Electronic Devices 2marks Ans

8/8/2019 Electronic Devices 2marks Ans

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V.M.K.V. ENGINEERING COLLEGE

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

ELECTRONIC DEVICES MATERIALBy

P.LOGANATHAN M.E.,AP / EEE,

DEPARTMENT OF ELCTRICAL AND ELECTRONICS ENGINEERING.

V.M.K.V. ENGINEERING COLLEGE,PERIYA SEERAGAPADI, SALEM 636 308.


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P.LOGANATHAN, AP / EEE V.M.K.V.E.C2

+ + + + + + +

- - - - - - - - - -

A

B

Vd

+

-

UNIT-I: BASICS OF SEMICONDUCTORS

(2 MARKS)

1. What is the path taken by the electron when it enters a uniform electric field?

When the charged particle (q Charge) moves in the direction of electric field (Eintensity), A force F=q*E .the path will be straight line. An electron placed at plate A will

be attracted towards the positively charged plate B. The tracing path of electron is straightline.

2. What is the path taken by the electron when it is placed in a uniform magneticfield with zero initial velocity?

When a charge particle moves in a magnetic field, it experience a force whose

direction is perpendicular both to the direction of motion of the particle and to the direction

of the field.

3. What is the path taken by the electron when it enters a uniform magnetic fieldwith an initial velocity at an angle with the direction of the field?

An electron is injected with an initial velocity of v m/sec at a small angle to thedirection of a magnetic field of flux density B wb/m2 as a result of these concurrent motions,

the electron traces a helical path.

X X X X

X X X X

X X X X

X X X X

X

X

CIRCULAR PATH


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4. What is the path taken by the electron when it is placed in perpendicular electric

and magnetic fields with zero initial velocity?

Let electric and magnetic fields align along y-direction and velocity vector is aligned

along positive x-direction. Let the charge be positive and initial velocity be vo In this case,velocity and magnetic field vectors are perpendicular to each other. Applying Right hand vector

cross product rule, we determine that magnetic force is acting in positive z-direction. If electric

field is not present, then the particle revolves along a circle in xz plane as shown in the figure

below. Motion of a charged particle in electric and magnetic fields

The radius of each of the circular element and other periodic attributes like time period,

frequency and angular frequency are same as for the case of circular motion of charged particle

in perpendicular to magnetic field

R = v / B ; T = 2 / B ; = B / 2 ; = B

5. An electron is accelerated through a potential of 40 V before it enters a magnetic

field density of 0.91 Wb/m2

at an angle of 30 degrees with the field. Find the

position of the electron after which it has completed one revolution in the field.

The velocity of electron is = 2qva/ m = 3.68 x 106 m / s.

The time taken for one revolution is T = 2m / Bq = 4 x 10-11 s.

The pitch = T v cos = 1.27 x 10-4 m.

Thus the electron has traveled the position after completed one revolution in the field is

1.27 x 10-4 m.

6. Differentiate intrinsic and extrinsic semiconductors.S.NO Intrinsic Semiconductor Extrinsic Semiconductor

1 It is a pure form of semiconductor An impurity of doping agent is addedin the pure semiconductor formsextrinsic semiconductor.

2 Number of electrons and holesare equal Number of electrons and holes arenot equal because of doping3 Conductivity is poor Conductivity is improved


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7. Differentiate P-type and N-type semiconductors.S.NO P - type Semiconductor N - type Semiconductor

1 An trivalent impurities is added topure Si or Ge to form P type

Semiconductor

Pentavalent impurities are added topure Si or Ge to form N type

Semiconductor.2 Doping agents : gallium, indium

etc,Doping agents : Arsenic, Antimonyetc,

3 Holes are majority carriers:Electrons are minority carriers

Electrons are majority carriers: areHoles minority carriers

8. Draw the energy band diagram of a semiconductor.

9. Define Fermi Level in semiconductor.The Fermi level is defined as the maximum energy level, which is occupied by an

electron at absolute zero temperature.

10. Illustrate the Fermi level of a P type semiconductor.

DISTANCE

FERMI LEVEL

ENERGYL

EV

EL

FORBIDDEN ENERGY GAP = 1.1 eV

CONDUCTION BAND

VALANCE BAND

D

E

DISTANCE

FERMI LEVEL

ENERGYL

EVEL

FORBIDDEN ENERGY GAP

CONDUCTION BAND

VALANCE BAND

D

E

E

EC

EA

E VALANCE BAND

DISTANCE

FERMI LEVEL

ENERGY LEVEL


CONDUCTION BAND

D

Excess Holes


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ED

E

RS

E VALANCE BAND

DISTANCE

FERMI LEVEL

ENERGY LEVEL


CONDUCTION BAND

D

Excess Electrons

Where EA = Acceptor Energy Level

EV = Valance band energy Level

EC = Conduction band energy Level

11. Illustrate the Fermi level of an N type semiconductor.

12. What is the effect of temperature on position of Fermi level?

In an N type semi conductor, as temperature T increases, more number ofElectrons hole pairs are formed. At very high temperature T, the concentration ofthermally generated electrons in conduction band will be far greater than theconcentration of donor electrons. In such case, as concentration of holes and electrons

become equal, the semi conductor becomes essentially intrinsic and EF returns to themiddle of the forbidden energy gap, hence it is concluded that as the temperature of P

and N type semiconductor increases, EF progressively moves towards the middle of theforbiddenenergygap.


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13. In a P-type semiconductor, the Fermi level is 0.3eV above the valence bandat a room temperature of 300 K. Determine the new position of the Fermi level for

a temperature of 400 K.

The Fermi level in a P Type material is given by

EF =EV + kTln NV / NA

Therefore, (EF EV) = kTlnNV / NA

At T = 300k, 0.3 = 300 k lnNV / NA

(a) At T = 350k, (EF1 EV) = 350k lnNV/ NA

Hence, from the above equation,

EF1 EV / 0.3 = 350 / 300

Therefore, EF1 EV = 350 / 300 x 0.3 = 0.35 eV

(b) At T = 400k, (EF2 EV) = 400k lnNV/ NA

Hence, from the above equation,

EF2 EV / 0.3 = 400 / 300

Therefore, EF2 EV = 400 / 300 x 0.3 = 0.4 eV

14.What is Drift current?It is a current when an electric field is applied across the semiconductor, the holes move

towards the negative terminal of the battery and electrons move towards the positive of the battery. This combined effect causes a current flow in the circuit. This current is called drift

current.

15.What is Diffusion current?A concentration gradient exists, if the number of either electrons or holes is greater in one

region of a semi conductor as compared to the rest of the region. The holes and electrons then

tend to move from region of higher concentration gradient to the region of lower concentration.

This process is called diffusion and the electric current produced due to this process is known asdiffusion current.

16.Define diffusion lengthThe minority carrier densities decay exponentially with the distance from the junction,

with a characteristics decay length of LP for holes and LN for electrons. It can be shown that the

average distance a hole diffuses before recombining is equal to LP so that it is called the diffusion

length.Where

LP = DP TP called the hole diffusion LengthLN = DN TN is called the electron diffusion length.


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UNIT-II: CHARACTERISTICS OF DIODES

(2 MARKS)

1. Define breakdown voltage and cut-in voltage of a PN junction diodeBreakdown Voltage :

It is a reverse biased voltage; in a PN junction diode anode is negative with respect

to cathode, when the voltage is increased (VR) at which the PN junction breakdown is

occur. This reverse voltage is called as breakdown voltage.

Cut - in Voltage :

It is a forward biased voltage; in a PN junction diode anode is positive with

respect to cathode, when the voltage is increased (VF) at which the current flow through

the PN junction starts increasing rapidly is known as cut-in voltage. It is also called asknee voltage or threshold voltage.

2. Define PIV of a PN junction diodePIV is a Peak Inverse Voltage. It is the maximum allowable voltage applied across

the diode when it is connected in reverse bias without destroying the device.

3. Give the diode current equation

WhereI is the diode current,IS is a scale factor called the saturation current,VD is the voltage across the diode,VT is the thermal voltage,

And n is the emission coefficient, also known as the ideality factor. The emission

coefficient n varies from about 1 to 2 depending on the fabrication process and semiconductor

material and in many cases is assumed to be approximately equal to 1 (thus the notation n is

omitted). The thermal voltage VT is approximately 25.85 mV at 300 K, a temperature close to

room temperature commonly used in device simulation software. At any temperature it is a

known constant defined by:

where

q is the magnitude of charge on an electronk is Boltzmanns constant,T is the absolute temperature of the p-n junction in kelvins


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4. Give the circuit symbol of a varactor diode

Equivalent circuit

5. Give the applications of tunnel diodes

1. Tunnel diode is used as mixers.

2. Being high speed devices, tunnel diode also lend than themselves to high speed

switching and logic operations as flip flop and gate.

3. They are used as low power oscillator up to 1000GHz, because of their simplicity,

frequency stability and immunity to radiation.

4. It may be used throughout microwave range as moderate to low noise pre amplifiers in

all kind of receivers.

6. Give the advantages of tunnel diodes.

1. Low Cost

2. Simplicity construction

3. High speed operation

4. Low temperature sensitivity, low noise and low power

5. Environmental Immunity.

7. Compare avalanche breakdown and zener breakdown?

S.NO Avalanche breakdown Zener breakdown

1 This occurs in lightly doped diode This occurs in heavily doped diode

2 These depletion region is very wide This depletion region is very small

3 Electric field is low electric field is strong.

4 It has sufficient kinetic energy and

disrupts covalent bond in crystal andreleasing the valance electron.

It break covalent bond and create

new electron hole pair.

C

RRRS


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8. Give the structure of a PIN diode.

9. What is the advantage of a PIN diode over an ordinary PN junction diode?

PIN diode is a high speed switching device, because its, highly improved

switching time compare with PN diode and it have high resistivity intrinsic layer is

sandwiched between the heavily doped P and N layer. It used as MPST switch.

10. What are multi junction photodiodes?

The multijunction diode is a high-impedance source and provides a negative biased

signal. To actuate a conventional low-impedance meter movement, an impedance transformation

device would be used. Such devices are within the current state of the art. Operational amplifiers

of unity gain may be employed. If has been found that the multi junction diode of this invention,

under open circuit conditions, has an output independent of the junction area. This permits the

use of extremely small units. For example, the device whose response is shown above is about

one-third of the area of the 1/16 th by 1/8 th inch hole normally used in tabulating cards.

11. What is a photovoltaic cell?

Photovoltaic cell is called as solar cell. It works on the principle of photovoltaic effect.

Which is the light energy is converted to electrical energy, and it is a self generating device,

when the light signal is strikes the device voltage is generated.

12. What are the advantages and disadvantages of LEDs? Advantage:

1. Less power consumption

2.

Very small size and weight3. Operating voltage and Current is less4. Variety of Spectral output colors

IP N


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5. Very fast action and Long life Disadvantage:

1. Very sensitive device, (it damage by over voltage and over current)2. Radiated output power is temperature dependent.3. Low efficiency.

13. What are the advantages and disadvantages of LCDs? Advantage:

1. Power consumption is very small compared with LED2. It is used in Watches, pocket calculators, portable instrument displays3. contrast is high4. very small size and weight Disadvantage:

1. The life time is Very less compare with LED

2. The response time is more than LED3. Operates slowly.

14. What are thermistors?

A temperature-sensitive resistor is called a Thermistor. The resistance of most commontypes of thermistor decreases as the temperature rises. It is a bulk semi conductor devicehaving negative temperature coefficient of resistivity. These are frequently used astemperature sensors in the range of - 100 to 300. The resistance decreasingexponentially with increasing temperature. They are called negative temperaturecoefficient, or NTC, thermistors. As the temperature rises, more charge carriers become

available and the resistance falls. Although less often used, it is possible to manufacture positivetemperature coefficient, orPTC, thermistors. These are made of different materials and showan increase in resistance with temperature.

15. Give the applications of thermistors.1. In electronic circuit used as temperature compensating devices2. as a temperature sensor in electronic thermometers.

3. as a sensing element in microwave power measuring equipment.4. as thermal delay.5. in control devices actuated by charges in temperature.


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UNIT III-BIPOLAR JUNCTION TRANSISTORS

(2 MARKS)

1. Give the circuit representation of a PNP type transistor.The name transistor is derived from the words transfer and resistor. It has 3 terminal 3

layer device as shown in figure,

2. Give the circuit representation of an NPN type transistor.

3. Define emitter efficiencyEmitter injection ratio or emitter efficiency is the ratio of the electron current to the total

current

The emitter efficiency is obtained from:


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4. Define transport factorBase transportation factor is the ratio of number of electrons arriving at the collector to the

number of injected electrons.

The base transport factor equals:

5. Define large signal current gainThe large signal current gain of a common base transistor is defined as the ratio of the

negative of the collector current increment to the emitter current change from cutoff (IE = 0) to IE

= - (Ic ICBO) / IE 0

Where ICBO is the reverse saturation current flowing through the reverse biased collector

base junction. As a magnitude of ICBO is negligible when compared to IE the above term canexpress as = Ic / IE

Since Ic and IE are flowing in opposite directions, is always positive. Typical value of range from 0.90 to 0.955. Also is not a constant but varies with IE collector voltage VCB andtemperature.

6. Give the relation between large signal current gain, emitter efficiency andtransport factor.

Any of three basic configurations, there is a definite relationship, as pointed out earlier,

between alpha (), beta (), and gamma (). These relationships are listed as below

7.

Give the circuit representation of a CB transistor configuration

The above figure represents the circuit diagram of common base configuration

transistor. Base terminal is common to both input characteristics and outputcharacteristics.

(OR


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8. Give the circuit representation of a CC transistor configuration

9. Give the circuit representation of a CE transistor configuration

10.Define early effectIt is defined as the variation of effective base with by collector base voltage is

known as base width modulation or early effect. When the collector to base voltage is

made to increase, it increases the depletion region across the collector base junction. With

the result the effective width of base layer is decrease. It is called as early effect.

11.Define alpha and beta of a transistor.Current amplification factor ():

The ratio of change in collector current IC to the change in Emitter current IE at

constant collector base voltage VCB is known as current amplification factor () = IC / IE at constant VCB


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Base Current amplification factor ():

It is defined as the ratio between change in collector current IC to the change in

Base current IB

= IC / IB

12.Write the h parameter equations for CB configuration.

13.Write the h parameter equations for CC configuration.

14.Write the h parameter equations for CE configuration.

15.How a transistor can be used as a switch?When used as an AC signal amplifier, the transistors Base biasing voltage is applied so

that it operates within its "Active" region and the linear part of the output characteristics curves

are used. However, both the NPN & PNP type bipolar transistors can be made to operate as an

"ON/OFF" type solid state switch for controlling high power devices such as motors, solenoids

or lamps. If the circuit uses the Transistor as a Switch, then the biasing is arranged to operate in

the output characteristics curves seen previously in the areas known as the "Saturation" and

"Cut-off" regions as shown below.


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16.What are RF transistors?The RF bipolar transistor is planner in the form and mostly Si, NPN type operating

up to 5db. At high frequency (RF) the reactance due to junction capacitance depend onthe depletion width and bias voltage etc., RF current handling capability, high power gain,

low base resistance, and low output capacitance. This device is intended for Class A, B, or C

amplifier, oscillator, or frequency multiplier circuits and is specifically designed for operation in

the VHF-UHF region.

17.What are power transistors?Power transistor is a semiconductor and widely used in electronic devices as an amplifier or a

switch. It handles high voltage and high current rating. These transistors are mostly used in

switching mode. A heavy-duty transistor designed for power-amplifier and power-control

service. It has following specification

1) High current capability2) Small reverse leak current3) High reliability4) High quality and low price


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UNIT IV-FET AND UJT

(2 MARKS)

1. Why a Field Effect Transistor is called so?The field-effect transistor (FET) controls the current between two points but does so

differently than the bipolar transistor. The FET operates by the effects of an electric field on the

flow of electrons through a single type of semiconductor material. This is why the device is

called as field effect transistor. It is sometimes called a unipolar transistor.

2. What are the types of FET?Field-effect transistors exist in two major classifications.

1. Junction FET (JFET)and2. Metal-Oxide- Semiconductor FET (MOSFET).

I. Depletion and Enhancement MOSFET (DE MOSFET)

II. Enhancement only MOSFET or E only MOSFET

3. Define pinch off voltage of a JFETIt is the minimum drain source voltage at which the drain current essentially

become constant. When a voltage is applied between drain and source (VDS) current flows and

the silicon channel acts rather like a conventional resistor. Now if VDS is increased (with VGS

held at zero volts) towards what is called the pinch off value VP, the drain current ID also at first,

increases. The transistor is working in the "ohmic region" as shown in Fig

4. List the advantages and disadvantages of JFET.Advantage:

1. it has a very high input impedance ( in the order of 100M)2. The operation of JFET is depends upon the bulk material current carriers that do

not cross the junctions. There for inherent noise is absent.3. It has negative temperature co efficient of resistance4. It has very power gain, small in size, long life and high efficiency.


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Disadvantage:

1. High leakage current. Low input impedance than MOSFET.2. Less transconductance.3. Amplification factor is not sufficient.

4. Define Tran conductance of JFET.It is the ratio of change in drain current (ID) to the change in gate source

voltage (VGS) at constant drain source voltage (VDS).

Trans conductance gfs = ID / VGS at constant VDS in mA or micromho

5. Define drain resistance of JFET.It is the ratio of change in drain source voltage (VDS) to the change in drain

current (ID) at constant gate source voltage VGS

Drain resistance rD = VDS / ID at constant VGS

6. Define amplification factor of JFET.It is the ratio of change in drain source voltage (VDS) to the change in gate

source voltage VGS (VGS) at constant drain current ID

Amplification factor = VDS / VGS at constant ID

7. List the applications of JFETa. The JFET Differential Amplifier

b. A Low-Noise Amplifier

c. The JFET Constant Current Source

d. The JFET Analog Switch

e. The JFET Voltage Controlled Resistor

8. State the principle of operation of MOSFETs.The basic principle of the device is a voltage on the oxide-insulated gate electrode (G)

can induce a conducting channel between the two other contacts called source (S) and drain (D).

The channel can be of n-type or p-type, and is accordingly called an nMOSFET or a pMOSFET

the gate terminal is formed as a small capacitor. One plate of this capacitor is the gate and the

other plate is the channel with metal oxide as the dielectric. When the negative voltage isapplied on the gate, electrons accumulate on it. These electrons repel the conduction band


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electrons in the n-channel therefore less number of conduction electrons are made available for

current conduction through the channel. The greater negative voltage on the gate, the lesser is

the current conduction from source to drain. If the gate is positive more electrons are made

available in n-channel. Consequently current from source to drain.

9. Give the circuit symbol of a P-channel enhancement MOSFET

10. Give the circuit symbol of a P-channel depletion MOSFET

11.Compare N channel and P Channel MOSFETsS.NO N channelMOSFET P channelMOSFET

1 ON resistance is smaller thanPMOS, thus it is smaller in timeand occupies space.

The mobility of electron in silicon atnormal electric field is about 2.5 times thatof holes thus its on resistance is highhence it occupies more space in ICfabrication.

2 Size is small as well as itsjunction capacitance also smallresult in which very fast

operation.

Size is greater than NMOS, the internalcapacitance reduces the speed ofoperation.

3 It require positive gate drainsupply,

It require negative gate drain supply.


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12. Define threshold voltage.Threshold voltage is defined as the minimum voltage that required making the transistor ON.

Transistor may be either NMOS or PMOS. For NMOS the value of threshold voltage is positive

value and for PMOS the value of threshold voltage is negative value.

13. What is Gate capacitance?The gate electrode forms a parallel plate capacitor with the channel, where Sio2 layer act

as dielectric. This formation of gate is called as gate capacitance. The gate definition spacer

defines the walls of a second etched trench that is used to remove a portion of the p-n junction,

thereby further reducing the junction capacitance

14.What are the applications of CCD?Charge-coupled devices (CCDs) have made possible a revolution in image processing

They consist of a series of light-sensitive elements, called pixels, arranged in a square or

rectangular array. When CCDs are exposed to light, an image of the object being observed is

formed; this image can be extracted from the CCD and stored on a computer for later analysis.

CCDs are used in a variety of modern instruments, ranging from scanners and photocopiers to

video cameras and digital still cameras. They have transformed the way scientists measure and

chart the universe. Because CCDs are available in a wide price range, they are accessible to

amateurs as well as professionals, and enable both to make significant contributions to modern

astronomy.

The potential of the charge-coupling concept for digital memory has been recognized.

Applications to image sensing and signal processing requirements have, however, preceded the

application of CCD to memory. This occurred because relatively simple charge-coupled devices

offered new performance potentials for image sensing and signal processing. Nonetheless, the

basic shift-register nature of CCD implies that its greatest opportunity for widespread application

lies in the highly competitive area of high density semiconductor memory. Today, sophisticated

CCD memory components are emerging.

15. What are BBDs?The BBD consists of series MOSFET switches with capacitor connected at their nodes

sampled values of input signal are stored as charge on the capacitor. It can be transferred from

one stage to next by controlling the switches using a clock pulses.
http://science.jrank.org/pages/1374/Charge-Coupled-Device.htmlhttp://science.jrank.org/pages/6419/Square.htmlhttp://science.jrank.org/pages/3929/Light.htmlhttp://science.jrank.org/pages/1374/Charge-Coupled-Device.htmlhttp://science.jrank.org/pages/1374/Charge-Coupled-Device.htmlhttp://science.jrank.org/pages/583/Astronomy.htmlhttp://science.jrank.org/pages/583/Astronomy.htmlhttp://science.jrank.org/pages/1374/Charge-Coupled-Device.htmlhttp://science.jrank.org/pages/1374/Charge-Coupled-Device.htmlhttp://science.jrank.org/pages/3929/Light.htmlhttp://science.jrank.org/pages/6419/Square.htmlhttp://science.jrank.org/pages/1374/Charge-Coupled-Device.html


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A capacitive delay device comprising a sequence of capacitances, in which information is

transferred by charge transfer. The device is provided with at least one auxiliary store which has

been connected between a first and a second capacitance, whilst after a charge transfer between

the capacitances a residual charge transfer takes place between the first capacitance and the

auxiliary store, after which the charge stored in the auxiliary store and in the second capacitance

is transferred to a capacitance succeeding the second capacitance.

16. What is the difference between a UJT and a BJT?S.NO UJT BJT

1 It is 2 layer 3 terminal device It is 3 layer 3 terminal device

2 It has no ability to amplify the

signal.

It has the ability to amplify the signal

3 Current conduction is only dueto one type of charge carriers.Majority charge carriers

Current conduction is both types of chargecarriers. In electron and holes.

4 It is used as oscillator It is used as amplifier

17. Define intrinsic ratio-off ratio of a UJT.The intrinsic standoff ratio is the ratio of voltage drop across RB1 (VA) to the battery

voltage RBBO (VBB). It varies from 0.4 to 0.8 for different devices. The schematic symbol is

Figure below

Intrinsic standoff ratio = VA / VBB


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UNIT 5: SCR AND PRINCIPLES OF IC

(2 MARKS)

1. Give the basic structure and circuit symbol of an SCR

(OR)

2. Draw the two transistor version of SCR.

3. Define latching current of an SCR

It is the minimum current flowing from anode to cathode when SCR goes from OFF state

to ON state and remains in ON state even after gate bias is removed. It is greater than but very

close to Holding current.

4. Define holding current of an SCR

It is that value of current below which the SCR switches from the conduction state (ONstate) to the forward blocking state.

5. Define forward current rating and breakdown voltage of SCR.

Forward current rating of SCRis defined as that the maximum anode current that it

can handle without destruction.

Break down voltage is the reverse voltage in which the breakdown occurs, and

breakings happened in junction J1

and unction3when Anode is negative with respect to cathode.


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6. Why an SCR is operated only in forward biased condition?

The reverse bias condition is represented by -V. A reverse bias exists when the potential

applied across the SCR results in the cathode being more positive than the anode. In this

condition the SCR is non-conducting and the application of a trigger voltage will have no effect

on the device. In the reverse bias mode, the knee of the curve is known as the Peak Inverse

Voltage PIV (or Peak Reverse Voltage - PRV) and this value cannot be exceeded or the device

will break-down and be destroyed. A good Rule-of -Thumb is to select a device with a PIV of at

least three times the RMS value of the applied voltage. Therefore current conduction in reverse

bias is negligible. This is a fact that SCR operated only in forward bias.

7. Give the basic structure and circuit symbol of a TRIAC

8. Give the two SCR version of a TRIACThe SCR will only operate when the anode is positive to the cathode. Two SCR's can be

placed back-to-back in order to operate with positive and negative voltages. A Triac is equivalent

to two SCR's back-to-back.

9. Give the basic structure and circuit symbol of a DIAC


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10. Give the two transistor analogy of GTO

11. What is meant by epitaxial layer?

In the Monolithic IC manufacturing process is consider the layer preparation. The second

layer of N type material which is grown as a single crystal extension on the substrate. For this

purpose the wafer put in diffusion furnace. A gas mixture of silicon atoms and pentavalent atoms

is passed over the wafers. This form a thin layer of n type semiconductor on heated surface of

the substrate. This thin layer is called epitaxial layer.

12. How electrical isolation is provided between the different components fabricated

in an IC?

Si02 layer is removed from the desired areas to penetrate into the N-type epitaxial layer

through the openings in Si02 layer and ultimately reach the P-type substrate. The temperature and

time period of diffusion are required to be carefully controlled. The process results in formation

of N-type regions, called the isolation islands. The name is given as they are separated by back-

to-back P-N junctions. Their purpose is to permit electrical isolation between various

components of IC. Each electrical element is later on formed in a separate isolation island. The

bottom of the N-type isolation island ultimately forms the collector of an N-P-N transistor.

13. What is the use of photolithography in IC manufacturing?

Photolithographic process is used to remove the Sio2 layer from selective area in which

impurities required the diffusion process.


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14. Why buried layer is needed in collector region?

The presence of a buried layer may cause the value of the collector breakdown voltages,

BVCES andBVCBO, to be considerably less than existing theory predicts. A method for forming a

buried layer below the collector region of a transistor of an integrated circuit uses a second

doping agent (Phosphorus) in addition to a main doping agent (antimony). The use of the second

doping agent solves the problem of undesired intermediate or phantom layers caused by the

external diffusion (out-diffusion) of the doping agent in a P-type doped isolation layer below the

buried layer. The use of the second doping agent also solves the problem of providing a collector

region for transistors of the integrated circuit with a concentration of impurities which is

typically 10 times higher than that of the collector region of a power transistor integrated

monolithically on the same chip. The use of the second doping agent finally makes it possible to

achieve collector regions for the transistors of the integrated circuit with a concentration of

impurities which differs from transistor to transistor in accordance with the circuit functions to

be carried out.

14.What is the advantage of CMOS over NMOS?The most important advantage of CMOS is the very low static power consumption in

compare with NMOS technology. Two important characteristics of CMOS devices are high

noise immunity and low static power consumption. On the other hand, CMOS technology is

more complex to fabricate then NMOS technology, so it is more expensive. However, almost

every todays digital circuits are CMOS. You want to use NMOS only when you want to

fabricate fast and low-cost a simple circuit.

16. What are the advantages of thick film hybrid circuits?

Advantage:

1. Relatively simple processing assembly techniques.

2. Low development cost and high reliable

3. Higher frequency performance

4. Reduced size leading to reduced weight and high density

5. High thermal conductivity of substrates.

Electronic Devices 2marks Ans

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