Fundamentals of microelectronics

834 Pages·2008·5.27 MB·English

by  Behzad Razavi

BR Wiley/Razavi/FundamentalsofMicroelectronics [Razavi.clsv.2006] June30,2007at13:42 1(1)
1
IntroductiontoMicroelectronics
Overthepastfivedecades,microelectronicshasrevolutionizedourlives.Whilebeyondtherealm
ofpossibilityafewdecadesago,cellphones,digitalcameras,laptopcomputers,andmanyother
electronicproductshavenowbecomeanintegralpartofourdailyaffairs.
Learning microelectronicscan be fun. As we learn how each device operates, how devices
comprisecircuitsthatperforminterestingandusefulfunctions,andhowcircuitsformsophisti-
catedsystems,webegintoseethebeautyofmicroelectronicsandappreciatethereasonsforits
explosivegrowth.
Thischaptergivesanoverviewofmicroelectronicssoastoprovideacontextforthematerial
presentedinthisbook.Weintroduceexamplesofmicroelectronicsystemsandidentifyimportant
circuit“functions”thattheyemploy.Wealsoprovideareviewofbasiccircuittheorytorefresh
thereader’smemory.
1.1 Electronics versus Microelectronics
Thegeneralareaofelectronicsbeganaboutacenturyagoandprovedinstrumentalintheradio
andradarcommunicationsusedduringthetwoworldwars.Earlysystemsincorporated“vacuum
tubes,”amplifyingdevicesthatoperatedwith theflowofelectronsbetweenplatesinavacuum
chamber.However,thefinitelifetimeandthelargesizeofvacuumtubesmotivatedresearchers
toseekanelectronicdevicewithbetterproperties.
Thefirsttransistorwasinventedinthe1940sandrapidlydisplacedvacuumtubes.Itexhibited
averylong(inprinciple,infinite)lifetimeandoccupiedamuchsmallervolume(e.g.,lessthan1
inpackagedform)thanvacuumtubesdid.
3
cmBut it was not until 1960s that the field of microelectronics, i.e., the science of integrating
manytransistorsononechip,began.Early“integratedcircuits”(ICs) containedonlya handful
of devices, but advances in the technology soon made it possible to dramatically increase the
complexityof“microchips.”
Example 1.1
Today’smicroprocessorscontain about100 million transistors in a chip area of approximately
3 cm. (Thechipisafewhundredmicronsthick.)Supposeintegratedcircuitswerenot
invcemnted3andweattemptedtobuildaprocessorusing100million“discrete”transistors.Ifeach
(cid:2)
device occupies a volume of 3 mm mm mm, determine the minimum volume for the
processor.Whatotherissueswouldaris3einsuch3animplementation?
(cid:2) (cid:2)
Solution
Theminimumvolumeisgivenby27mm ,i.e.,acube1.4moneachside!Ofcourse,the
3 8
10
(cid:2) 1
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2 Chap.1 IntroductiontoMicroelectronics
wiresconnectingthetransistorswouldincreasethevolumesubstantially.
Inadditiontooccupyingalargevolume,thisdiscreteprocessorwouldbeextremelyslow;the
signalswouldneedtotravelonwiresaslongas1.4m!Furthermore,if eachdiscretetransistor
costs1centandweighs1g,eachprocessorunitwouldbepricedatonemilliondollarsandweigh
100tons!
Exercise
Howmuchpowerwouldsuchasystemconsumeifeachtransistordissipates10 W?
(cid:22)
Thisbookdealswithmostlymicroelectronicswhileprovidingsufficientfoundationforgen-
eral(perhapsdiscrete)electronicsystemsaswell.
1.2 Examples ofElectronic Systems
At this point, we introducetwo examplesof microelectronicsystems and identifysome of the
importantbuildingblocksthatweshouldstudyinbasicelectronics.
1.2.1 CellularTelephone
Cellulartelephonesweredevelopedinthe1980sandrapidlybecamepopularinthe1990s.To-
day’scellphonescontainagreatdealofsophisticatedanaloganddigitalelectronicsthatliewell
beyondthescopeofthisbook.Butourobjectivehereistoseehowtheconceptsdescribedinthis
bookproverelevanttotheoperationofacellphone.
Supposeyouarespeakingwithafriendonyourcellphone.Yourvoiceisconvertedtoanelec-
tricsignalbyamicrophoneand,aftersomeprocessing,transmittedbythe antenna.The signal
producedbyyourantennaispickedupbytheyourfriend’sreceiverand,aftersomeprocessing,
appliedtothespeaker[Fig.1.1(a)].Whatgoesonintheseblackboxes?Whyaretheyneeded?
Transmitter (TX) Receiver (RX)
Microphone Speaker
? ?
(a) (b)
Figure1.1 (a)Simplifiedviewofacellphone,(b)furthersimplificationoftransmitandreceivepaths.
LetusattempttoomittheblackboxesandconstructthesimplesystemshowninFig.1.1(b).
Howwelldoesthissystemwork?Wemaketwoobservations.First,ourvoicecontainsfrequen-
ciesfrom20Hzto20kHz(calledthe“voiceband”).Second,foranantennatooperateefficiently,
i.e.,toconvertmostoftheelectricalsignaltoelectromagneticradiation,itsdimensionmustbea
significantfraction(e.g., )ofthewavelength.Unfortunately,afrequencyrangeof20Hzto
20kHztranslatestoawa2v5e%length of mto m,requiringgiganticantennas
1 7 4
foreachcellphone.Conversely,toobta1in:5area1s0onablea1n:t5enna10length,e.g.,5cm,thewavelength
(cid:2) (cid:2)
mustbearound20cmandthefrequencyaround1.5GHz.
Recallthatthewavelengthisequaltothe(light)velocitydividedbythefrequency.
1
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Sec.1.2 ExamplesofElectronicSystems 3
Howdowe“convert”thevoicebandtoagigahertzcenterfrequency?Onepossibleapproachis
tomultiplythevoicesignal, ,byasinusoid, [Fig.1.2(a)].Sincemultiplication
inthetimedomaincorresponxd(st)toconvolutioninAthcoesf(r2e(cid:25)qufcetn)cydomain,andsincethespectrum
x(t) Acos( 2 p fCt) Output Waveform
Voice
Signal
t t t
(a)
Spectrum of Cosine Output Spectrum
X(f)
Voice
Spectrum
Hz 0 Hz f −fC 0 +fC f −fC 0 +fC f
k k
0 0
2 2
− +
(b)
Figure1.2 (a) Multiplication of avoicesignal by asinusoid, (b) equivalent operation inthe frequency
domain.
ofthesinusoidconsistsoftwoimpulsesat ,thevoicespectrumissimplyshifted(translated)
to [Fig.1.2(b)].Thus,if GHz,thfecoutputoccupiesabandwidthof40kHzcentered
(cid:6)
at1GfcHz.Thisoperationisanfecx=am1pleof“amplitudemodulation.”
(cid:6) 2
We therefore postulate that the black box in the transmitter of Fig. 1.1(a) contains a
multiplier, asdepictedinFig.1.3(a).Buttwootherissuesarise.First,thecellphonemustdeliver
3
Power
Amplifier
Acos( 2 p fCt) Oscillator
(a) (b)
Figure1.3 (a)Simpletransmitter,(b)morecompletetransmitter.
arelativelylargevoltageswing(e.g.,20 )totheantennasothattheradiatedpowercanreach
acrossdistancesofseveralkilometers,thVeprpebyrequiringa “poweramplifier”betweenthemul-
tiplierandtheantenna.Second,thesinusoid, ,mustbeproducedbyan“oscillator.”
WethusarriveatthetransmitterarchitecturesAhocwons2in(cid:25)Ffcigt.1.3(b).
Let us now turn our attention to the receive path of the cellphone, beginningwith the sim-
ple realization illustrated in Fig. 1.1(b). Unfortunately,This topology fails to operate with the
principleofmodulation:ifthesignalreceivedbytheantennaresidesaroundagigahertzcenter
frequency,theaudiospeakercannotproducemeaningfulinformation.Inotherwords,ameansof
Cellphonesinfactuseothertypesofmodulationtotranslatethevoicebandtohigherfrequencies.
2
Alsocalleda“mixer”inhigh-frequencyelectronics.
3
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4 Chap.1 IntroductiontoMicroelectronics
translatingthespectrumbacktozerocenterfrequencyisnecessary.Forexample,asdepictedin
Fig.1.4(a),multiplicationbyasinusoid, ,translatesthespectrumtoleftandrightby
Acos(2(cid:25)fct) Output Spectrum
Received Spectrum Spectrum of Cosine
−f 0 +f f −f 0 +f f −2f 0 +2f f
C C C C C C
(a)
Low−Noise
Amplifier Amplifier
Low−Pass Low−Pass
Filter Filter
oscillator oscillator
(b) (c)
Figure 1.4 (a) Translation of modulated signal to zero center frequency, (b) simple receiver, (b) more
completereceiver.
,restoringtheoriginalvoiceband.Thenewly-generatedcomponentsat canberemoved
bfcyalow-passfilter.WethusarriveatthereceivertopologyshowninFig.1.42(fbc).
(cid:6)
Ourreceiverdesignis still incomplete.Thesignalreceivedby theantennacan beaslowas
a few tens of microvolts whereas the speaker may require swings of several tens or hundreds
of millivolts. That is, the receiver must providea great deal of amplification (“gain”)between
theantennaandthespeaker.Furthermore,sincemultiplierstypicallysufferfromahigh“noise”
andhencecorruptthereceivedsignal,a“low-noiseamplifier”mustprecedethemultiplier.The
overallarchitectureisdepictedinFig.1.4(c).
Today’s cellphones are much more sophisticated than the topologies developed above. For
example,thevoicesignalinthetransmitterandthereceiverisappliedtoadigitalsignalprocessor
(DSP)toimprovethequalityandefficiencyofthecommunication.Nonetheless,ourstudyreveals
someofthefundamentalbuildingblocksof cellphones,e.g.,amplifiers,oscillators, andfilters,
with the last two also utilizing amplification. We therefore devote a great deal of effort to the
analysisanddesignofamplifiers.
Havingseen thenecessity of amplifiers,oscillators, andmultipliersin bothtransmitandre-
ceivepathsofacellphone,thereadermaywonderif“thisisoldstuff”andrathertrivialcompared
tothestateoftheart.Interestingly,thesebuildingblocksstillremainamongthemostchallenging
circuitsincommunicationsystems.Thisisbecausethedesignentailscriticaltrade-offsbetween
speed(gigahertzcenterfrequencies),noise,powerdissipation(i.e.,batterylifetime),weight,cost
(i.e., price of a cellphone), and many other parameters. In the competitiveworld of cellphone
manufacturing,a given design is never “good enough”and the engineers are forced to further
pushtheabovetrade-offsineachnewgenerationoftheproduct.
1.2.2 DigitalCamera
Another consumer productthat, by virtue of “going electronic,” has dramatically changed our
habits and routines is the digital camera. With traditional cameras, we received no immediate
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Sec.1.2 ExamplesofElectronicSystems 5
feedback on the quality of the picture that was taken, we were very careful in selecting and
shootingscenestoavoidwastingframes,weneededtocarrybulkyrollsoffilm,andwewould
obtain the final result only in printed form. With digital cameras, on the other hand, we have
resolvedtheseissuesandenjoymanyotherfeaturesthatonlyelectronicprocessingcanprovide,
e.g.,transmissionof picturesthroughcellphonesorability to retouchoralter picturesby com-
puters.Inthissection,westudytheoperationofthedigitalcamera.
The“frontend”ofthecameramustconvertlighttoelectricity,ataskperformedbyanarray
(matrix)of“pixels.” Eachpixelconsistsofanelectronicdevice(a“photodiode”thatproduces
4
a currentproportionalto the intensity of the light that it receives. As illustrated in Fig. 1.5(a),
thiscurrentflowsthroughacapacitance, ,foracertainperiodoftime,therebydevelopinga
CL
Amplifier
2
5
0
0
C
ol
u
m
n
s
s
w
o
IDiode R
Light 0
0
5
2
CL Vout
Signal
Photodiode Processing
(a) (b) (c)
Figure1.5 (a)Operationofaphotodiode, (b)arrayofpixelsinadigitalcamera,(c)onecolumnofthe
array.
proportionalvoltageacrossit.Eachpixelthusprovidesavoltageproportionaltothe“local”light
density.
Nowconsideracamerawith,say,6.25-millionpixelsarrangedina array[Fig.
1.5(b)]. How is the output voltage of each pixel sensed and processed2?5I0f0eac2h5p0i0xel contains
(cid:2)
itsownelectroniccircuitry,theoverallarrayoccupiesaverylargearea,raisingthecostandthe
power dissipation considerably. We must therefore “time-share” the signal processing circuits
amongpixels.Tothisend,wefollowthecircuitofFig.1.5(a)withasimple,compactamplifier
andaswitch(withinthepixel)[Fig.1.5(c)].Now,weconnectawiretotheoutputsofall2500
pixels in a “column,” turn on only one switch at a time, and apply the correspondingvoltage
tothe“signalprocessing”blockoutsidethecolumn.Theoverallarrayconsistsof2500ofsuch
columns,witheachcolumnemployingadedicatedsignalprocessingblock.
Example 1.2
Adigitalcameraisfocusedonachessboard.Sketchthevoltageproducedbyonecolumnasa
functionoftime.
Theterm“pixel”isanabbreviationof“picturecell.”
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6 Chap.1 IntroductiontoMicroelectronics
Solution
The pixels in each column receive light only from the white squares [Fig. 1.6(a)]. Thus, the
V
column
V
column
t
(a) (b) (c)
Figure1.6 (a)Chessboardcapturedbyadigitalcamera,(b)voltagewaveformofonecolumn.
columnvoltagealternatesbetweenamaximumfor suchpixelsandzero forthose receivingno
light.TheresultingwaveformisshowninFig.1.6(b).
Exercise
Plotthevoltageifthefirstandsecondsquaresineachrowhavethesamecolor.
Whatdoeseachsignalprocessingblockdo?Sincethevoltageproducedbyeach pixelis an
analogsignalandcanassumeallvalueswithinarange,wemustfirst“digitize”itbymeansofan
“analog-to-digitalconverter”(ADC).A6.25megapixelarraymustthusincorporate2500ADCs.
Since ADCs are relatively complex circuits, we may time-share one ADC between every two
columns(Fig.1.7),butrequiringthattheADCoperatetwiceasfast(why?).Intheextremecase,
ADC
Figure1.7 SharingoneADCbetweentwocolumnsofapixelarray.
wemayemployasingle,veryfastADCforall2500columns.Inpractice,theoptimumchoice
liesbetweenthesetwoextremes.
Once in the digital domain, the “video”signal collected by the camera can be manipulated
extensively.Forexample,to“zoomin,”thedigitalsignalprocessor(DSP)simplyconsidersonly
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Sec.1.3 BasicConcepts 7
(cid:3)
asectionofthearray,discardingtheinformationfromtheremainingpixels.Also,toreducethe
requiredmemorysize,theprocessor“compresses”thevideosignal.
Thedigitalcameraexemplifiestheextensiveuseofbothanaloganddigitalmicroelectronics.
Theanalogfunctionsincludeamplification,switchingoperations,andanalog-to-digitalconver-
sion,andthedigitalfunctionsconsistofsubsequentsignalprocessingandstorage.
1.2.3 AnalogversusDigital
AmplifiersandADCsareexamplesof“analog”functions,circuitsthatmustprocesseachpoint
onawaveform(e.g.,avoicesignal)withgreatcaretoavoideffectssuchasnoiseand“distortion.”
Bycontrast,“digital”circuitsdealwithbinarylevels(ONEsandZEROs)and,evidently,contain
no analogfunctions.Thereadermaythen say,“I havenointentionofworkingfora cellphone
orcameramanufacturerand,therefore,neednotlearnaboutanalogcircuits.”Infact,withdigital
communications, digital signal processors, and every other function becoming digital, is there
anyfutureforanalogdesign?
Well, some of the assumptions in the above statements are incorrect. First, not every func-
tioncanberealizeddigitally.ThearchitecturesofFigs.1.3and1.4mustemploylow-noiseand
poweramplifiers,oscillators,andmultipliersregardlessofwhethertheactualcommunicationis
inanalogordigitalform.Forexample,a20- Vsignal(analogordigital)receivedbytheantenna
cannotbedirectlyappliedto a digitalgate.(cid:22)Similarly,thevideosignalcollectivelycapturedby
thepixelsinadigitalcameramustbeprocessedwithlownoiseanddistortionbeforeitappears
inthedigitaldomain.
Second,digitalcircuitsrequireanalogexpertiseasthespeedincreases.Figure1.8exemplifies
thispointbyillustratingtwobinarydatawaveforms,oneat100Mb/sandanotherat1Gb/s.The
finiterisetimeandfalltimeofthelatterraisesmanyissuesintheoperationofgates,flipflops,and
otherdigitalcircuits,necessitatinggreatattentiontoeachpointonthewaveform.
10 ns
x (t)
1
t
1 ns
x (t)
2
t
Figure1.8 Datawaveformsat100Mb/sand1Gb/s.
1.3 BasicConcepts
(cid:3)
Analysisofmicroelectroniccircuitsdrawsuponmanyconceptsthataretaughtinbasiccourses
onsignalsandsystemsandcircuittheory.Thissectionprovidesabriefreviewoftheseconcepts
so as to refreshthe reader’smemoryand establish the terminologyused throughoutthis book.
The reader may first glance through this section to determine which topics need a review or
simplyreturntothismaterialasitbecomesnecessarylater.
1.3.1 AnalogandDigitalSignals
Anelectricsignalisawaveformthatcarriesinformation.Signalsthatoccurinnaturecanassume
allvaluesinagivenrange.Called“analog,”suchsignalsincludevoice,video,seismic,andmusic
Thissectionservesasareviewandcanbeskippedinclassroomteaching.
(cid:3)
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8 Chap.1 IntroductiontoMicroelectronics
waveforms.ShowninFig.1.9(a),ananalogvoltagewaveformswingsthrougha“continuum”of
V(t( V(t( + Noise
t t
(a) (b)
Figure1.9 (a)Analogsignal,(b)effectofnoiseonanalogsignal.
valuesandprovidesinformationateachinstantoftime.
While occurring all around us, analog signals are difficult to “process” due to sensitivities
to such circuit imperfections as “noise” and “distortion.” As an example, Figure 1.9(b) illus-
5
tratestheeffectofnoise.Furthermore,analogsignalsaredifficultto“store”becausetheyrequire
“analogmemories”(e.g.,capacitors).
Bycontrast,adigitalsignalassumesonlyafinitenumberofvaluesatonlycertainpointsin
time.DepictedinFig.1.10(a)isa“binary”waveform,whichremainsatonlyoneoftwolevelsfor
V(t( ONE V(t(+ Noise
ZERO
t t
T T
(a) (b)
Figure1.10 (a)Digitalsignal,(b)effectofnoiseondigitalsignal.
eachperiod, .SolongasthetwovoltagescorrespondingtoONEsandZEROsdiffersufficiently,
logicalcircuiTtssensingsuchasignalprocessitcorrectly—evenifnoiseordistortioncreatesome
corruption[Fig.1.10(b)].We thereforeconsiderdigitalsignalsmore“robust”thantheiranalog
counterparts.Thestorageofbinarysignals(inadigitalmemory)isalsomuchsimpler.
Theforegoingobservationsfavorprocessingofsignalsinthedigitaldomain,suggestingthat
inherently analog information must be converted to digital form as early as possible. Indeed,
complexmicroelectronicsystemssuch asdigitalcameras,camcorders,andcompactdisk (CD)
recordersperformsomeanalogprocessing,“analog-to-digitalconversion,”anddigitalprocessing
(Fig.1.11),withthefirsttwofunctionsplayingacriticalroleinthequalityofthesignal.
Digital
Analog Analog Analog−to−Digital Processing
Signal Processing Conversion and Storage
Figure1.11 Signalprocessinginatypicalsystem.
It is worth notingthat many digital binary signals must be viewed and processed as analog
waveforms.Consider,forexample,theinformationstoredonaharddiskinacomputer.Uponre-
trieval,the“digital”dataappearsasadistortedwaveformwithonlyafewmillivoltsofamplitude
Distortionarisesiftheoutputisnotalinearfunctionofinput.
5
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Sec.1.3 BasicConcepts 9
(cid:3)
(Fig.1.12).SuchasmallseparationbetweenONEsandZEROsprovesinadequateifthissignal
~3 mV
Hard
Disk t
Figure1.12 Signalpickedupfromaharddiskinacomputer.
is to drivea logicalgate, demandinga great dealof amplificationand other analog processing
beforethedatareachesarobustdigitalform.
1.3.2 AnalogCircuits
Today’smicroelectronicsystemsincorporatemanyanalogfunctions.Asexemplifiedbythecell-
phoneand the digitalcamera studied above,analog circuits often limit the performanceof the
overallsystem.
Themostcommonly-usedanalogfunctionisamplification.Thesignalreceivedbyacellphone
orpickedupbyamicrophoneprovestoosmalltobeprocessedfurther.Anamplifieristherefore
necessarytoraisethesignalswingtoacceptablelevels.
Theperformanceofanamplifierischaracterizedbyanumberofparameters,e.g.,gain,speed,
andpowerdissipation.Westudytheseaspectsofamplificationingreatdetaillaterinthisbook,
butitisinstructivetobrieflyreviewsomeoftheseconceptshere.
Avoltageamplifierproducesanoutputswinggreaterthantheinputswing.Thevoltagegain,
,isdefinedas
Av
(1.1)
vout
Av = :
vin
Insomecases,weprefertoexpressthegainindecibels(dB):
(1.2)
vout
Av dB =20log :
j vin
Forexample,avoltagegainof10translatesto20dB.Thegainoftypicalamplifiersfallsinthe
rangeof to .
1 5
10 10
Example 1.3
Acellphonereceivesasignallevelof20 V,butitmustdeliveraswingof50mVtothespeaker
thatreproducesthevoice.Calculatethere(cid:22)quiredvoltagegainindecibels.
Solution
Wehave
(1.3)
50mV
Av =20log
20(cid:22)V
(1.4)
68dB:
(cid:25)
Exercise
Whatistheoutputswingifthegainis50dB?