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EL 124 EL 124 Electronic Devices and CircuitsElectronic Devices and Circuits

Chapter 3Chapter 3

A. Professor A. Professor SajidSajid Ansari , Ansari , P.EngP.Eng. PMP.. PMP.

Chapter 3Chapter 3SpecialSpecial--Purpose DiodesPurpose Diodes

Department of Electrical EngineeringDepartment of Electrical EngineeringUsman Institute of TechnologyUsman Institute of Technology

ObjectivesObjectivesDescribe the characteristics of a Describe the characteristics of a zenerzener diodediode and and

analyze its operationanalyze its operationExplain how a Explain how a ZenerZener is used in voltage regulation is used in voltage regulation

and limitingand limitingDescribe the Describe the Varactor diodeVaractor diode and its variable and its variable

capacitance characteristicscapacitance characteristicsDiscuss the operation and characteristics of Discuss the operation and characteristics of LEDs LEDs

andand PhotodiodesPhotodiodesDiscuss the basic characteristics of the Discuss the basic characteristics of the Current Current

Regulator diodeRegulator diode, , the PIN diodethe PIN diode, the , the StepStep--Recovery Recovery diodediode, the , the Tunnel diodeTunnel diode, and the , and the LASER diodeLASER diode..

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The basic function of The basic function of ZenerZener DiodeDiode is to maintain a specific voltage is to maintain a specific voltage across its terminals within given limits of line or load change. Typically across its terminals within given limits of line or load change. Typically it is used for providing a stable reference voltage for use in power it is used for providing a stable reference voltage for use in power supplies and other equipmentsupplies and other equipment

ZENER DIODEZENER DIODE

supplies and other equipment.supplies and other equipment.

RRLoadLoad

This particular This particular ZenerZener circuit will work to maintain 10 V across the load.circuit will work to maintain 10 V across the load.

Zener Diodes Zener Diodes –– Operating RangeOperating RangeA A ZenerZener DiodeDiode is much is much like a normal diode, the like a normal diode, the exception being is that itexception being is that itexception being is that it exception being is that it is placed in the circuit is placed in the circuit in in reverse biasreverse bias and and operates operates in reverse breakdownin reverse breakdown. . This typical characteristic This typical characteristic curve illustrates the curve illustrates the operating range for a operating range for a ZenerZener. Note that its . Note that its forward characteristics forward characteristics are just like a normal are just like a normal diode.diode.

Operating rangeOperating range

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ZenerZener Diodes Diodes –– Regulation RangesRegulation RangesThe The zenerzener diode’s diode’s breakdown breakdown characteristicscharacteristics are are determined bydetermined by the the

ZenerZener zonezone Diode zoneDiode zone

yydopingdoping process. process.

Low voltage Low voltage zenerszeners((<5V<5V), operate in the ), operate in the zenerzener breakdown rangebreakdown range. . Those designed to Those designed to operate operate >5 V>5 V operate operate

l il i l hl hmostly in mostly in avalanche avalanche breakdown rangebreakdown range. .

ZenersZeners are available with are available with voltage breakdowns of voltage breakdowns of 1.8 V to 200 V.1.8 V to 200 V.

This curve illustrates the minimum and This curve illustrates the minimum and maximum ranges of current operation that the maximum ranges of current operation that the ZenerZener can effectively maintain its voltagecan effectively maintain its voltage..

AvalancheAvalanchezonezone

5V.5V.

ZenerZener Diodes Diodes –– Breakdown CharacteristicsBreakdown Characteristics

Note very small reverse current (before “knee”).

Breakdown occurs @ knee.Breakdown Characteristics:• VZ remains near constant• VZ provides:

-Reference voltage-Voltage regulationg g

• IZ escalates rapidly• IZMAX is achieved quickly• Exceeding IZMAX is fatal

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ZenerZener Diodes Diodes –– Voltage RegulationVoltage Regulation

Regulation occurs between:

VZK - knee voltageto

VZM - Imax

IdealIdealZenerZener Diodes Diodes –– Equivalent CircuitEquivalent Circuit• Ideal Zener exhibits a

constant voltage, regardless of current draw.

• Ideal Zener exhibits no resistanceresistancecharacteristics.

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PracticalPracticalZenerZener Diodes Diodes –– Equivalent CircuitEquivalent Circuit

• Zener exhibits a near• Zener exhibits a nearconstant voltage, varied by current draw through the series resistance ZZ.

• As Iz increases, Vz also increases.

Zener Diodes Zener Diodes –– Characteristic CurveCharacteristic Curve

• ∆Vz results• ∆Vz results from ∆Iz.

• ∆Iz thru Zz produce this.produce this.

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Zener DiodesZener DiodesZenerZener diodes have given characteristics such as;diodes have given characteristics such as;•• Temperature coefficientsTemperature coefficients –– describes the % describes the % ∆∆VzVz for for ∆∆Temp (Temp (00C)C)

00 //∆∆VzVz = = VzVz x Tx T00CC xx ∆∆T T %/%/ooCC

•• Power ratingsPower ratings –– the the zenerzener incurs power dissipation based on incurs power dissipation based on IzIzand and ZzZz P = IP = I22ZZ

Power Power deratingderating factor specifies the reduced power rating for factor specifies the reduced power rating for device operating temperatures in excess of the “rated device operating temperatures in excess of the “rated maximum temperature”maximum temperature”maximum temperature”.maximum temperature”.PPD(D(deratedderated)) = P= PD(max)D(max) –– ((mWmW//00C)C)∆∆T T mWmW

The data sheet provides this information.The data sheet provides this information.

Zener Diode Zener Diode -- ApplicationsApplicationsRegulation When VRegulation When VININ changeschanges

In this simple illustration of zener regulation circuit, the zener diode In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages. will “adjust” its impedance based on varying input voltages. Zener Zener current will increase or decrease directly with voltage input changes.current will increase or decrease directly with voltage input changes.The zener current, Iz, will vary to maintain a constant Vz. The zener current, Iz, will vary to maintain a constant Vz.

Note: Note: The zener has a finite range of current operation.The zener has a finite range of current operation.

VVZener Zener remains remains constantconstant

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Zener Diode Zener Diode -- ApplicationsApplications

VVININ IIZZ & & VVoutout

VV IIZZ && VVVVININ IIZZ & & VVoutout

ZenerZener Diode Diode -- ApplicationsApplications

1N47401N4740

PPDDMAXMAX = 1W and V= 1W and VZZ = 10V= 10V

Calculate Calculate VVZZRegulateRegulate: (pg.91): (pg.91)

VV = V= V + V+ V = 55mV + 10V = 10 055V= 55mV + 10V = 10 055V

MAXMAX

IIZKZK = 0.25mA to I= 0.25mA to IZMZM = 100mA= 100mA

VVRRminmin = I= IZK ZK R= .25mA x 220 = 55mVR= .25mA x 220 = 55mV

VVRmaxRmax = = IIZMZM R= 100mA x 220 = 22V= 100mA x 220 = 22V

VVininMINMIN = V= VRR + V+ VZZ = 55mV + 10V = 10.055V.= 55mV + 10V = 10.055V.

VVinin(max)(max) = 22V + 10V = 32V= 22V + 10V = 32V

∴∴VVRegReg is ≈10.055V to 32V.is ≈10.055V to 32V.

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Example 3Example 3--66Determine the minimum and maximum load currents for which the zener diode in Figure 3-12 will maintain regulation. What is the minimum value of RL that can be used?What is the minimum value of RL that can be used?VZ = 12V, IZK = 1 mA, and IZM = 50 mA.Assume ZZ = 0 , and VZ remains a constant 12 V over the range of current values, for simplicity.

470ΩR

RL24V

Example Example 33--6 (6 (Solution)Solution)

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Example 3Example 3--77For the circuit in Figure 3-13.(a) Determine VOUT at IZK and IZM.(b) Calculate the value of R that should be used.(c) Determine the minimum value of RL that can

be used.

RL24V

R

RL24V1N4744

Example Example 33--7 (Solution7 (Solution))

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ZenerZener LimitingLimitingZenerZener diodes can used for limiting just as normal diodes. Recall diodes can used for limiting just as normal diodes. Recall in previous chapter studies about limiters. The difference to in previous chapter studies about limiters. The difference to consider for a consider for a zenerzener limiter is its limiter is its zenerzener breakdown breakdown characteristicscharacteristics (operates in reverse bias)(operates in reverse bias)characteristics characteristics (operates in reverse bias).(operates in reverse bias).

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Varactor DiodesVaractor DiodesA A varactorvaractor diodediode is best explained as a is best explained as a Variable Capacitor DiodeVariable Capacitor Diode. . It operates in reverse bias and the doping is to maximize inherent It operates in reverse bias and the doping is to maximize inherent capacitance of the depletion region. Think of the depletion region as capacitance of the depletion region. Think of the depletion region as a variable dielectric The dielectric is “adjusted” by reverse biasa variable dielectric The dielectric is “adjusted” by reverse biasa variable dielectric. The dielectric is adjusted by reverse bias a variable dielectric. The dielectric is adjusted by reverse bias voltage changes. voltage changes.

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Varactor Varactor Diode ApplicationDiode ApplicationThe Varactor diode can be useful in filter circuits as the The Varactor diode can be useful in filter circuits as the adjustable component for resonance frequency selection.adjustable component for resonance frequency selection.

VaractorVaractor

Bias adjustBias adjust

Optical DiodesOptical Diodes

1.1. Light Emitting Diodes (LEDs)Light Emitting Diodes (LEDs)2.2. PhotodiodePhotodiode

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LightLight--Emitting Diode (LED) Emitting Diode (LED) TheThe LightLight--EmittingEmitting DiodeDiode (LED)(LED) emitsemits photonsphotons asas visiblevisible lightlight.. ItsItspurposepurpose isis forfor indicationindication andand otherother displaydisplay solutionssolutions.. VariousVarious impuritiesimpuritiesareare addedadded duringduring thethe dopingdoping processprocess toto varyvary thethe colorcolor outputoutput..gg p gp g pp yy pp

LightLight--Emitting Emitting DiodesDiodesElectroluminescence: Electroluminescence: The process of The process of emitting photons from a parent material emitting photons from a parent material (substrate), is the basis for LEDs.(substrate), is the basis for LEDs.

Colors result from the choice of substrate Colors result from the choice of substrate material and the resultingmaterial and the resulting wavelength.wavelength.material and the resulting material and the resulting wavelength.wavelength.

Today's LEDs (green, red, yellow) are Today's LEDs (green, red, yellow) are based on based on indium gallium aluminum indium gallium aluminum phosphidephosphide

BlueBlue uses uses silicon carbide or gallium nitridesilicon carbide or gallium nitride

White uses SapphireWhite uses SapphireIR (infrared)IR (infrared) –– GaAsGaAs (gallium arsenide)(gallium arsenide) A tA t F d biF d biIR (infrared) IR (infrared) GaAsGaAs (gallium arsenide)(gallium arsenide)LED Biasing:LED Biasing: 1.2V to 3.2V is typical.1.2V to 3.2V is typical.

Note: Some newer LED’s run at higher Note: Some newer LED’s run at higher voltages and emit immense light energy. voltages and emit immense light energy. Applications: Applications: Traffic signalsTraffic signals

Outdoor video screensOutdoor video screensRunway markersRunway markers

A strong A strong Forward biasForward biasencourages conductionencourages conduction--band electrons in the Nband electrons in the N--material to leap the material to leap the junction and recombine junction and recombine with available holes with available holes releasing light and heat.releasing light and heat.

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LED LED –– Spectral CurvesSpectral Curves(the range of wavelengths human EYE can perceive, approximately from 390(the range of wavelengths human EYE can perceive, approximately from 390 nm to 750nm to 750 nm)nm)

Note the wavelengths of the various colors and infrared.Note the wavelengths of the various colors and infrared.

Example of LEDsExample of LEDs(Seven Segment Display )(Seven Segment Display )

The seven segment display is an example of LEDs used for The seven segment display is an example of LEDs used for display of decimal digits.display of decimal digits.

See “Light Emitting Diodes.pdf”See “Light Emitting Diodes.pdf”Acrobat Document

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PhotodiodesPhotodiodesUnlike LED’s, photodiodes receive light rather than produce light. The Unlike LED’s, photodiodes receive light rather than produce light. The photodiodephotodiode varies it’s current in response to the amount of light that varies it’s current in response to the amount of light that strikes it. It is placed in the circuit in strikes it. It is placed in the circuit in Reverse BiasReverse Bias. As with most . As with most diodes, no current flows when in reverse bias, but when light strikes diodes, no current flows when in reverse bias, but when light strikes the exposed junction through a tiny window reverse currentthe exposed junction through a tiny window reverse currentthe exposed junction through a tiny window, reverse current the exposed junction through a tiny window, reverse current increases proportional to light intensity (irradiance).increases proportional to light intensity (irradiance).

Note: Note: All All Photodiodes exhibit a Photodiodes exhibit a “reverse leakage “reverse leakage current” which current” which appears as an inverseappears as an inverseappears as an inverse appears as an inverse variable resistance. variable resistance. Irradiance causes the Irradiance causes the device to exhibit a device to exhibit a reduction in the reduction in the variable resistance variable resistance characteristic. characteristic.

Others DiodesOthers Diodes

1.1. Current Regulator DiodeCurrent Regulator Diode2.2. SchottkySchottky DiodeDiode3.3. PIN DiodePIN Diode44 SS R Di dR Di d4.4. StepStep--Recovery DiodeRecovery Diode5.5. Tunnel DiodeTunnel Diode

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Current Regulator DiodeCurrent Regulator DiodeSymbolSymbol

Current regulator diodesCurrent regulator diodes(constant current (constant current diodes) keep a constant diodes) keep a constant current value over a current value over a specified range of specified range of forward biasforward bias voltages voltages ranging from about 1.5V ranging from about 1.5V to 6Vto 6V

OperatingOperating RangeRangeto 6V. to 6V.

This device exhibits very This device exhibits very high impedances.high impedances.

SchottkySchottky DiodeDiodeTheThe SchottkySchottky diode’sdiode’s (hot(hot--carriercarrier diodes)diodes) significantsignificant characteristiccharacteristic isis itsitsfastfast switchingswitching speedspeed.. ThisThis isis usefuluseful forfor highhigh frequenciesfrequencies andand digitaldigitalapplicationsapplications..

ItIt isis notnot aa typicaltypical diodediode inin thatthat itit doesdoes notnot havehave aa pp--nn junctionjunction.. Instead,Instead,itit consistsconsists ofof aa lightlylightly--dopeddoped nn--materialmaterial andand heavilyheavily--dopeddoped (conduction(conduction--bandband electrons)electrons) metalmetal boundedbounded togethertogether.. ItsIts BarrierBarrier PotentialPotential isistypicallytypically aroundaround 00..33 VV..

TheThe SchottkySchottky DiodeDiode operatesoperates onlyonly withwith majoritymajority carrierscarriers..

Lightly dopedLightly doped Heavily doped (conductionHeavily doped (conduction--band electrons)band electrons)

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PIN DiodePIN DiodeTheThe isis alsoalso usedused inin mostlymostly microwavemicrowave frequencyfrequency applicationsapplications.. ItsIts variablevariableforwardforward seriesseries resistanceresistance characteristiccharacteristic isis usedused forfor attenuation,attenuation,modulation,modulation, andand switchingswitching.. InIn reversereverse biasbias itit exhibitsexhibits aa nearlynearly constantconstantcapacitancecapacitance..

PINPIN DiodeDiode consistconsist ofof heavilyheavily dopeddoped PP andand NN regionsregions separatedseparated byby ii regionregion..

Fixed CapacitanceFixed CapacitanceCurrentCurrent--controlled controlled ResistanceResistance

StepStep Recovery DiodeRecovery DiodeStepStep--Recovery DiodeRecovery DiodeTheThe StepStep--RecoveryRecovery DiodeDiode usesuses gradedgraded dopingdoping wherewhere thethe dopingdoping levellevel ofofsemisemi--conductiveconductive materialmaterial isis reducedreduced asas thethe pnpn--junctionjunction isis approachedapproached..

TheThe diodediode recoversrecovers veryvery quickly,quickly, makingmaking itit usefuluseful inin VeryVery HighHigh--FrequencyFrequency(VHF)(VHF) applicationsapplications..

Tunnel DiodeTunnel DiodeTheThe tunneltunnel diodediode exhibitsexhibits negativenegative resistanceresistance.. ItIt willwill actuallyactually conductconductwellwell withwith lowlow forwardforward biasbias.. WithWith furtherfurther increasesincreases inin biasbias itit reachesreachesthethe negativenegative resistanceresistance rangerange wherewhere currentcurrent willwill actuallyactually gogo downdown..ThisThis isis achievedachieved byby heavilyheavily--dopeddoped pp andand nn materialsmaterials thatthat createcreate aaveryvery thinthin depletiondepletion regionregion whichwhich permitspermits electronselectrons toto “tunnel”“tunnel” thruthruthethe barrierbarrier regionregion..

Germanium or GalliumGermanium or Gallium

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LASER DiodeLASER DiodeTheThe LASERLASER diodediode ((LLightight AAmplificationmplification byby SStimulatedtimulated EEmissionmission ofofRRadiation)adiation) producesproduces aa monochromaticmonochromatic (single(single color)color) “coherent”“coherent”lightlight.. LaserLaser diodesdiodes inin conjunctionconjunction withwith photodiodesphotodiodes areare usedused totoretrieveretrieve datadata fromfrom compactcompact discsdiscs..pp

ForwardForward biasbias thethe diodediode andand electronselectrons movemove throughthrough thethe junction,junction,recombinationrecombination occursoccurs (as(as ordinary)ordinary).. RecombinationRecombination resultsresults ininphotonphoton release,release, causingcausing aa chainchain reactionreaction ofof releasesreleases andandavalanchingavalanching photonsphotons whichwhich formform anan intenseintense laserlaser beambeam..

Sajid Ansari , P.Eng. PMP.A. Professor (UIT).A. Professor (UIT).