of a-p-n junction contains:

(a) Electrons only

(b) A rectifier

(c) Electrons and holes(d) Neither electrons nor holes ( )

10 A p-n junction can be used as:(a) An amplifier (b) A rectifier

(c) An oscillator (d) A modulator ( )

11 Depletion layer increases:(a) When p-n junction is formed

(b) On applying forward bias

(c) On applying the reverse bias

(d) When temperature is decreases ( )12 The number of free electrons and holes in an intrinsic semiconductor increase when

the temperature:(a) Decreases (b) Increases

(c) Stays the same (d) None of the above ( )

13 The electronic configuration of Si is:

(a) 1s2, 2s

2, 2p

6, 3s

2, 3p

4 (b) 1s

2, 2s

2, 2p

6, 3s

2, 3p

2

(c) 1s2, 2s

2, 2p

6, 3S

1, 3p

3 (d) 1s

2, 2s

2, 2p

6, 3s

0, 3p

4 ( )

14 To make an n-type material, following is added to Ge:(a) Al (b) B

(c) P (d) In ( )


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15 The energy needed to detach the fifth valence electron from the Antimony (Sb)impurity atom surrounded by Ge atoms is approximately:

(a) 0.001 eV (b) 0.01 eV(c) 0.1 eV (d) 1.0 eV ( )

16 In an N-type semiconductor, the position of Fermi level:

(a) Is lower than the centre of energy gap

(b) Is at the centre of the energy gap

(c) Is higher than the centre of energy gap(d) Can be anywhere in energy gap depending upon the

concentration of impurity atoms ( )17. The resistivity of a semiconductor at Ok is:

(a) Zero (b) Infinite(c) 10

5(d) None of the above ( )

18 The unit of current density is:

(a) C/m2 (b) C/m

3

(c) Ampere/m2 (d) Ampere/m

3( )

19 The merging of a free electron with a hole is called:

(a) Neutralization (b) Decomposition(c) Recombination (d) None of the above ( )

20 The example of intrinsic semiconductor is:(a) Ge (b) Ca(c) Sb (d) Ag ( )

21 Cut in voltage of Si diode is of the order of:

(a) 0.2 V (b) 0.6 mV(c) 0.2 mV (d) 0.6 V

( )

S.NO. Q:01 02 03 04 05 06 07 08 09

Ans A B D B A B B C CS.NO. 10 11 12 13 14 15 16 17 18

Ans B C B B C B C B C

S.NO. Q:19 20 21

Ans C A B


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Chapter-3

Semiconductor Diodes

Q.1 What is a P-N Junction Diode?

Ans.: P-N Junction : When a P-type semi conducting material is joined to a N-type

material is the atomic sense, the device is called P-N junction. During theformation of a P-N Junction a potential barrier is developed at the junction. This

potential barrier abstracts the flow of free charge carriers (electrons & holes). PN

junction has a special feature that it allows the flow of current easily in one

direction but obstruct the flow of current in opposite direction. Thus P-N

Junction acts as a rectifier diode therefore this device is called P-N junction

Diode.

Q.2 Explain the working of P-N Junction Diode. Discuss its Forward and Reverse

Bias.

Ans.: Working of P-N Junction Diode :P-N Junction diode has a very special featurethat it allows the flow of current only in our direction and obstruct the flow ofcurrent ins other direction so this device is used in rectifies circuits.

Forward Biasing of a P-N Junction Diode :

P-N junction, If P-terminal is joined topositive terminal of battery and N-terminal tonegative terminal then the diode is said to beForward Biased.

By a Forward Bias, the height of the

potential barrier at the junction is reduced because the field applied by the

external source at the depletion larger is opposite to the field present there due to

the potential barrier.


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InForward Bias, current flows in the circuit from

P-region to N-region of the diode; By increasingthe biasing voltage current increase exponentiallythe dynamic resistance offered by the P-Njunction is very low of the order of 100 ohm.

Reverse Bias :If the P-terminal of P-N junction is

connected to negative terminal of the battery and N-terminal of the diode to

positive terminal of the battery, the applied bias is calledReverse Bias.

In Reverse Bias, the height of potentialbarrier as well as the thickness of depletion layer

is increased. Thus the availability of free

charge carriers is reduced and their flow is

obstructed by the barrier the current in this state

is almost zero only a small current in

microamperes flow due to the thermally generated holes in a N-material and free

electrons in P-region.

InReverse Bias, state the dynamic resistance of P-

N Junction diode is very large (106ohms) Thus P-N Junction allows current to flow easily in

forward bias state and obstructs the flow of

current in reveal bias state.


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Q.3 Give circuit diagrams to obtain characteristic curves of a P-N Junction Diode.

Ans.:

The circuit ofForward Biasing of aP-N JunctionDiode

The circuit of ReverseBiasing of a P-N JunctionDiode

Volt-AmpereCharacteristics of a P-NJunction Diode


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Q.4 Explain the phenomena of Avalanche Breakdown and of Zener Breakdown.Ans. Avalanche Breakdown : This breakdown takes place due to strong electric field

in depletion layer. Due to this, there is a direct rupture of covalent bands and

electron-hole pairs so generated acquire a large amount of kinetic energy from

this field and collide with immobile ions, thereby generating further electron-

hole pairs. This process is cumulative in nature and results in avalanche of

charge carriers in very short time. This mechanism is called Avalanche

Multiplication.

Zener Breakdown :When P and N regions are heavily doped, the change charge

carrier density at the junction is abrupt and depletion layer of very thin width isformed. The charge carriers of opposite nature are concentrated on both sides of

this layer. Due to this, strong electric field is generated is the depletion layer due

to its very small width even at nearly zero bias.

When diode is reverse biased, field across depletion layer becomes very high. In

this state the valance band of P-region comes in front of conduction band of N-

region and the electrons start flowing from valance band of P-region to

conduction band of N-region by tunnel effect due to strong electric field. This

type of breakdown is called Zener Breakdown.

Q.5 Explain the working of Zener Diode. How is it used for the VoltageStabilization?

Ans.: In reverse bias state, when applied voltage exceeds a certain limit, the currentthrough the P-N diode increases abruptly. This reverse voltage at which currentincreases sharply, is called Zener Voltage.The ordinary diode may got damaged.But zener diode is made to work in this breakdown region.

Symbol of Zener Diode

To obtain constant DC Voltage, regulated power supply is used. Zener diode is

used in many different type of Voltage Regulators for Voltage Stabilization.


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28

Example : In Transistor, Shunt Regulator, Transistor Series Regulator, Zener

Diode Shunt Regulator.

Zener Diode Shunt Regulator :

As shown in circuit diagram -

Vi= VZ+ RSI

or VZ= ViRSI _ _ _ _ (1)

and current through zenerdiode will be

IZ= I IL _ _ _ _ _ (2)

We know that variation in input voltage Vi and load current ILmakes output

voltage unregulated.

When a zener diode is connected in parallel with load RL, then zener diode keeps

the output voltage constant is all conditions for example, If voltage variation DC

takes place at any instant in the input voltage Vi. Let it changes the current by an

amount dI. This current variation simultaneously changes the current through

zener with the same amount, but the load current I Lremains constant and thus

voltage across load remains equal to zener voltage Vz. Similarly if the load

current changes, the zener current increases or decreases to keep the currentthrough RS. Constant as seen from equation (1).

Q.6 Explain the principle of Varacter Diode.

Ans.: Varacter Diodes:The varacter is a reverse biased diode having a useful property

that the diodes in reverse bias state that the diodes in reverse bias state acts as a

capacitor whose capacitance varies with the applied reverse voltage.

The width of depletion layer increases with the

reverse voltage. In this state P and N region ofthe diode are like the plates of the capacitor and

the depletion layer is like the dielectric. We

know that capacitance is inversely

Circuit Symbol


29/120


proportional to the distance between the plates. Consequently larger the reverse

voltage, the larger is the width of depletion layer and hence the smaller thecapacitance. It means that the barrier capacitance is controlled by the reverse

voltage.

The varacter diode is widely used in FMRadio, TV receiver etc.

Q.7 Explain the principle and working of Photodiode.

Ans.: Photodiode : It is a light a sensitive device which converts light signals intoelectrical signals. It is a special type of diode which operates in reverse bias.

Construction ofPhotodiode :Photodiode is made of asemiconductor P-Ndiode kept in a sealedopaque plastic of glasscasing. It has a smalltransparent window

through which light falls atjunction.

Working of Photodiode :When light falls or junction of PN diode, electron-hole

pairs are generated in the semiconductor material. Due to movement of these

pairs, current flow in reverse bias. The magnitude of current depends on the

intensity of light. Higher the intensity of light IL, higher is the reverse current that

flows through the circuit.

Use of Photodiode : Photodiode is generally used in photo-detection devices,

logic circuits etc.

Q.8 Explain working of Light Emitting Diode. Mention its applications.

Ans.: Light Emitting Diode (LED) :P-N junction diode emits light when it is forward

biased. Such type of diode is calledLight Emitting Diode.

Characteristic Curve

of Varacter Diode


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30

These diodes are made from Gallium Phosphide (Ga) Materia or Gallium

Arsenide Phosphide (GaAsP) material.

The amount of light emitted by the diode is directly proportional to the forward

current i.e. higher the forward current, higher is the intensity of light emitted.

When LED is

forward biased, the

electrons from N

region and holes from

P region move

towards the junction andrecombine. In this process the electrons lying in the conduction band of N-region

fall into the holes in the valance band of P region. As a result energy of electrons

decreases and the energy equivalent to the difference of energy between the

conduction band and valance band is radiated in the form of light in LED.

Q.9 Describe the construction and circuit symbol of a Photovoltaic Cell.

Ans.: Photovoltaic Cell : If the Potential difference is generated between the ends of

the semiconductor material due to light radiator fall on it, then this effect iscalled photovoltaic effect and semiconductor device using this effect is called

Photovoltaic Cell. The generated potential difference depends upon the intensity

of light and the load. A Photovoltaic Cell consists of semiconductor material

such as silicon, germanium etc. which is joined with the metal plate as shown in

fig. below :


31/120


Multiple Choice Questions:

1 A P-N junction in reverse bias acts as a:(a) Very high resistance (b) Very low resistance

(c) Zero resistance (d) None of the above ( )

2 The characteristic curve of a P-N junction diode:

( )

3 Symbol of P-N junction diode is:

( )

4. The varactor diode is used as:


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32

(a) Rectifier (b) Regulator

(c) Oscillator (d) Switching circuits ( )

5 The color of light emitted by a LED depends upon:(a) its reverse bias

(b) its forward bias

(c) the amount of forward current

(d) the type of semiconductor material used ( )

6 Zener diode is used as:

(a) Amplifier (b) Oscillator

(c) Regulator (d) Rectifier ( )

7 The ratio of the resistance of a-p-n junction diode under forward biased and reversebiased condition is:

(a) 102:1 (b) 10

2:1

(c) 1 : 104

(d) 1 : 104 ( )

8 The voltage where the avalanche occurs is called the:

(a) Barrier potential (b) Peek voltage(c) Knee voltage (d) Breakdown voltage ( )

9 In LED, blue light is emitted by:

(a) GaAs (b) GaP(c) GaAsP (d) GaN ( )

10 Zener diode used in circuit is always biased:(a) forward (b) reverse(c) zero (d) none of the above

( )

11. Zener diode is:

(a) Rectifier diode (b) constant current device(c) constant voltage device (d) forward biased diode ( )

12. Zener diode is used is:

(a) amplifier (b) oscillator(c) regulator (d) rectifier ( )

13 For highly doped diode:(a) Zener break down is likely to take place(c) Avalanche break down is likely to take place

(c) Either will take place

(c) Neither will take place ( )


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14 Zener diode is generally operated:(a) In forward bias (b) In reverse bias

(c) In zero bias (d) Both in same bias ( )

S.NO. Q:01 02 03 04 05 06 07 08 09

Ans A B C B A C D D D

S.NO. 10 11 12 13 14

Ans B A B B B


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34

Chapter-4

Rectification & Power Supply

Q.1 Define Rectification and Rectifier. What are the types of Rectifier?

Ans.: Rectification : Rectification is the process by which alternating voltage is

converted into direct voltage.

Rectifier :The electrical device which rectifies an alternating voltage is called a

Rectifier. In a rectifier the characteristic property of a diode of unidirectional

conduction is used by which a low resistance is offered to the flow current in on

direction and a high resistance is offered for flow in opposite direction.

Rectifiers are mainly of two types :

(i) Half Wave Rectifier :In which only one diode is used and only half of the

cycle of input alternating voltage is used.

(ii) Full Wave Rectifier : In which the full wave of the input alternating

voltage is used.

Q.2 Describe working of Half Wave Rectifier with the help of circuit diagram.

Ans.: Half Wave Rectifier :


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As shown in figure, in Half Wave Rectifier,the source of alternating voltage isconnected to primary coil of a transformer and the secondary coil to a diode Dand load resistance in series.

Suppose voltage on transformer is Vi = Ep sin t the voltage induced in

secondary is Vs = n EpSin t = Em sin t, where n ratio of turns insecondary and primary coil.

For positive half cycle of Vi, the end A of secondary coil is at positive voltage andB at negative voltage. In this state, diode is forward biased and current i flow inthe circuit. During other half cycle, A is at negative voltage and end B is atpositive voltage. In this state P-N diode is reverse biased and no current slows.Thus thorough the load RL connected in the output circuit current flows only forhalf cycle of the cycle of input voltage Vi.

The output voltage is always positive (unidirectional) but varies with time i.e.,

the output voltage is pulsating.

Q.3 Determine following quantities in Full Wave Rectifier :

(i) Average Value of Pulsating Current

(ii) Efficiency

(iii) Ripple Factor

Ans.: Full Wave Rectifier :

(i) Average Value of Pulsating Current :

Idc =1

T

0

T

idt


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36

=1

T

/2

10

T

i dt+2

/2

T

T

i dt

OR Idc =1

T

/2

0

sinT

mI tdt +/2

( )sinT

m

T

I tdt

= mI

T

/2

0

TCos t

-/2

T

T

Cos t

= mI

t

/2

0cos

Tt -

/2cos

T

Tt

= mI2

2 2 2. 0 . .2 2T TCos Cos Cos T Cos

T T T

Idc=2

mI

2

T

(ii) Efficiency :

Rectification Efficiency of Full Wave RectifierDOutput C Power

Input AC Power dc

ac

P

P

Output DC Power of Full Wave Rectifier

22

2

4m

dc dc L L

I

P I R R

Input AC Power of Full Wave Rectifier2

2 ( ) ( )2

mac rms L L

IP I R R R R

Efficiency

2

2

2 2

4

8 0.812

( ) ( )1

2

m L

L

m L L

L

I R

R

I R R R R R

R

Percentage Efficiency

81.2%

1L

R

R

Maximum Efficiency of Full Wave Rectifier will be 81.2% when R < < RL.


37/120


(iii) Ripple Factor : The alternating variation present in the output of Full

Wave Rectifier is calledRipple. It is measured by a constant calledRippleFactor.

Ripple Factor ac

dc

IThe effective valueof ac component in outputr

The dc component in output I

OR

1/22

2 1rms

dc

Ir

I

Substituting the value of Irmsand Idcfor a Full Wave Rectifier

1/22 2

2. 12 4m

m

Ir

I

1/22

18

r

r = 0.482

r =48.2%

Q.4 Draw the circuit diagram of a Bridge Rectifier and explain its working.

Ans.: Bridge Rectifier :

As shown in figure Bridge Rectifier consists of four diodes D1, D2, D3& D4whichare connected to form a network just like wheat stone bridge.


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38

Working of Bridge Rectifier :When AC voltage Vi = Epsin t is applied across

the ends of the primary coil of transformer then V s = nEp sin t is inducedbetween the ends E and F of its secondary coil. During the half cycle of Inputvoltage when E is positive with respect to Point F, diodes D1& D3 are forwardbiased and D2& D4are reverse biased. Consequently a current slows through thediodes D1, & D3. During Next half cycle, diode D2 & D4 conduct and currentflows only through then. Thus it is clear that for both half cycles the currentsflows through RCin one direction only.

Q.5 What are Filters? Describe the working of Shunt Capacitor Filter.

Ans.: Filter : Filter is an electric circuit used for smoothing output current i.e. toseparate the AC component from the pulsating output current of the rectifier. Itis made by combining inductors and capacitors.

Shunt Capacitor Filter :

In this type of filter, a capacitor C is connected is parallel with the load resistance

RL. Filtration of AC component from diode output voltage depends upon the

energy storage capacity of the capacitor C. When diode (Suppose D1) output

voltage increases (Say from t1to t2as shown in figure) the capacitor stores energy

by charging to the peak B of the corresponding Input cycle upto time t2. Beyond

t2when input voltage decreases from its peak value, the diode D1disconnects

from the source and load. So it stops conducting and capacitor starts discharging

energy thorough the load RL.

In the next half cycle of Input, this process takes place through diode D 2. The

process continues for other cycles and the rectifier with capacitor filter gives the

wave form of the output voltage like ABCD curve shown in figure.


39/120


Q.6 Explain the working of Series Inductor Filter.

Ans.: Series Inductor Filter : The ripple in the rectifier output can be reduced byconnecting high independence inductor L in series with lead Resistance RL.

Working of Series Inductor Filter : When diode output current tends to riseabove the average value, of the inductor L stores energy in due to rise in thecurrent in the form of magnetic energy and offers impedance to AC componentand no resistance to DC component. Thus AC component is blocked and onlyDC component reaches at the load. When the load current decreases belowaverage, it develops additional only in the inductor due to outflow of magneticenergy and resists the decrement of the output load current. In this way thefluctuation in the output current decreases by the inductor L.

Q.7 Sketch block diagram of SMPS.

Ans.: SMPS :

Block Diagram of Switching Mode Power Supply


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40

Multiple Choice Questions

Q:1 Which rectifier requires four diodes?

(a) Half-wave rectifier (b) Bridge rectifier

(c) Full-wave rectifier (d) All of the above ( )

2 Output wave of full-wave rectifier given:

( )

3 The efficiency of a half-wave rectifier is:

(a) 50% (b) 81%(c) 40% (d) 100% ( )

4 The current obtained from a rectifier without filter is:

(a) Varying d.c. (b) Constant d.c.(c) Pulsating d.c. (d) d.c. mixed with a.c. ( )

5 The filter circuit which has better voltage regulation is:

(a) Choke input filter (b) -filter

(c) Capacitor filter (d) LC filter ( )


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6. The output of a half-wave rectifier is suitable only for:

(a) Charging batteries (b) Running a.c. motors(c) Running car radios (d) Running TV ( )

The output voltage for a half wave rectifier is:(a) Pure AC (b) Pure DC

(c) Pulsating (d) None of the above ( )

7 A full wave rectifier is connected with an AC source of frequency 50 Hz. The outputcurrent contains a ripple of what frequency?

(a) 25 Hz (b) 50 Hz

(c) 100 Hz (d) 60 Hz ( )

8 Which of the following V-I graphs is correct for a p-n junction?(a)

I

v

(b)

I

v(c)

I

v

(d)I

v

S.NO. Q:01 02 03 04 05 06 07 08 09

AnsB B C C B A C C


42/120

42

Chapter-5

Bipolar Junction - Transistor

Q.1 What is Transistors? Draw notations for PNP and NPN Transistors.

Ans.: Transistor : When a third doped element is added to semiconductor diode in

such a way that two P-N Junctions are formed, the resulting device is known

so Transistor.

Transistor is small in size and very light in weight. It needs a very low voltage

source and has longer life.

There are two types of Transistor :

(i) PNP Transistor


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(ii) NPN Transistor

Q.2 Explain different types of Transistor Biasing.

Ans.: The connection of correct polarity of voltage across its two junctions for proper

working of transistor its known as Transistor Biasing. There are three ways of

biasing transistor :

(i) Forward Active : In this biasing, emitter base junction is made forward

biased and collector base junction is reverse biased. This biasing is called

Forward Ativeand generally used inAmplification.

(ii) Saturation :IN this biasing, both emitter base junction and collector basejunction are forward biased. In this mode the transistor has very largecurrent to flow and used as a closed switch.


44/120

44

(iii) Cut Off : In this biasing both junction are reversed biased and transistorhas practically no current to flow and used as open switch.

Q.3 Discuss sign convention and signs of various voltage and current in a Junction

Transistor.Ans.: Sign Conventions for Voltages and Currents in Transistor : In Junction

Transistorone of its terminal for input and output circuits.

Sign Convention for Current : Normally, current flowing into a Junction

Transistor is taken positive where as current flowing out of it is takes as

negative.

Sign Convention for Voltages :The voltage is taken as positive when terminals

symbol of transistor used as first subscript in symbol indicating voltage is

positive with respect to terminals symbol of transistor used as second subscriptin voltages symbol.


45/120


In NPN In PNP

VEB - +

VCB + -

VCE + -

Q.4 What do you understand from Junction Transistor Configurations andCharacteristic Curves.

Ans.: In Junction Transistor, all the electrical quantities at the output are in general

controlled by the electrical quantities at the Input. The four electrical quantities

related with input and output are :

a. Input Current

b. Input Voltage

c. Output Current

d. Output Voltage

The dependence of these quantities on one another is represented by different

curves called Characteristic Curves.

There are three basic configuration of Transistor :

(i) Common Base Configuration

(ii) Common Emitter Configuration

(iii) Common Collector Configuration

Q.5 Give the circuit diagram of Common Base NPN Transistor and give the

method of plotting input and output characteristics.

Ans.: Common Base Configuration : In this configuration base terminal is common ininput and output circuits. To represent the behaviour of NPN Transistorin this

configuration two sets of Characteristic Curvesare drawn :


46/120

46

(i) Input Characteristics : In thisconfiguration IE is inputcurrent and VEB is inputvoltage. IC is output current andVCBis output voltage. Keeping VCBconstant, the graph between IE andVEB are called InputCharacteristics.

(ii) Output Characteristics : Incommon base configuration, Ic isoutput current and VCB isoutput voltage.

Collector Current ICis a function of IEand VCBi.e.

Ic = (VCB, IE)


47/120


Keeping IEconstant the set of waves drawn between ICand VCBare called

Output Characteristics. The output characteristics of transistor in CBconfiguration may be divided into three region of operation :

(i) Active

(ii) Saturation

(iii) Cutoff

Q.6 What do you understand from and of Transistor. Establish relation

between them.

Ans.: Current Amplification Factor () :In common base configuration, this is defined

as the ratio of collector current ICand emitter current IEkeeping VCBconstant.

C

E

I

I

Current Amplification Factor () :In common emitter configuration, the ratio ofICand IBfor constant VCEis calledDC Current Gainand given as

tanCE

C

B V Cons t

I

I

Relation between and :

ForJunction Transistor, we know that

IE = IB+ IC _ _ _ _ _ (i)

Dividing this equation by the IC, we get

1=

B

C

I

I + 1 _ _ _ _ _ (ii)

1

=1

+ 1

1=

1


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48

=1

_ _ _ _ _ (iii)

Equation (ii) can be written as

1

1 =

1- 1

1=

1

=1

_ _ _ _ _ (iv)

Q.7 Describe Hybrid Parameters of a Transistor.

Ans.: Hybrid Parameters of Transistor : To stand a four terminal network two

variables are taken as independent variables while two are taken as dependent

variables. In transistor Input current I1 and output voltage V2 control the

behaviour of internal circuit so there are taken as independent variables and

remaining the two are dependant variables.

V1 = f (I1, V2) _ _ _ _ _ (i)

I2= f ( I1, V2) _ _ _ _ _ (ii)

On taking total differentials

dV1 =1

1

V

I dI1+

1

2

V

VdV2 _ _ _ _ _ (iii)

And dI1 =2

1

I

IdI1 +

2

2

I

Vdv2 _ _ _ _ _ (iv)

OR V1= h11 I1+ h12 V2 _ _ _ _ _ (v)

I2 = h21 I1+ h22 v2 _ _ _ _ _ (vi)


49/120


Here coefficient h11, h12, h21& h22are of different dimension so they are referredas hybrid parameters.

hi= h11=1

1

V

I Short circuited input impedance when V2=0 its unit is

ohm.

hr = h12=1

2

V

V Open circuited reverse voltage ratio when I1=0 it

dimension less parameter.

hf= h21=2

1

I

I Short circuited forward current gain when V2=0 it is

dimension less parameter.

H0= h22=2

2

I

V Open circuited output admittance when I1=0 its unit

is mho.


50/120

50


Q:1 In transistor, the relation among emitter current gI , base current BI and Collector

Current BI is:

(a) c E BI I I (b) E B CI I I

(c) E B CI I I (d) E C BI I I

2 In a transistor leakage current mainly depends on:

(a) Temperature (b) Doping of base(c) Size of emitter (d) Rating of transistor ( )

3. Transistor is known as bipolar transistor because it has:

(a) Electrons (b) Holes(c) Electrons and holes (d) None of the above ( )

4. In NPN or PNP transistor:

(a) c EI I (b) c EI I

(c) c EI I (d) None of the above ( )

5. Which of the following transistor amplifiers has the highest power gain?

(a) Common base (b) Common emitter(c) Emitter follower (d) Common collector ( )

6. The voltage gain of a common base amplifier is:

(a) Zero (b) Less than unity(c) Unity d) Greater than unity ( )

7 In N-P-N transistor both emitter and collector junctions are forward biased. The

transistor will operate in:(a) Active region (b) Saturation region

(c) Cut-off region (d) Inverted region ( )

8. Who invented the first junction transistor?

(a) Bell (b) Faraday

(c) Marconi (d) Schockley ( )


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9 The current amplification factor in common collector configuration as:

(a) (b)

(c) (d) ( )

10. The base of an n-p-n transistor is thin and:

(a) Heavily doped

(b) Lightly doped(c) Marconi

(d) Doped by pentavalent material ( )

11. I bell is equal to:(a) 1 db (b) 10 db

(c) 100 db (d) 1000 db ( )

12 In a transistor the value of a is 100, the value of is:

(a) 0.01 (b) 0.1(c) 0.99 (d) 1 ( )

13 The common-emitter gain ( ) of a transistor is defined as:

(a) (b)

(c) (d) None of the above ( )

14 Figure represents a:(a) PNP transistor

(b) NPN transistor

(c) Unipolar transistor(d) None of the above

15. In case of a bipolar transistor, is:

(a) +ve and greater than one

(b) +ve and less than one

(c) -ve and less than one

(d) None of the above ( )

16. The relation between and is given by:

(a) = (b) =


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52

(c) (d) None of the above ( )

17. In a transistor leakage current mainly depends on:

(a) Temperature (b) Doping(c) Size (d) None of the above ( )

18 The smallest part of a transistor is:

(a) Emitter (b) Base(c) Collector (d) None of the above ( )

19. The unit of impedance is given by:(a) Ampere (b) Ohm

(c) Volt (d) None of the above ( )

20. The current gain of CC amplifier is:(a) Zero (b) Greater than unity

(c) Unity (d) Less than unity ( )

21. The decibel is a measure of:(a) Power ration (b) Voltage ratio

(c) Current ratio (d) None of the above ( )

22 In a Cc amplifier voltage gain:

(a) Remains constant(b) Equal to one(c) Is independent of input

(d) Depends on output ( )

23 For a n-p-n transistor the collector voltage is:(a) ve (b) +ve(c) O (d) a.c. voltage

( )

24 Highly doped region in a transistor is:(a) Emitter (b) Collector(c) Base (d) All are equally doped

( )

25. Thinnest region in a transistor is:(a) Emitter (b) Base

(c) Both of the above (d) All of the above

( )


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26 Transistor is a:(a) Voltage operated device (b) Current operated device

(c) Both of the above (d) None of the above ( )

27 Transistor can be used as:(a) Switch (b) Amp lifer

(c) Oscillator (d) All of the above

( )

S.NO. Q:01 02 03 04 05 06 07 08 09Ans B B C B A D B A C

S.NO Q 10 11 12 13 14 15 16 17 18

Ans B C C A B A B B B

S.NO Q 19 20 21 22 23 24 25 26 27

Ans B B A B B B A B B


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54

Chapter-6

Amplifiers and Oscillators

Q.1 Explain how a Transistor works as an Amplifier?

Ans.: A Transistorraises the strength of a weak signal and thus acts as anAmplifier.

The weak signal is applied betweenemitter base junction and output is taken across the load RL connected in thecollector circuit. In order to achieve faithful amplification, emitter base junctionshould always remain forward biased. To do so, a DC voltage EE is applied inthe input circuit in addition to the signal.

As the Input circuit has low resistance c being forward biased), a small change insignal voltage causes an appreciable change in emitter current. This almostcauses same change in collector current due to transistor action. The collector

current flowing through load resistance RL produces large voltage across it.Thus, a weak signal applied in the input circuit appears in the amplified form inthe collector circuit. It is in this way that a transistor acts as an amplifier.


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Q.2 Draw circuit diagrams for Transistor Amplifier Circuits in differentconfiguration.

Ans.: (1) Common Emitter Amplifier :

(2) Common Base Amplifier :

(3) Common Collector Amplifier :


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56

Q.3 Describe R-C Coupled Transistor Amplifier.

Ans.: This amplifier is usually employed for voltage amplification, in this, a couplingcapacitor Cc is used its connect the output of first stage to the base (i.e. input) ofsecond stage and so on. As the coupling from Due stage to next stage is achievedby a coupling capacitor followed by a connection to a shunt resistor, therefore,such amplifiers are called resistance capacitance coupled amplifiers.

As shown in fig. R1, R2, & RE form the biasing and stabilization network. Theemitter by pass capacitor CE offers low reactance path to signal. The couplingcapacitor Cctransmits ac signal but blocks DC.

Operation :When A.C. signal is applied to the base of first transistor, it appears

in amplified form across its collector load Rc. The amplified signal across Rc isgiven to base of next stage through coupling capacitor C c. The second stage doesfurther amplification o the signal. In this way cascaded stages amplify the signaland overall gain is considerably increased.

First stage gain G1 = AC

in

R

R

Where RAC= RC11Rin =c in

C in

R xR

R RRin input resistance of second stage

Second stage gain G2= C

in

R

R

Total gain G = G1x G2

OUTPUT

CC

CE


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The Rc coupled amplifier has excellent audio fidelity one a wide range offrequency. Therefore it is widely used in voltage amplifiers. The main drawbackof R-C coupled amplifier is that it has poor impedance matching.

Q.3 What do you understand by Feedback? Describe its types.

Ans.: Feedback :Transfer of energy from output of the electric network to its input iscalledFeedback, when a fraction of total voltage or current of the output of theamplifier is fed back to its input, its working efficiency changes.

There are two types of Feedback :

(i) Positive Feedback :When the Input signal and feedback signal have the

same phase, the magnitude of the input signal increases by the addition offeedback signal. This type of feed is also calledRegenerative Feedback.

(ii) Negative Feedback :When the input signal and the feedback signal havethe opposite phase, the magnitude of the input signal decreases, such typeof feed back is calledNegativeorDegenerative Feedback.

Q.4 Prove that for an Amplifier with Feedback.

Ans.: AF =1

A

A

As shown in fig., if the resultantvoltage at the input of the amplifiers

Vif= Vi+ B1 Vf

{ With Positive Feedback }

If the input voltage Vi is amplified directly by a factor A to the value of theoutput voltage then

A =0

i

V

V

. (i)

If Vifis input voltage with feedback and V0 is the output voltage then

A = 0'

if

V

V


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58

Vif = 0'V

A..(ii)

If the fraction of output voltage V0 is fed in to the input of amplifier then

Vf = V0

Vif = Vi+ V0 ..(iii)

Using equation (ii) and (iii), we have

0

1V

A= Vi + V0

V0= 1iAV

A

The Amplifier gains with feedback is given by

Af =0'

i

V

V

Af =1

A

A Hence proved.

Q.5 Explain that stability in gain and bandwidth increases due to NegativeFeedback.

Ans.: Gain Stability:We know that

Af =1

A

A { for Negative Feedback }

For Negative Feedback in an Amplifier to be effecting the designer deliberatelymakes AB>>1 therefore above relation becomes

Af =1

This means that gain now depends upon the feedback fraction B i.e.characteristics of feed back circuit. As feed back circuit is usually a resistivenetwork, the gain of the amplifier becomes independent of the ageing oftransistor, variation in supply voltage, temperature and frequency.


59/120


Frequency Response :As shown above Af=1

. This means that voltage gain of

the amplifier is independent of signal frequency. The result is that voltage gainsof the amplifier will be substantially constant over a wide range of signalfrequency. The Negative Feedback, therefore improves frequency response i.e.,increases bandwidth.

Q.6 Describe Oscillator. Draw the circuit diagram of a general Oscillator.

Ans.: Oscillator :It is an active electronic circuit which produces electrical oscillationsof any desired frequency or we can say it converts energy from DC source into a

periodically varying voltage or current.Oscillators are self excited active electronic circuits as no external signal isapplied at the input of the oscillators.

An oscillator employs positive feedback for its operation. Whereas negativefeedback tends to reduce the magnitude of the signal applied to the amplifier,positive feedback has the opposite effect.

Consider the block diagram of Fig.(1), here A is the voltage gain of the amplifierand m is the feedback fraction. The output voltage is E2= AE1where E1 is theinput voltage. Clearly, feedback voltage = m E2 = AmE1. If the link L were

removed and the feedback circuit output connected to the input circuit with +veto +ve and ve to ve, the feedback would be positive. This means that feedbackvoltage (= A mE1) is in phase with the input voltage. Since E1 is the requiredinput voltage to produce the output voltage E2, it means that for continuous


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output (i.e., oscillations), the feedback voltage must be equal to the input voltage

i.e.Feedback Voltage = E1

Or A m E1= E1

Or A m = 1

Under these conditions (i.e., Am=1), even if there were no input generator (i.e.,E1), a continuous output will be obtained across RLimmediately after connectingthe necessary power supplies (i.e, VCC). The general layout of an oscillator isshown in Fig. 31.2.

It is clear from Fig.(2) that continuous output (E2) will be obtained with noextremely applied input signal. It provides its own input signal via the feedbackcircuit. Thus, output voltage E2 = AE1 and feedback voltage = A mE1 = E1(because Am = 1).

An amplifier with infinite gain is one that can produce an output signal withoutany extremely applied input signal. It provides its own input signal via thefeedback circuit. Such a circuit is known as an oscillator.


1 Which of the following transistor amplifiers has the highest power gain?(a) Common base (b) Common emitter

(c) Emitter follower (d) Common collector ( )

2 The voltage gain of a common base amplifier is:

(a) Zero (b) Less than unity

(c) Unity d) Greater than unity ( )3 An oscillator converts:

(a) A.C. power into D.C. power

(b) D.C. power into A.C. power

(c) Mechanical power into A.C. power(d) A.C. power into rediation power ( )

4 In a Cc amplifier voltage gain:(a) Remains constant

(b) Equal to one


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(c) Is independent of input(d) Depends on output ( )

5. Ina practical oscillator, A is:

(a) slightly less than one(b) I

(c) 1

(d) slightly greater than one ( )

6 In a Cc amplifier voltage gain:(a) Remains constant

(b) Equal to one

(c) Is independent of input(d) Depends on output ( )

7 The gain of an amplifier with feedback is given by:

(a) (b)

(c) (d)

( )

8. The value of feedback fraction is:

(a) (b)

(c) (d) ( )

9 The negative feedback increase:

(a) gain (b) bandwidth

(c) noise (d) All of the above ( ).10 The gain of an amplifier with feedback is given by:

(a) Af = (b) Af =

(c) Af =

(c) Af =

( )

S.NO. Q:01 02 03 04 05 06 07 08 09

Ans A D A C A B B A C

S.NO. 10

Ans B


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62

Chapter-7

FETs, Thyristors andOpto-electronic Devices

Q.1 Describe Field Effect Transistor and its types.

Ans.: Field Effect Transistor (FET) : It is a semiconductor device in which flow ofcurrent through semiconductor is controlled by an electric field (voltage) i.e. it isa field (voltage) controlled device.

FET is a unipolar device i.e. current flow by only one type of majority carrierelectrons or holes.

Types of FET :

FET

JFET MOSFET

(Junction Field Effect Transistor) (Metal Oxide Semiconductor FieldEffect Transistor)

N-channel JFET P-Channel JPET E-MOSFET D-MOSFET

(Enhancement (DeplctionMOSFET) MOSPET)

Q.2 Describe the construction of JFET.


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Ans.: JFET : It consists of N or P type semiconductor bar whose ends are heavilydoped with same type impurity.

Types of JFET :JFET is of two types :

N-Channel JFET : If the bar is made from Ntype semiconductor Then the transistor is said tobeN-Channel JFET.

P-Channel JFET : When bar is made from P-typeJFET.

In JFET one end of bar is said to beSource (S). Majority carriers of barenter the bar from this terminal.Another end of bar is said to be drainmajority carriers leave the bar through thisterminal. Gate (G) is contracted onlateral surface of bar by doping heavily withopposite type impurity.

Q.3 State advantages of a JFET over a Bipolar Junction Transistor.Ans.: Advantage of JFET over Bipolar Junction Transistor :

JFET is a voltage-controlled device.


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JFET operation depends only one type

of majority charge carriers(electrons or holes) therefore it iscalled unipolar transistor.

Its thermal stability is quite good.

JFET are much easier to fabricate andare particularly suitable for ICs.

FETs are costlier than BJTs.

Q.4 Describe how the characteristics of a JFET are determined experimentally.Ans.: Characteristics Curve of JFET :

In JFET drain current (ID)

depend upon the drain

source voltage (VDS) and

the gate-source

voltage Vgs.

The dependence of one

quantity over the other can be represented by plotting the curves between them.

These curves are called Characteristic Curves of JFET. These Characteristic

Curvesare of two types:-

(i) Drain characteristics : Keeping gate-source voltage VGS constant, graph

plotted between drain current ID and drain source voltage VDS is called

Drain Characteristics.

These characteristics can be divided into three regions :


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(a) OhmicRegion

(b) Saturation Region

(c) Break-down Region

(ii) Transfer Characteristics : Keepingoutput voltage VDSconstant in the pinch off region, ifa graph is plotted between the saturated draincurrent IDSand the gate voltage VGS, then the familyof curves so obtained, is called Transfer

Characteristics.

Q.5 What is the principle of MOSFET? Explain itstypes.

Ans.: Metal Oxide Semiconductor Field Effect Transistor (MOSFET) : MOSFETworks on the principle that the thickness and hence the resistance of theconducting channel of a Semiconductor material can be controlled by atransverse electric field across the insulator deposited on the semi conductingmaterial.

Types of MOSFET : MOSFET is of two types :

(i) E-MOSFET (Enhancement MOSFET) : In this the controlling transverseelectric field across the insulator deposited on the semi conducting


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In a liquid crystal cell, liquid crystal material is sandwiched between twotransparent glass plates with transparent SnO2 film electrodes deposited in insidefaces.

When both glass sheets are transparent, the cell is known as Transmittive TypeCell. When only one glass sheet is transparent and other has a reflecting coating,then cell is known as Reflective Type Cell. When cell is not activated, bothtransmittive and reflective type cell do not appear bright.

When cell is activated, the incident light is scattered in all directions by both typeof cells and the cell appears quite bright. This type of LCD is known as DynamicScattering Type LCD.

Another type of LCD isField Effect LCD.

Q.8 Draw and explain a Seven Segment and Dot Matrix Display.

Ans.: Seven Segment Display :This can display all numerals and nine alphabets. Itconsists of 7 rectangular LED/LCD which can be turned on/off to form desireddigit.

(1) (2)

As shown in Fig.(2), external resistance is required to limit the current throughLED/LCD.


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Dot Matrix Display : The another method of displaying numerals and letters is a

dot matrix formation of LEDs / LCDs in a monolithic structure.Commonly used Dot Matrix for display are 5 x 7, 5 x 8, 7 x 9. Which can display64 different alphanumeric characters by deriving the appropriate horizontal andvertical inputs.


Q:1 A FET consists of a:(a) Drain

(b) Source(c) Gate(d) All of the above ( )


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2. The best electronic device for switching is:(a) Triode (b) BJT

(c) FET (d) MOSFET ( )

3 An SCR is a semiconductor device consisting of:(a) One PN junction (b) Two PN junction

(c) Three PN junctions (d) Four PN junction ( )

4 A thyristor is basically:(a) PNPN device (b) a set of TRIACs

(c) Two TRIACs (d) Two DIACs ( )

5 The gate- source diode of a JEFT should:(a) be forward biased

(b) be reverse biased(c) be either forward or reverse biased(d) not be biased at all ( )

6 A JFET:

(a) Is a voltage controlled device

(b) is a current controlled device(c) Has low input resistance

(d) Has a very large voltage gain ( )

7. The gate-source diode of a JFET should:

(a) Be forward biased

(b) be reverse biased(c) Be either forward or reverse biased(d) Not be biased at all ( )

8. When a JFET is operated above pinch-off voltage its drain current:(a) Increase sharply (b) Becomes constant

(c) Starts increasing (d) Becomes zero ( )

9. Positive feedback is used in:(a) Rectifier (b) Detector

(c) Amplifier (d) Oscillator ( )

10. A Shockley diode is the same as:

(a) A four layer diode (b) An SCR

(c) A diac (d) A triac ( )


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11. Which is an optoelectronic device?

(a) Phototransistor (b) LED(c) LCD (d) All of the above ( )

12. Which one of the following has the negative resistance region?

(a) Tunnel diode (b) Schottky diode

(c) Step recovery diode (d) Optocoupler ( )

13 A thyristor is basically:(a) A PNPN device

(b) A combination of DIAC and TRIAC

(c) A set of TRIACs

(d) None of the above ( )

14. Which of the following needs light to be activated?

(a) LED (b) LCD(c) LASCR (d) All of the above ( )

15. An FET consists of a:

(a) Drain (b) Source(c) Gate (d) All of the above ( )

16. After drain source voltage reaches pinch off value in a JFET, drain currentbecomes:

(a) Zero (b) Low(c) Saturated (d) Reversed ( )

17. Compared to a bipolar transistor JFET has a much higher:

(a) Voltage gain (b) Input resistance

(c) Supply voltage (d) Drain current ( )18 Thyristors are used in:

(a) Power supply (b) Amplifiers

(c) Oscillators (d) Display devices ( )


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72

BACHELOR OF COMPUTER APPLICATIONS

(Part-I) EXAMINATION

(Faculty of Science)

(Three Year Scheme of 10+2+3 Pattern)

PAPER - 117

SEMICONDUCTOR PHYSICS,ELECTRONICDEVICES & CIRCUITS

OBJECTIVE PART- I

Year - 2011

Time allowed : One Hour Maximum Marks : 20

The question paper contains 40 mul tiple choice questions with f our choices and student wi ll

have to pick the corr ect one (each carr ying mark).

1. According to Daltons atomic theory, an atom can:(a) be destroyes

(b) be created

(c) either be destroyed ar created(d) neither be destroyed nor created ( )

2. A matter has neither definite volume nor fixed shape is:

(a) solid (b) liquid(c) gas (d) None of the above ( )

3. The maximum number of electrons possible in the shell of principal quantum number

1n is

(a) 2n (b) 22n

(c) 2n (d) n(n+1) ( )



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(a) Ionic bond (b) Covalent bond(c) Molecular bond (d) Metallic bond ( )

5. For producing a p-type semiconductor which of these would used:

(a) Acceptor atoms (b) Donar atoms(c) Pentavalent impurity (d) Arsenic ( )

6. The merging of a free electron with a hole is called:

(a) decomposition (b) recombination(c) neutralization (d) covalent bonding ( )

7. Free electron exits in:(a) First band (b) Forbidden band

(c) Valence band (d) Conduction band ( )

8. Depletion layer has:

(a) Both free electrons and holes (b) None of the free charge carriers

(c) Only free electrons (d) Only free holes ( )

9. A P-N junction in reverse bias acts as a:

(a) Very high resistance (b) Very low resistance

(c) Zero resistance (d) None of the above ( )

10. The characteristic curve of a P-N junction diode:


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( )

11. Symbol of P-N junction diode is:

( )

12. The varactor diode is used as:(a) Rectifier (b) Regulator

(c) Oscillator (d) Switching circuits ( )


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13. The color of light emitted by a LED depends upon:

(a) its reverse bias(b) its forward bias

(c) the amount of forward current(d) the type of semiconductor material used ( )

14. Zener diode is used as:

(a) Amplifier (b) Oscillator(c) Regulator (d) Rectifier ( )

15. Which rectifier requires four diodes?(a) Half-wave rectifier (b) Bridge rectifier

(c) Full-wave rectifier (d) All of the above ( )

16. Output wave of full-wave rectifier given:

( )

17. The efficiency of a half-wave rectifier is:

(a) 50% (b) 81%

(c) 40% (d) 100% ( )

18. The current obtained from a rectifier without filter is:

(a) Varying d.c. (b) Constant d.c.


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(c) Pulsating d.c. (d) d.c. mixed with a.c. ( )

19. The filter circuit which has better voltage regulation is:

(a) Choke input filter (b) -filter

(c) Capacitor filter (d) LC filter ( )

20. The output of a half-wave rectifier is suitable only for:

(a) Charging batteries (b) Running a.c. motors

(c) Running car radios (d) Running TV ( )

21. In transistor, the relation among emitter current gI , base current BI and Collector

Current BI is:

(a) c E BI I I (b) E B CI I I

(c) E B CI I I (d) E C BI I I

22. In a transistor leakage current mainly depends on:

(a) Temperature (b) Doping of base

(c) Size of emitter (d) Rating of transistor ( )

23. Transistor is known as bipolar transistor because it has:

(a) Electrons (b) Holes

(c) Electrons and holes (d) None of the above ( )

24. In NPN or PNP transistor:

(a) c EI I (b) c EI I

(c) c EI I (d) None of the above ( )

25. Which of the following transistor amplifiers has the highest power gain?

(a) Common base (b) Common emitter(c) Emitter follower (d) Common collector ( )

26. The voltage gain of a common base amplifier is:(a) Zero (b) Less than unity(c) Unity d) Greater than unity ( )

27. A PET consists of a:(a) Drain


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(b) Source(c) Gate

(d) All of the above ( )

28. The best electronic device for switching is:(a) Triode (b) BJT

(c) FET (d) MOSFET ( )

29. An SCR is a semiconductor device consisting of:(a) One PN junction (b) Two PN junction

(c) Three PN junctions (d) Four PN junction ( )

30. A thyristor is basically:

(a) PNPN device (b) a set of TRIACs(c) Two TRIACs (d) Two DIACs ( )

31. Semiconductor materials have bonds:

(a) Ionic (b) Metallic

(c) Covalent (d) Mutual ( )

32. Current flow in a semiconductor depends on the phenomenon:

(a) Drift(b) Diffusion

(c) Recombinationt


33. Electronic configuration of sodium is:

(a)2 2 6 1

1 2 2 3s s p s (b)2 2 6 2

1 2 2 3s s p s

(c)2 2 2 5

1 2 2 3s s p s (d)2 2 5 11 2 2 3s s p s ( )

34. An example of p-type semiconductor:(a) Pure germanium

(b) Pure silicon

(c) Silicon doped with phosphorous

(d) Germanium with Indium ( )

35. The energy gap is the largest in the case of:(a) Metals

(b) Semiconductor

(c) Insulators


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(d) Superconductors ( )

36. For a P-N junction diode, the approximate ratio of resistances in reverse and forward bias

is:

(a) 1 : 1 (b) 310 :1

(c) 310 :1 (d) 610 :1 ( )

37. The device which converts d.c. into a.c. is called:

(a) Rectifier

(b) Oscillator(c) Amplifier

(d) Modulator ( )

38. In filter circuit which component is connected in serier to load?

(a) R (b) L

(c) Diode (d) C ( )

39. Low power display device is:

(a) LED (b) LCD

(c) Monitor (d) Seven-segment display ( )

40. Electrical energy is converted into light by:

(a) Electronic device

(b) Optoelectronic device(c) Optical device


Answer Key

1. ( ) 2. ( ) 3. ( ) 4. ( ) 5. ( ) 6. ( ) 7. ( ) 8. ( ) 9. ( ) 10. ( )

11. ( ) 12. ( ) 13. ( ) 14. ( ) 15. ( ) 16. ( ) 17. ( ) 18. ( ) 19. ( ) 20. ( )

21. ( ) 22. ( ) 23. ( ) 24. ( ) 25. ( ) 26. ( ) 27. ( ) 28. ( ) 29. ( ) 30. ( )

31. ( ) 32. ( ) 33. ( ) 34. ( ) 35. ( ) 36. ( ) 37. ( ) 38. ( ) 39. ( ) 40. ( )


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_____________

DESCRIPTIVE PART-II

Year- 2011

Time allowed : 2 Hours Maximum Marks : 30

Attempt any four questions out of the six. All questions carry 7 marks each.

Q.1 (a) Name and explain different types of bonds in solids with examples.(b) Explain the difference among conductors, insulators and semiconductors.

Q.2 (a) Explain intrinsic and extrinsic semiconductors with examples.

(b) What do you mean by mobility of charge carrier?

(c) Explain formation of depletion layer in PN junction semiconductor/diode.

Q.3 (a) Explain the working of P-N junction diode. Discuss its forward and reverse biased

characteristics.

(b) Describe the construction and working of light emitting diode (LED) and mention

its two applications.


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Q.4 (a) What is meant by bipolar junction transistor? Draw a neat circuit diagram to study

the characteristics of a PNP transistor in common base configuration. Sketch the

input and output characteristics curves and indicate the active cut-off and

saturation regions.

(b) Prove the relation:1

Q.5 (a) Describe the working of a half-wave rectifier. Find out its:

(i) Average value of pulsating current and voltage.

(ii) RMS value of pulsating current and voltage

(iii) Efficiency

(iv) Ripple factor.

(b) Write down short notes on:

(i) F.E.T.;

(ii) S.M.P.S;

(iii) Opto-electronic devices.

______


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SEMICONDUCTOR PHYSICS,ELECTRONICDEVICES & CIRCUITS

PAPER - 117

OBJECTIVE PART- I

Year - 2010

Time allowed : One Hour Maximum Marks : 20

The question paper contains 40 mul tiple choice questions with f our choices and student wi ll

have to pick the correct one (each carr ying mark).

1. The reason for inter-molecular forces is:

(a) Only dipole-dipole interactions(b) Only induced dipole interactions(c) Dispersive forces



(a) lonic (b) Cova

Documents

Semiconductor Physics, Electronic Devices and Circuits