Electric Circuits and Electron Devices[1]

8/3/2019 Electric Circuits and Electron Devices[1]

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EC 25 ELECTRIC CIRCUITSANDELECTRON DEVICES

SEM:II Branch: CSE

Staff-in-Charge: M.UMA SORNA RANI


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UNIT I CIRCUIT ANALYSIS TECHNIQUES

INTRODUCTIONCircuit Definitions

Node any point where 2 or more circuit elements are connected togetherWires usually have negligible resistanceEach node has one voltage (w.r.t. ground)Branch a circuit element between two nodesLoop a collection of branches that form a closed path returning to the same nodewithout going through any other nodes or branches twiceVoltage-current characteristic of ideal resistor:A Node is a point of connection between two or more circuit elementsNodes can be spread out by perfect conductors


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Kirchoffs Current Law (KCL)

The algebraic sum of all currents entering (or leaving) a node is zeroEquivalently: The sum of the currents entering a node equals the sum of the

currents leaving a nodeMathematically:...Nkkti10)(When applying KCL, the current directions (entering or leaving a node) are basedonthe assumed directions of the currentsAlso need to decide whether currents entering the node are positive or negative;

this dictates the sign of the currents leaving the nodeAs long all assumptions are consistent, the final result will reflect the actualcurrent directions in the circuitKirchoffs Voltage Law (KVL)

.The algebraic sum of all voltage differences around any closed loop is zero

.Equivalently: The sum of the voltage rises around a closed loop is equal to the

sum of thevoltage drops around the loop

.Mathematically:

..

.Nkktv10)(.Voltage polarities are based on assumed polarities.

If assumptions are consistent, the final results will reflect the actual polarities.The algebraic sum of voltages around each loop is zero.Beginning with one node, add voltages across each branch in the loop (if you encounter a

+ sign first) and subtract voltages (if you encounter a sign first).Svoltage drops -S

voltage rises = 0.Or S


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voltage drops = Svoltage rises


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NETWORK THEOREMS

This chapter introduces important fundamental theorems of network analysis. Theyarethe

Superposition theoremThvenin.s theoremNorton.s theoremMaximum power transfer theoremSuperposition Theorem

Used to find the solution to networks with two or more sources that are not in s

eries orparallel.The current through, or voltage across, an element in a network is equal to thealgebraicsum of the currents or voltages produced independently by each source.Since the effect of each source will be determined independently, the number ofnetworks to be analyzed will equal the number of sources.The total power delivered to a resistive element must be determined using the totalcurrent through or the total voltage across the element and cannot be determined

by asimple sum of the power levels established by each source.Thvenins Theorem

Any two-terminal dc network can be replaced by an equivalent circuit consistingof avoltage source and a series resistor.Thvenin.s theorem can be used to:

Analyze networks with sources that are not in series or parallel.Reduce the number of components required to establish the same characteristics at theoutput terminals.


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Investigate the effect of changing a particular component on the behavior of a networkwithout having to analyze the entire network after each change.Procedure to determine the proper values of RTh and ETh

Preliminary

Remove that portion of the network across which the Thvenin equation circuit is to befound. In the figure below, this requires that the load resistor RL be temporarily removedfrom the network.Mark the terminals of the remaining two-terminal network. (The importance of this stepwill become obvious as we progress through some complex networks.)

RTh:

Calculate RTh by first setting all sources to zero (voltage sources are replacedby shortcircuits, and current sources by open circuits) and then finding the resultant resistancebetween the two marked terminals. (If the internal resistance of the voltage and/or currentsources is included in the original network, it must remain when the sources areset tozero.)ETh:

Calculate ETh by first returning all sources to their original position and finding the open-circuit voltage between the marked terminals. (This step is invariably the one that willlead to the most confusion and errors. In all cases, keep in mind that it is theopen-circuitpotential between the two terminals marked in step 2.)Draw the Thvenin equivalent circuit with the portion of the circuit previously removed

replaced between the terminals of the equivalent circuit. This step is indicatedby theplacement of the resistor RL between the terminals of the Thvenin equivalent circuit.Nortons Theorem

Norton.s theorem states the following:


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Any two-terminal linear bilateral dc network can be replaced by an equivalent circuitconsisting of a current and a parallel resistor.The steps leading to the proper values of IN and RN.Preliminary steps:

Remove that portion of the network across which the Norton equivalent circuit isfound.Mark the terminals of the remaining two-terminal network.Finding RN:

Calculate RN by first setting all sources to zero (voltage sources are replacedwith shortcircuits, and current sources with open circuits) and then finding the resultant

resistancebetween the two marked terminals. (If the internal resistance of the voltage and/orcurrent sources is included in the original network, it must remain when the sources areset to zero.) Since RN = RTh the procedure and value obtained using the approachdescribed for Thvenin.s theorem will determine the proper value of RN.Finding IN :

Calculate IN by first returning all the sources to their original position and then finding

the short-circuit current between the marked terminals. It is the same current that wouldbe measured by an ammeter placed between the marked terminals.Conclusion:

Draw the Norton equivalent circuit with the portion of the circuit previously removedreplaced between the terminals of the equivalent circuit.Maximum Power Transfer Theorem

For loads connected directly to a dc voltage supply, maximum power will be delivered tothe load when the load resistance is equal to the internal resistance of the source; that is,when: RL = Rint

The maximum power transfer theorem states the following:

A load will receive maximum power from a network when its total resistive valueisexactly equal to the Thvenin resistance of the network applied to the load. Thatis,

RL = RTh


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Series resistors & voltage division

Series: Two or more elements are in series if they are cascaded or connected sequentiallyand consequently carry the same current.

The equivalent resistance of any number of resistors connected in a series is the sum ofthe individual resistances

..

........NnnNeqRRRRR121v2122RRRv..v2111RRRv..Parallel resistors & current division

Parallel: Two or more elements are in parallel if they are connected to the sametwo nodes andconsequently have the same voltage across them.

The equivalent resistance of a circuit with N resistors in parallel is:

NeqRRRR111121.......iRRRi2112...iRRRi2121..


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Delta -> Star transformation

)(1cbacbRRRRRR..

.)(2cbaacRRRRRR...)(3cbabaRRRRRR...Star -> Delta transformation

2133221RRRRRRRRb..

.1133221RRRRRRRRa

..

.3133221RRRRRRRRc...


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UNIT II TRANSIENT RESONANCE IN RLC CIRCUITS

INTRODUCTION

The fundamental passive linear circuit elements are theresistor (R),capacitor (C)inductor (L).

These circuit elements can be combined to form an electrical circuit in four distinct ways:the RC circuit,the RL circuit,the LC circuitthe RLC circuit

These circuits exhibit important types of behaviour that are fundamental to analogue

electronics.

RL CIRCUIT

A resistor-inductor circuit (RL circuit), or RL filter or RL network, is one ofthe simplestanalogue infinite impulse response electronic filters. It consists of a resistorand an inductor,either in series or in parallel, driven by a voltage source.

The complex impedance ZL (in ohms) of an inductor with inductance L (in henries)is

The complex frequency s is a complex number,

where


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.j represents the imaginary unit:

j2 = - 1

.

is the exponential decay constant (in radians per second), and

.is the angular frequency (in radians per second).

RC circuit

The simplest RC circuit is a capacitor and a resistor in series.When a circuit composes of only a charged capacitor and a resistor,

then the capacitor would discharge its energy into the resistor.This voltage across the capacitor over time could be found through KCL, where thecurrent coming out of the capacitor must equal the current going through the resistor.This results in the linear differential equationNatural response

The simplest RC circuit is a capacitor and a resistor in series.When a circuit composes of only a charged capacitor and a resistor,

then the capacitor would discharge its energy into the resistor.This voltage across the capacitor over time could be found through KCL,where the current coming out of the capacitor must equal the current going through theresistor.This results in the linear differential equationRLC circuit

Time dependences

Now we study an AC circuit, where the resistor R, coil L, and capacitor C are inseriesconnection. The circuit is ideal, because the internal resistances of the coil and the capacitor areignored. The connections are given in figure 10-1. The power source gives a periodic voltage, u= sin.t, where the frequency, f = ./2., can be adjusted.

Figure 10-1. RLC circuit


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When the circuit is closed and the system has stabilized, the current is given by (compare witheqn. U = RI .I = U/R)

),sin()(....tZti

where

22)1(CLRZ....)/1arctan(RCL....

..

The impedance Z and phase difference .(between voltage and current) depend on the frequencyof the source voltage. The angle .actually tells how much the current i comes after the totalvoltage (source voltage) u. The time dependences of i and u are given in figure10-2, where .>

0.Resonance

The circuit is said to be in resonance, if the impedance is the same as R, i.e.the effects due to theinductance and the capacitance cancel each other. In the resonance, the power transferred fromthe source to the circuit is in maximum. From figure 10-3 is seen that the requirement forresonance is

.L = 1/.CFrom the same figure is seen that .= 0 and Z = Zmin. The current is given by

I = U/Z = U/R= Imax.


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Q factor

. The ratio of the inductance L to the resistance R of a coil remains constant for different windingarrangements in the same volume and shape. It makes sense to define this value as a figure of

merit to distinguish different coil structures. The quality factor Q is definedby this ratio.The voltage, which is induced by the same current in an inductor scales with thefrequency f andthus the apparent power in the device. The general definition of the quality factor is based on theratio of apparent power to the power losses in a device. From this definition, the quality factor ofa coil results to:

with . = 2pf

RC and RL Transient Analysis BasicsTransient State:

If a network contains energy storage elements, with change in excitation, the current andvoltages change from one state to another state is called transient state. The behavior of thevoltage or current when it is changed from one state to another state is calledtransient state.

Transient Time:

The time taken for the circuit to change from one steady state to another steadystate iscalled the transient time.

Natural response:

If we consider a circuit containing storage elements which are independent of sources, theresponse depends upon the nature of the circuit, it is called natural response.

Transient response:

The storage elements deliver their energy to the resistances, hence the responsechangeswith time, gets saturated after sometime, and is referred to the transient response.Laplace Transform:

The Laplace transform of any time dependent function f(t) is given by F(s).

Where S.A complex frequency given by S=s + j.


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Inverse Laplace Transform:

Inverse Laplace transforms permits going back in the reverse direction i.e. froms domainto time domain.

Order of a System:

The order of the system is given by the order of the differential equation governing the

th th

system. If the system is governed by norder differential equation, than the system is called norder system.

n n-1 n-2

Q(s) =a0 s + a1s + a2s + ..+an-1 s +anthe order of the system is equal to n..Initial Value Theorem

The initial value theorem states that if x (t) and x. (t) both are laplace transformable, then

Final Value Theorem

The final value theorem states that if x (t) and x. (t) both are laplace transformable, then

Driving Point impedance

The ratio of the Laplace transform of the voltage at the port to the laplace transform ofthe current at the same port is called driving point impedance.

Transfer Point impedance

The ratio of the voltage transform at one port to the current transform at the other port is

called transfer point impedance.

Resonant Circuit

.The circuit that treat a narrow range of frequencies very differently than all otherfrequencies are referred to as resonant circuit..The gain of a highly resonant circuit attains a sharp maximum or minimum at itsresonantfrequency.


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Resonance

Resonance is defined as a phenomenon in which applied voltage and resulting current arein phase.

Bandwidth

The Bandwidth is defined as the frequency difference between upper cut-off frequency(f2) and lower cut-off frequency (f1).

Half Power frequencies

The upper and lower cut-off frequencies are called the half-power frequencies. At thesefrequencies the power from the source is half of the power delivered at the resonant frequency.

Selectivity

Selectivity is defined as the ratio of bandwidth to the resonant frequency of resonantcircuit.

Q factor

The quality factor, Q, is the ratio of the reactive power in the inductor or capacitor to thetrue power in the resistance in series with the coil or capacitor.

Series Resonance in RLC circuit

.In series RLC circuit resonance may be produced by either varying frequency forgiven

constant values of L and C or varying either L and C or both for a given frequency.

.At resonance inductive reactance is equal to the capacitive reactance.

.If f < f0 the current I leads the resultant supply voltage V and so the circuitbehaves as a

capacitive circuit at the frequencies which are less than f0.

.At f = f0, the voltage and current are in phase. The circuit behaves as pure resistive circuit

at the resonant frequency with unit power factor.

.

If f > f0, the current I lags the resultant supply voltage V and so the circuitbehaves as an


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inductive circuit at the frequencies which are more than f0.

.At resonance series RLC circuit acts as a voltage amplifier.

.Series resonance circuit is always driven by a voltage source with very small in

ternal

resistance to maintain high selectivity of the circuit.

Parallel Resonance

.A parallel circuit is said to be in resonance when applied voltage and resultingcurrent are

in phase that gives unity power factor condition.

.Parallel resonance is also known as Anti resonance.

.At anti resonance the parallel resonant circuit acts as current amplifier.


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Reactance curves

The graph of individual reactance versus the frequency is called Reactance Curve.

Types of Tuned circuits

Single tuned circuitDouble tuned circuitSingle tuned circuit

In RF circuit design, tuned circuits are generally employed for obtaining maximum

power transfer to the load connected to secondary or for obtaining maximum possible value of

secondary voltage.

A single tuned circuit is used for coupling an amplifier and radio receiver circuits.

Double tuned circuit

.In double tuned circuits, a variable capacitor is used at input as well as output side..With the help of adjustable capacitive reactance, impedance matching is possible

if thecoupling is critical, sufficient or above..It is also possible to adjust phase angle such that impedance at generator sidebecomesresistive..The magnitude matching can be achieved by adjusting mutual inductance to the criticalvalue, which effectively fulfills maximum power transfer condition.


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UNIT III SEMICONDUCTOR DIODES

Review of intrinsic and extrinsic semiconductors

Intrinsic semiconductor

An intrinsic semiconductor is one, which is pure enough that impurities donot appreciably affect its electrical behavior. In this case, all carriers are created due to thermallyor optically excited electrons from the full valence band into the empty conduction band. Thusequal numbers of electrons and holes are present in an intrinsic semiconductor.Electrons andholes flow in opposite directions in an electric field, though they contribute to current in thesame direction since they are oppositely charged. Hole current and electron current are not

necessarily equal in an intrinsic semiconductor, however, because electrons andholes havedifferent effective masses (crystalline analogues to free inertial masses).

The concentration of carriers is strongly dependent on the temperature. At lowtemperatures, the valence band is completely full making the material an insulator. Increasing thetemperature leads to an increase in the number of carriers and a corresponding increase inconductivity. This characteristic shown by intrinsic semiconductor is differentfrom the behaviorof most metals, which tend to become less conductive at higher temperatures dueto increased

phonon scattering.

Both silicon and germanium are tetravalent, i.e. each has four electrons (valenceelectrons) in their outermost shell. Both elements crystallize with a diamond-like structure, i.e. insuch a way that each atom in the crystal is inside a tetrahedron formed by the four atoms whichare closest to it. Each atom shares its four valence electrons with its four immediateneighbours, so that each atom is involved in four covalent bonds.


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Extrinsic semiconductor

An extrinsic semiconductor is one that has been doped with impurities to modifythenumber and type of free charge carriers. An extrinsic semiconductor is a semiconductor that has

been doped, that is, into which a doping agent has been introduced, giving it different electricalproperties than the intrinsic (pure) semiconductor.

Doping involves adding dopant atoms to an intrinsic semiconductor, which changestheelectron and hole carrier concentrations of the semiconductor at thermal equilibrium. Dominantcarrier concentrations in an extrinsic semiconductor classify it as either an n-type or p-typesemiconductor. The electrical properties of extrinsic semiconductors make them essential

components of many electronic devices.

A pure or intrinsic conductor has thermally generated holes and electrons. Howeverthese are relatively few in number. An enormous increase in the number of chargecarriers canby achieved by introducing impurities into the semiconductor in a controlled manner. The resultis the formation of an extrinsic semiconductor. This process is referred to as doping. There arebasically two types of impurities: donor impurities and acceptor impurities. Donor impurities aremade up of atoms (arsenic for example) which have five valence electrons. Accept

or impuritiesare made up of atoms (gallium for example) which have three valence electrons.

The two types of extrinsic semiconductor

N-type semiconductors

Extrinsic semiconductors with a larger electron concentration than hole concentrationare known as n-type semiconductors. The phrase 'n-type' comes from the negativecharge of theelectron. In n-type semiconductors, electrons are the majority carriers and hole

s are the minoritycarriers. N-type semiconductors are created by doping an intrinsic semiconductorwith donorimpurities. In an n-type semiconductor, the Fermi energy level is greater than that of the intrinsicsemiconductor and lies closer to the conduction band than the valence band.


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Arsenic has 5 valence electrons, however, only 4 of them form part of covalentbonds. The 5th electron is then free to take part in conduction. The electrons are said to be themajority carriers and the holes are said to be the minority carriers.

P-type semiconductors

As opposed to n-type semiconductors, p-type semiconductors have a larger holeconcentration than electron concentration. The phrase 'p-type' refers to the positive charge of thehole. In p-type semiconductors, holes are the majority carriers and electrons are the minoritycarriers. P-type semiconductors are created by doping an intrinsic semiconductorwith acceptorimpurities. P-type semiconductors have Fermi energy levels below the intrinsic Fermi energylevel. The Fermi energy level lies closer to the valence band than the conduction band in a p-type

semiconductor.

Gallium has 3 valence electrons, however, there are 4 covalent bonds to fill. The 4thbond therefore remains vacant producing a hole. The holes are said to be the majority carriersand the electrons are said to be the minority carriers

Theory of PN junction diode

On its own a p-type or n-type semiconductor is not very useful. However when combined veryuseful devices can be made. The p-n junction can be formed by allowing a p-type

material todiffuse into a n-type region at high temperatures.

The p-n junction has led to many inventions like the diode, transistors and integrated circuits.


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Free electrons on the n-side and free holes on the p-side can initially diffuseacross thejunction. Uncovered charges are left in the neighbourhood of the junction. Thisregion isdepleted of mobile carriers and is called the DEPLETION REGION (thickness 0.5 1.0 m).

The diffusion of electrons and holes stop due to the barrier potential difference (p.d across thejunction) reaching some critical value. The barrier potential difference (or thecontact potential)depends on the type of semiconductor, temperature and doping densities.

At room temperature, typical values of barrier p.d. are:

Ge ~ 0.2 0.4 V

Si ~ 0.60.8 V

FORWARD BIAS P-N JUNCTIONWhen an external voltage is applied to the P-N junction making the P side positive withrespect to the N side the diode is said to be forward biased (F.B). The barrierp.d. is decreasedby the external applied voltage. The depletion band narrows which urges majoritycarriers toflow across the junction. A F.B. diode has a very low resistance.

REVERSE BIAS P-N JUNCTIONWhen an external voltage is applied to the PN junction making the P side negative withrespect to the N side the diode is said to be Reverse Biased (R.B.). The barrierp.d. increases.The depletion band widens preventing the movement of majority carriers across the junction.A R.B. diode has a very high resistance.

REVERSE BIAS P-N JUNCTION

Only thermally generated minority carriers are urged across the p-n junction. Therefore themagnitude of the reverse saturation current (or reverse leakage current) dependson thetemperature of the semiconductor.

When the PN junction is reversed biased the width of the depletion layer increases, howeverif the reverse voltage gets too large a phenomenon known as diode breakdown occurs.


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I-V CHARACTERISTICSI-V CHARACTERISTICSWhen the diode is F.B., the current increases exponentially with voltage exceptfor a small

range close to the origin.When the diode is R.B., the reverse current is constant and independent of the applied reversebias.Turn-on or cut-in (threshold) voltage V.: for a F.B. diode it is the voltage when the currentincreases appreciably from zero.It is roughly equal to the barrier p.d.:For Ge, V .~ 0.2 0.4 V (at room temp.)For Si, V.

~ 0.60.8 V (at room temp.)

Energy Band structure

The highest electronic energy band in a semiconductor or insulator which can befilled withelectrons. The electrons in the valence band correspond to the valence electronsof theconstituent atoms. In a semiconductor or insulator, at sufficiently low temperatures, the valenceband is completely filled and the conduction band is empty of electrons. Some of

the high energylevels in the valence band may become vacant as a result of thermal excitation of electrons to


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higher energy bands or as a result of the presence of impurities. The net effectof the valenceband is then equivalent to that of a few particles which are equal in number andsimilar in motionto the missing electrons but each of which carries a positive electronic charge.These particles

are referred to as holes.

In solids, the valence band is the highest range of electron energies in which electrons arenormally present at absolute zero temperature.The valence electrons are bound toindividualatoms, as opposed to conduction electrons (found in conductors and semiconductors), which canmove freely within the atomic lattice of the material. On a graph of the electronic band structureof a material, the valence band is located below the conduction band, separated

from it ininsulators and semiconductors by a band gap. In metals, the conduction band hasno energy gapseparating it from the valence band.


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Energy-band diagram of pn junction in thermal equilibrium in which both the n and p region aredegenerately doped.

Zener diode

A Zener diode is a type of diode that permits current not only in the forward directionlike a normal diode, but also in the reverse direction if the voltage is largerthan the breakdownvoltage known as "Zener knee voltage" or "Zener voltage". The device was named after ClarenceZener, who discovered this electrical property.

A conventional solid-state diode will not allow significant current if it is reverse-biasedbelow its reverse breakdown voltage. When the reverse bias breakdown voltage is

exceeded, aconventional diode is subject to high current due to avalanche breakdown. Unlessthis current islimited by circuitry, the diode will be permanently damaged due to overheating.In case of large


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forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to itsjunction built-in voltage and internal resistance. The amount of the voltage drop depends on thesemiconductor material and the doping concentrations.

A Zener diode exhibits almost the same properties, except the device is speciallydesigned so as to have a greatly reduced breakdown voltage, the so-called Zenervoltage. Bycontrast with the conventional device, a reverse-biased Zener diode will exhibita controlledbreakdown and allow the current to keep the voltage across the Zener diode closeto the Zenervoltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltagedrop of very nearly 3.2 V across a wide range of reverse currents. The Zener diode is therefore

ideal for applications such as the generation of a reference voltage (e.g. for an amplifier stage),or as a voltage stabilizer for low-current applications.

Zener diode characteristics

A zener diode is much like a normal diode, the exception being is that it is placed in thecircuit in reverse bias and operates in reverse breakdown. This typical characteristic curveillustrates the operating range for a zener. Note that its forward characteristics are just like anormal diode.

The zener diode.s breakdown characteristics are determined by the dopingprocess. Low voltage zeners (>5V), operate in the zener breakdown range. Those designed tooperate


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Note very small reverse current (before knee).

Breakdown occurs @ knee.Breakdown Characteristics:

VZ remains near constantVZ provides:-Reference voltage-Voltage regulationIZ escalates rapidlyIZMAX is achieved quicklyExceeding IZMAX is fatalZener Diodes

Equivalent Circuit

Ideal Zener exhibits a constant voltage, regardless of current draw.Ideal Zener exhibits no resistance characteristics.Zener exhibits a near constant voltage, varied by current draw through the seriesresistance ZZ.As Iz increases, Vz also increases.Zener Diodes

Characteristic Curve

.Vz results from .Iz..Iz thru Zz produce this.


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Zener diodes have given characteristics such as;

Temperature coefficients describes the % .Vz for .Temp (0C).Vz = Vz x T0Cx .T %/oCPower ratings the zener incurs power dissipation based on Iz and Zz

P = I2ZPower derating factor specifies the reduced power rating for device operating temperatures in

excess of the rated maximum temperature.

0

PD(derated) = PD(max) (mW/C).T mW


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UNIT IV TRANSISTORS

TRANSISTOR CHARACTERISTICS:

The basic of electronic system nowadays is semiconductor device.The famous and commonly use of this device is BJTs

(Bipolar Junction Transistors).It can be use as amplifier and logic switches.BJT consists of three terminal:

collector : Cbase : B.emitter : ETwo types of BJT : pnp and npn

Transistor Construction

3 layer semiconductor device consisting:2 n-and 1 p-type layers of material npn transistor2 p-and 1 n-type layers of material .pnp transistorThe term bipolar reflects the fact that holes and electrons participate in the injectionprocess into the oppositely polarized material

A single pn junction has two different types of bias:forward biasreverse biasThus, a two-pn-junction device has four types of bias.


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Position of the terminals and symbol of BJT.

Base is located at the middle and more thin from the level of collectorand emitter

The emitter and collector terminals are made of the same type ofsemiconductor material, while the base of the other type of materialTransistor currents

-The arrow is always drawn on the emitter The arrow always pointtoward the n-type

-The arrow indicates the direction of the emitter current:pnp:EBnpn: B

EIC=the collector currentIB= the base currentIE= the emitter current


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Transistor Operation

The basic operation will be described using the pnp transistor. The operation ofthe pnptransistor is exactly the same if the roles played by the electron and hole are

interchanged.One p-n junction of a transistor is reverse-biased, whereas the other is forward-biasedForward-biased junction Reverse-biased junction of a pnp transistorof a pnp transistor

Both biasing potentials have been applied to a pnp transistor and resulting majority andminority carrier flows indicated.Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-typematerial.A very small number of carriers (+) will through n-type material to the base terminal.Resulting IB is typically in order of microamperes.The large number of majority carriers will diffuse across the reverse-biased junction intothe p-type material connected to the collector terminal.


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-Active region defined by the biasing arrangements-Cutoff region region where the collector current is 0A-Saturation region-region of the characteristics to the left of VCB = 0V

The curves (output characteristics) clearly indicate that a first approximationto therelationship between IE and IC in the active region is given byIC IE

Once a transistor is in the on. state, the base-emitter voltage will be assumed to beVBE = 0.7VIn the dc mode the level of IC and IE due to the majority carriers are related by a quantity

called alpha.=

IC = .IE + ICBO


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It can then be summarize to IC = .IE (ignore ICBO due to small value)For ac situations where the point of operation moves on the characteristics curve, an acalpha defined by

ECII....Alpha a common base current gain factor that shows the efficiency by calculatingthecurrent percent from current flow from emitter to collector.The value of .is typical from0.9 ~ 0.998.COMMON EMITTER CONFIGURATION

It is called common-emitter configuration since :-emitter is common or reference to both input and output terminals.-emitter is usually the terminal closest to or at ground potential.Almost amplifier design is using connection of CE due to the high gain for current andvoltage.Two set of characteristics are necessary to describe the behavior for CE ;input(baseterminal) and output (collector terminal) parameters.


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Input characteristics for a common-emitter NPN transistor

Output characteristics for a common-emitter npn transistor

For small VCE (VCE < VCESAT, IC increase linearly with increasing of VCEVCE > VCESAT IC not totally depends on VCEconstant ICIB(uA) is very small compare to IC (mA). Small increase in IB cause big increasein ICIB=0 A ICEO occur.Noticing the value when IC=0A. There is still some value of current flows.COMMON COLLECTOR CONFIGURATION

Also called emitter-follower (EF).It is called common-emitter configuration since both thesignal source and the load share the collector terminal as a common connection point.


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The output voltage is obtained at emitter terminal.The input characteristic of common-collector configuration is similar with common-emitter. configuration.

Common-collector circuit configuration is provided with the load resistor connected fromemitter to ground.It is used primarily for impedance-matching purpose since it has high input impedanceand low output impedance.Notation and symbols used with the common-collector configuration:

(a) pnp transistor ; (b) npn transistor

For the common-collector configuration, the output characteristics are a plot ofIE vs VCE for arange of values of IB.


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Limits of Operation

Many BJT transistor used as an amplifier. Thus it isimportant to notice the limits of operations.

At least 3 maximum values is mentioned in data sheet.There are:a) Maximum power dissipation at collector: PCmax or PD

b) Maximum collector-emitter voltage: VCEmax sometimes named as VBR(CEO) or VCEO.

c) Maximum collector current: ICmax

There are few rules that need to be followed for BJT

transistor used as an amplifier. The rules are:i) transistor need to be operate in active region!

ii) IC < ICmax

ii) PC < PCmax

FIELD EFFECT TRANSISTOR (FET)

Field effect devices are those in which current is controlled by the action of an electron

field, rather than carrier injection.Field-effect transistors are so named because a weak electrical signal coming inthroughone electrode creates an electrical field through the rest of the transistor.The FET was known as a unipolar transistor.The term refers to the fact that current is transported by carriers of one polarity(majority), whereas in the conventional bipolar transistor carriers of both polarities

(majority and minority) are involved.The family of FET devices may be divided into :

Junction FETDepletion Mode MOSFETEnhancement Mode MOSFET


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Junction FETs (JFETs

JFETs consists of a piece of high-resistivity semiconductor material (usually Si) which

constitutes a channel for the majority carrier flow.Conducting semiconductor channel between two ohmic contacts source & drainJFET is a high-input resistance device, while the BJT is comparatively low.If the channel is doped with a donor impurity, n-type material is formed and thechannelcurrent will consist of electrons.If the channel is doped with an acceptor impurity, p-type material will be forme

d and thechannel current will consist of holes.N-channel devices have greater conductivity than p-channel types, since electrons have highermobility than do holes; thus n-channel JFETs are approximately twice as efficient conductorscompared to their p-channel counterparts

The magnitude of this current is controlled by a voltage applied to a gate, which is areverse-biased.


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The fundamental difference between JFET and BJT devices: when the JFET junctionisreverse-biased the gate current is practically zero, whereas the base current ofthe BJT isalways some value greater than zero.

Basic structure of JFETs

In addition to the channel, a JFET contains two ohmic contacts: the source and the drain.The JFET will conduct current equally well in either direction and the source and drainleads are usually interchangeable.N-channel JFET

This transistor is made by forming a channel of N-type material in a P-type substrate.Three wires are then connected to the device.One at each end of the channel.One connected to the substrate.In a sense, the device is a bit like a PN-junction diode, except that there aretwo wiresconnected to the N-type side

The gate is connected to the source.Since the pn junction is reverse-biased, little current will flow in the gate connection.The potential gradient established will form a depletion layer, where almost alltheelectrons present in the n-type channel will be swept away.The most depleted portion is in the high field between the G and the D, and theleast-depletedarea is between the G and the S.


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Because the flow of current along the channel from the (+ve) drain to the (-ve)source isreally a flow of free electrons from S to D in the n-type Si, the magnitude of this currentwill fall as more Si becomes depleted of free electrons.

There is a limit to the drain current (ID) which increased VDS can drive throughthechannel.This limiting current is known as IDSS (Drain-to-Source current with the gate shorted tothe source).The output characteristics of an n-channel JFET with the gate short-circuited tothesource.The initial rise in ID is related to the buildup of the depletion layer as VDS increases.The curve approaches the level of the limiting current IDSS when ID begins to bepinchedoff.The physical meaning of this term leads to one definition of pinch-off voltage,VP , whichis the value of VDS at which the maximum IDSS flows.With a steady gate-source voltage of 1 V there is always 1 V across the wall of

thechannel at the source end.A drain-source voltage of 1 V means that there will be 2 V across the wall at the drainend. (The drain is up 1V from the source potential and the gate is 1V down, hence thetotal difference is 2V.)

The higher voltage difference at the drain end means that the electron channel is squeezed

down a bit more at this end.When the drain-source voltage is increased to 10V the voltage across the channelwalls atthe drain end increases to 11V, but remains just 1V at the source end.


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The field across the walls near the drain end is now a lot larger than at the source end.As a result the channel near the drain is squeezed down quite a lot.

Increasing the source-drain voltage to 20V squeezes down this end of the channelstillmore.As we increase this voltage we increase the electric field which drives electrons along theopen part of the channel.However, also squeezes down the channel near the drain end.This reduction in the open channel width makes it harder for electrons to pass.

As a result the drain-source current tends to remain constant when we increase the drain-source voltage.Increasing VDS increases the widths of depletion layers, which penetrate more intochannel and hence result in more channel narrowing toward the drain.The resistance of the n-channel, RAB therefore increases with VDS.The drain current: IDS = VDS/RABID versus VDS exhibits a sub linear behavior, see figure for VDS < 5V.

The pinch-off voltage, VP is the magnitude of reverse bias needed across the p+njunctionto make them just touch at the drain end.Since actual bias voltage across p+n junction at drain end is VGD, the pinch-offoccurwhenever: VGD = -VP.


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JFET: I-V characteristics

MOSFETs and Their Characteristics

The metal-oxide semiconductor field effect transistor has a gate, source, and drain justlike the JFET.The drain current in a MOSFET is controlled by the gate-source voltage VGS.There are two basic types of MOSFETS: the enhancement-type and the depletion-type.The enhancement-type MOSFET is usually referred to as an E-MOSFET, and thedepletion-type, a D-MOSFET.The MOSFET is also referred to as an IGFET because the gate is insulated from th

e channel


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n-channel, enhancement-type MOSFET

The p-type substrate makes contact with the SiO2 insulator.

Because of this, there is no channel for conduction between the drain and sourceterminals.


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Unit V SPECIAL SEMICONDUCTOR DIODES

Tunnel diode( Esaki Diode)

It was introduced by Leo Esaki in 1958.Heavily-doped p-n junctionImpurity concentration is 1 part in 10^3 as compared to 1 part in 10^8 in p-njunction diodeWidth of the depletion layer is very small(about 100 A).It is generally made up of Ge and GaAs.

It shows tunneling phenomenon.Circuit symbol of tunnel diode is :Tunnelling Effect

Classically, carrier must have energy at least equal to potential-barrier heightto cross thejunction .But according to Quantum mechanics there is finite probability that it can penetratethrough the barrier for a thin width.

This phenomenon is called tunneling and hence the Esaki Diode is known as TunnelDiode.


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CHARACTERISTIC OF TUNNEL DIODE

Ip:-Peak Current

Iv :-Valley Current

Vp:-Peak Voltage

Vv:-Valley Voltage

Vf:-Peak Forward Voltage

ENERGY BAND DIAGRAM

Energy-band diagram of pn junction in thermal equilibrium in which both the n and p region are

degenerately doped

AT ZERO BIASSimplified energy-band diagram and I-V characteristics of the tunnel diode at zero bias.


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-Zero current on the I-V diagram;-All energy states are filled below EF on both sides of the junction;

AT SMALL FORWARD VOLTAGE

Simplified energy-band diagram and I-V characteristics of the tunnel diode at aslightforward bias

-Electrons in the conduction band of the n region are directly opposite to the empty statesin the valence band of the p region.

So a finite probability that some electrons tunnel directly into the empty states resulting inforward-bias tunneling current.


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AT MAXIMUM TUNNELING CURENT

Simplified energy-band diagraam and I-V characteristics of the tunnel diode at aforward biasproducing maximum tunneling current.

-The maximum number of electrons in the n region are opposite to the maximum numberof empty states in the p region.-Hence tunneling current is maximum.TUNNEL DIODE EQUIVALENT CIRCUIT

This is the equivalent circuit of tunnel diode when biased in negative resistance region.At higher frequencies the series R and L can be ignored.Hence equivalent circuit can be reduced to parallel combination of junction capacitanceand negative resistance.VARACTOR DIODE

A varactor diode is best explained as a variable capacitor. Think of the depletion region as a

variable dielectric. The diode is placed in reverse bias. The dielectric is adjusted by reverse

bias voltage changes.

Junction capacitance is present in all reverse biased diodes because of the depletionregion.Junction capacitance is optimized in a varactor diode and is used for high frequencies andswitching applications.Varactor diodes are often used for electronic tuning applications in FM radios and

televisions.


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They are also called voltage-variable capacitance diodes.A Junction diode which acts as a variable capacitor under changing reversebias is known as VARACTOR DIODE

A varactor diode is specially constructed to have high resistance under reversebias. Capacitance for varactor diode are Pico farad. (10-12 ) range

CT = .A / Wd

CT =Total Capacitance of the junction. = Permittivity of the semiconductor materialA = Cross sectional area of the junctionWD= Width of the depletion layerCurve between Reverse bias voltage Vr across varactor diode and total junction capacitance

Ct and Ct can be changed by changing Vr.

Application for Varactor diode

Use of varactor diode in a tuned circuit. Capacitance of the varactor in parallel with theinductor.(LC circuit)


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Silicon Controlled Rectifier (SCR)

Three terminalsanode -P-layer

cathode -N-layer (opposite end)gate -P-layer near the cathodeThree junctions -four layersConnect power such that the anode is positive with respect to the cathode -no current will flow

A silicon controlled rectifier is a semiconductor device that acts as a true electronicswitch. it can change alternating current and at the same time can control the amount of powerfed to the load. SCR combines the features of a rectifier and a transistor.

CONSTRUCTION

When a pn junction is added to a junction transistor the resulting three pn junction deviceis called a SCR. ordinary rectifier (pn) and a junction transistor (npn) combined in one unit toform pnpn device. three terminals are taken : one from the outer p-type materialcalled anode asecond from the outer n-type material called cathode K and the third from the base of transistorcalled Gate. GSCR is a solid state equivalent of thyratron. the gate anode and cathode of SCRcorrespond to the grid plate and cathode of thyratron SCR is called thyristor


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WORKING PRINCIPLE

Load is connected in series with anode the anode is always kept at positive potential w.r.tcathode.

WHEN GATE IS OPEN

No voltage applied to the gate, j2 is reverse biased while j1 and j3 are FB . J1and J3 isjust in npn transistor with base open .no current flows through the load RL andSCR is cut off. ifthe applied voltage is gradually increased a stage is reached when RB junction J2 breakdown .theSCR now conducts heavily and is said to be ON state. the applied voltage at which SCRconducts heavily without gate voltage is called Break over Voltage.


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WHEN GATE IS POSITIVE W.R.T CATHODE.

The SCR can be made to conduct heavily at smaller applied voltage by applying small

positive potential to the gate.J3 is FB and J2 is RB the electron from n type material start movingacross J3 towards left holes from p type toward right. electrons from j3 are attracted acrossjunction J2 and gate current starts flowing. as soon as gate current flows anodecurrent increases.the increased anode current in turn makes more electrons available at J2 breakdown and SCRstarts conducting heavily. the gate looses all control if the gate voltage is removed anode currentdoes not decrease at all. The only way to stop conduction is to reduce the applied voltage to zero.

BREAKOVER VOLTAGE

It is the minimum forward voltage gate being open at which SCR starts conductingheavily i.e turned on

PEAK REVERSE VOLTAGE( PRV)

It is the maximum reverse voltage applied to an SCR without conducting in the reversedirection

HOLDING CURRENT

It is the maximum anode current gate being open at which SCR is turned off fromonconditions.

FORWARD CURRENT RATING

It is the maximum anode current that an SCR is capable of passing without destruction

CIRCUIT FUSING RATING

It is the product of of square of forward surge current and the time of durationof thesurge


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VI CHARACTERISTICS OF SCR

FORWARD CHARCTERISTICS

When anode is +ve w.r.t cathode the curve between V & I is called Forwardcharacteristics. OABC is the forward characteristics of the SCR at Ig =0. if thesupplied voltageis increased from zero point A is reached .SCR starts conducting voltage acrossSCR suddenlydrops (dotted curve AB) most of supply voltage appears across RL

REVERSE CHARCTERISTICS

When anode is ve w.r.t.cathode the curve b/w V&I is known as reverse characteristicsreverse voltage come across SCR when it is operated with ac supply reverse volta

ge is increasedanode current remains small avalanche breakdown occurs and SCR starts conductingheavily isknown as reverse breakdown voltage

SCR as a switch

SCR Half and Full wave rectifier

Application

SCR as a static contactorSCR for power control

SCR for speed control of d.c.shunt motorOver light detector


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Unijunction transistor circuits were popular in hobbyist electronics circuits inthe 1970sand early 1980s because they allowed simple oscillators to be built using just one active device.Later, as integrated circuits became more popular, oscillators such as the 555 timer IC became

more commonly used.

In addition to its use as the active device in relaxation oscillators, one of the most importantapplications of UJTs or PUTs is to trigger thyristors (SCR, TRIAC, etc.). In fact, a DC voltagecan be used to control a UJT or PUT circuit such that the "on-period" increaseswith an increasein the DC control voltage. This application is important for large AC current control.


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DIAC Diode A.C. switch

A Diac is two terminal , three layer bi directional device which can be switchedfrom itsoff state for either polarity of applied voltage.

Construction:

The diac can be constructed in either npn or pnp form.The two leads are connected to p-regions of silicon separated by an n region. the structure of diac is similar tothat of a transistordifferences are

There is no terminal attached to the base layer

The three regions are nearly identical in size. the doping concentrations are id

entical togive the device symmetrical properties.

Operation

When a positive or negative voltage is applied across the terminals of Diac onlya smallleakage current Ibo will flow through the device as the applied voltage is increased , the leakagecurrent will continue to flow until the voltage reaches breakover voltage Vbo atthis pointavalanche breakdown of the reverse biased junction occurs and the device exhibits negative

resistance i.e current through the device increases with the decreasing values of applied voltagethe voltage across the device then drops to breakback voltage Vw


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V-I CHARECTERISTICS OF A DIAC

For applied positive voltage less than + Vbo and Negative voltage less than -Vbo, a

small leakage current flows thrugh the device. Under such conditions the diac blocks flow ofcurrent and behaves as an open circuit. the voltage +Vbo and -Vbo are the breakdown voltagesand usually have range of 30 to 50 volts.

When the positive or negative applied voltage is equal to or greater than tha breakdownvoltage Diac begins to conduct and voltage drop across it becomes a few volts conduction thencontinues until the device current drops below its holding current breakover voltage and holding

current values are identical for the forward and reverse regions of operation.

Diacs are used for triggering of triacs in adjustable phase control of a c mainspower.Applications are light dimming heat control universal motor speed control

TRIAC

Triacs are three terminal devices that are used to switch large a.c. currents with a small triggersignal. Triacs are commonly used in dimmer switches, motor speed control circuits andequipment that automatically controls mains powered equipment including remote c

ontrol. Thetriac has many advantages over a relay, which could also be used to control mains equipment;the triac is cheap, it has no moving parts making it reliable and it operates very quickly.


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The three terminals on a triac are called Main Terminal 1.

MT2

(MT1), Main Terminal 2. (MT2) and Gate. (G). To turn on

the triac there needs to be a small current IGT flowing through

the gate, this current will only flow when the voltage between

G

G and MT1 is at least VGT. The signal that turns on the triac iscalled the trigger signal. Once the triac is turned on it will stay

MT1

on even if there is no gate current until the current flowingbetween MT2 and MT1 fall below the hold current IH.

The triac is always turned fully on or fully off. When the triac is on there isvirtually no pdbetween MT2 and MT1 so the power dissipated in the triac is low so it does not get hot or wasteelectrical power. When the triac is off no current flows between MT2 and MT1 sothe powerdissipated in the triac is low so it does not get hot or waste electrical power.This means thattriacs can be small and are very efficient.

Triacs can be used in d.c. circuits in which case whenthe triac is triggered it will stay on until power isremoved from the triac. It is easy to calculate the valueof gate resistor needed to turn on a triac using the gatecharacteristics and ohms law. The maximum value ofresistor can be found from the voltage across theresistor (VS -VGT) divided by the gate current IGT. So,R = (VS -VGT)/ IGT

VS

R

0v

In a.c. circuits the triac needs to be repeatedly triggered because the triac turns off when the a.c.current goes from positive to negative or negative to positive as the current become momentarily

zero. The triac is used in mains circuits to control theamount of power by only turning the triac on for partof the wave a bit like in pulse width modulation. This

can be done by varying the value of the gate resistorso that the triac does not turn on until the a.c signalreaches a particular voltage. The problem with this


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first dimmer is that there is a very high voltage acrossthe variable resistor and it will get hot as there is a lot

of power to dissipate (P=V2/R).

To get round the problem of needing a high power components the variable resisto

r is usuallyconnected between MT2 and G so current will only flow through the resistor to trigger thetriac(fig 4), once the triac is on the voltage at MT2 falls to zero so no current flows through the


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resistor. The other problem with these circuits is that the minimum power is half the maximumthis is because the highest voltage, which will give the latest trigger point, occurs half waythrough each half wave. Using a capacitor can solve this problem, the resistor is adjusted so that

the charging time allows the trigger to happen at any point in the half cycle (fig 5).

Figure 4 Figure 5PHOTO DIODE

A photodiode is a type of photodetector capable of converting light into eithercurrent orvoltage, depending upon the mode of operation.[1] The common, traditional solarcell used togenerate electric solar power is a large area photodiode.

Photodiodes are similar to regular semiconductor diodes except that they may beeitherexposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connectionto allow light to reach the sensitive part of the device. Many diodes designed for use specificallyas a photodiode will also use a PIN junction rather than the typical PN junction.

A photodiode is a PN junction or PIN structure. When a photon of sufficient energystrikes the diode, it excites an electron, thereby creating a free electron anda (positively charged

electron hole). If the absorption occurs in the junction's depletion region, orone diffusion lengthaway from it, these carriers are swept from the junction by the built-in field of the depletionregion. Thus holes move toward the anode, and electrons toward the cathode, anda photocurrentis produced.

P-N PHOTODIODE:

The simple p-n diode is illustrated in fig (3-1). The diode junction is reverse-biased, causing

mobile holes (electrons) from the p (n) region toward the n (p) region leaves behind theimmobile negative acceptor ions (positive donor ions), which, in turn, establishan electric fielddistribution in the vicinity of the junction called depletion region, as shown in fig (3-1-b).Because there are no free charges, the resistance in this region is high, so that the voltage dropacross the diode mostly occurs across the junction. When an incident photon is absorbed in thedepletion region after passing through the p-layer, it raises an electron from the valance to theconduction band the electron is now free to move, and a hole is left in the a va

lance band. In this


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way, free charge carrier pairs, commonly called photocurrents, are created by photon absorption.These moving carriers then cause current flow through the external circuit.

fig(3-1) (a) p-n diode . (b) Electric field distribution across the diode

In order to generate the electron-hole pair, the incident photon must have energy larger thanthat of the band gap Eg between the valance and the conduction bands, that is, hw0 > Eg. Interms of the cutoff wavelength .c, we have

.c = 1.24 / Eg

With Eg measured in electron volts, the cutoff wavelength is about 1.06m for silicon and 1.6m for germanium, where their band-gap energies are 1.1 and .67 eV, respectively.

Just as inthe case of the external photoelectric effect, not all wavelengths lower than .ccan generatephotocarrier, as the absorption of photons in the p and n regions is increased at the shorterwavelengths. After photocarrier are generated in the p and n region, most of thefree carrierswill diffused randomly through the diode and recombine before reaching the depletion

junction. The quantum efficiency . for the semiconductor junction diode can thenbe defined

as the number of electron-hole pairs per incident photon.

Two main factors limit the response time of a photodiode:


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1-the transit time of the photocarriers through the depletion region

2-the diffusion time of photocarriers (generated in the depletion region) through the diffusionregion.

Carrier diffusion is inherently a slow process. In order to have a high-speed photodiode, thecarriers should be generated in the depletion region in the high field intensityarea or close to itso that the diffusion times are less than the carrier transit times. This can beaccomplished byincreasing the bias voltage, but practical constraints limits the applied bias voltage.

Photovoltaic mode

When used in zero bias or photovoltaic mode, the flow of photocurrent out of the

device isrestricted and a voltage builds up. This mode exploits the photovoltaic effect,which is the basisfor solar cells in fact, a traditional solar cell is just a large area photodiode.

Photoconductive mode

In this mode the diode is often reverse biased, dramatically reducing the response time at theexpense of increased noise. This increases the width of the depletion layer, which decreases thejunction's capacitance resulting in faster response times. The reverse bias indu

ces only a smallamount of current (known as saturation or back current) along its direction while thephotocurrent remains virtually the same. For a given spectral distribution, thephotocurrent islinearly proportional to the illuminance (and to the irradiance).[2]

Although this mode is faster, the photoconductive mode tends to exhibit more electronic

[citation needed]

noise.The leakage current of a good PIN diode is so low (< 1nA) that the JohnsonNyquist noise of the load resistance in a typical circuit often dominates.

Other modes of operation

Avalanche photodiodes have a similar structure to regular photodiodes, but theyare operatedwith much higher reverse bias. This allows each photo-generated carrier to be multiplied byavalanche breakdown, resulting in internal gain within the photodiode, which increases theeffective responsivity of the device.

Phototransistors also consist of a photodiode with internal gain. A phototransistor is in essencenothing more than a bipolar transistor that is encased in a transparent case so


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that light can reachthe base-collector junction. The electrons that are generated by photons in thebase-collectorjunction are injected into the base, and this photodiode current is amplified bythe transistor'scurrent gain (or hfe). Note that while phototransistors have a higher responsivity for light they

[citation needed]

are not able to detect low levels of light any better than photodiodes.Phototransistorsalso have significantly longer response times.

Critical performance parameters of a photodiode include:


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They are also widely used in various medical applications, such as detectors forcomputedtomography (coupled with scintillators) or instruments to analyze samples (immunoassay). Theyare also used in pulse oximeters.

PIN diodes are much faster and more sensitive than ordinary p-n junction diodes,and hence areoften used for optical communications and in lighting regulation.

P-N photodiodes are not used to measure extremely low light intensities. Instead, if highsensitivity is needed, avalanche photodiodes, intensified charge-coupled devicesorphotomultiplier tubes are used for applications such as astronomy, spectroscopy,night visionequipment and laser rangefinding.

P-N photodiodes are used in similar applications to other photodetectors, such asphotoconductors, charge-coupled devices, and photomultiplier tubes.

Photodiodes are used in consumer electronics devices such as compact disc players, smokedetectors, and the receivers for remote controls in VCRs and televisions.

In other consumer items such as camera light meters, clock radios (the ones thatdim the displaywhen it's dark) and street lights, photoconductors are often used rather than photodiodes,although in principle either could be used.

Photodiodes are often used for accurate measurement of light intensity in science and industry.They generally have a better, more linear response than photoconductors.

They are also widely used in various medical applications, such as detectors forcomputedtomography (coupled with scintillators) or instruments to analyze samples (immunoassay). Theyare also used in pulse oximeters.

PIN diodes are much faster and more sensitive than ordinary p-n junction diodes,

and hence areoften used for optical communications and in lighting regulation.

LIGHT EMITTING DIODE (LED)

A light-emitting diode (LED) is a semiconductor light source. LEDs are used asindicator lamps in many devices, and are increasingly used for lighting. Introduced as a practicalelectronic component in 1962,[2] early LEDs emitted low-intensity red light, butmodern versionsare available across the visible, ultraviolet and infrared wavelengths, with very high brightness.

When a light-emitting diode is forward biased (switched on), electrons are abletorecombine with electron holes within the device, releasing energy in the form of


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photons. Thiseffect is called electroluminescence and the color of the light (corresponding to the energy of thephoton) is determined by the energy gap of the semiconductor. An LED is often small in area(less than 1 mm2), and integrated optical components may be used to shape its radiation

pattern.[3] LEDs present many advantages over incandescent light sources including lower energyconsumption, longer lifetime, improved robustness, smaller size, faster switching, and greater


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durability and reliability. LEDs powerful enough for room lighting are relatively expensive andrequire more precise current and heat management than compact fluorescent lamp sources ofcomparable output.

Light-emitting diodes are used in applications as diverse as replacements for aviationlighting, automotive lighting (particularly brake lamps, turn signals and indicators) as well as intraffic signals. The compact size, the possibility of narrow bandwidth, switching speed, andextreme reliability of LEDs has allowed new text and video displays and sensorsto bedeveloped, while their high switching rates are also useful in advanced communicationstechnology. Infrared LEDs are also used in the remote control units of many commercial

products including televisions, DVD players, and other domestic appliances.

SYMBOL OF LED

WORKING PRINCIPLE OF LED

DIODE I-V CURVE


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Used as numerical counters for counting production items.Analog quantities can also be displayed as a number on a suitable device. (e.g.)Digitalmultimeter.

Used for solid state video displays.Used for image sensing circuits.Used for numerical display in pocket calculators.


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Electric Circuits and Electron Devices[1]

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