Download - Chp 13 Electrical Properties
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ISSUESTOADDRESS...
CHAPTER13:
ELECTRICALPROPERTIES
Howareelectricalconductanceandresistance
characterized?
Whatarethephysicalphenomenathatdistinguish
conductors,semiconductors,andinsulators?
Formetals,howisconductivityaffectedby
imperfections,T,anddeformation?
Forsemiconductors,howisconductivityaffected
byimpurities
(doping)
and
T?
ScanningelectronmicroscopeimagesofanIC:
Al
Si
(a)(d)
VIEW
OF
AN
INTEGRATED
CIRCUIT
AdotmapshowinglocationofSi(asemiconductor):
Sishowsupaslightregions.
0.5mm45m
(doped)
(b)
Alshowsupaslightregions.
Fig.(a),(b),(c)fromFig.18.0,
Callister6e.
Fig.(d)fromFig.18.25,Callister6e.(Fig.18.25iscourtesy
NickGonzales,NationalSemiconductorCorp.,WestJordan,
UT.)
(c)
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Ohm's Law:V
=
I R
voltagedrop(volts) resistance(Ohms)
current(amps)
ELECTRICALCONDUCTION
VIe-
A(crosssect.area)
L
Resistivity, andConductivity,:geometryindependentformsofOhm'sLaw(recallnormalizationwith)
V IE:electric resistivit
LA
field
intensity
(Ohmm)
J:currentdensity
I
conductivity
Resistance:
R
L
A
L
A
Silver 6.8 x 107
METALS
Soda-lime glass 10-10
CERAMICSconductors
RoomTvalues(Ohmm)1
CONDUCTIVITY: COMPARISON
-14
Copper 6.0 x 107
Iron 1.0 x 107
-4
SEMICONDUCTORS
Concrete 10-9
Aluminum oxide
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100m
Cu wire I = 2.5A- +e-
EX: CONDUCTIVITYPROBLEM
Whatistheminimumdiameter(D)ofthewiresothat
V
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Metalsaregoodconductors
sincetheirvalencebandisonly
partiallyfilled.
Metals: Thermalenergyputs
manyelectronsinto
a hi her ener state.
+-
-
CONDUCTION&ELECTRONTRANSPORT
eeeqn ne =#ofe
qe =Chargeofe
=E mobility
EnergyStates:
thecasesbelow
formetalsshow
thatnearby
energystates
Energy
filled
emptyband
s
Energy
partlyfilled
emptyband
GAPMylevel
highest
filledstate
are
access ebythermal
fluctuations.
filledband
valenceband
filledstate
filledband
va enceband
filledstates
Fermihttp://www.bayarea.net/~kins/
AboutMe/GIFs/Fermi_2.jpg
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Insulators:Higherenergystatesnot
accessible dueto a .
Semiconductors:Higherenergystates
separatedbyasmallergap.
ENERGYSTATES: INSULATORSAND
SEMICONDUCTORS
Energy
filled
emptyband
es
GAP
Energy
filled
emptyband
es
GAP?Engineered
material:Gaps
aretunable
moretocome
filledband
band
filled
sta
filledband
va enceband
filled
stat
Insulators: Aparallelplatecapacitorinvolvesaninsulator,ordielectric,between
two
metal
electrodes.
The
charge
density
buildup
at
the
capacitor
surface
is
related
to
thedielectricconstant ofthematerial
E,Electricfieldstrength(V/m)
o
D,charge
density(C/m2)
o electricpermittivityinvacuum,
constant 8.9x1012C Vm
k,dielectricconstant
(materialproperty)
ko electricpermittivity
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Imperfectionsincreaseresistivitygrainboundaries
dislocations
impurityatoms
Theseacttoscatterelectronssothatthey
takealessdirectpath.
METALS: RESISTIVITYVST,IMPURITIES
vacancies
Cu+3
.32at%
Ni
Cu+2
.16at%
Ni
eforme
dCu+
1.12a
t%Ni
3
4
5
6
esistivity,
0-8Ohm-m)
at%Ni
)](1[ rto TT temperaturecoefficient ofresistivity,o is
intrinsic(noimpurity)resistivity
T (C)-200 -100 0
1R(1
0
Cu+.
Pure
Cu
increaseswith:temperature
wt%impurity
%CWAdaptedfromFig.18.8,Callister 6e. (Fig.18.8adaptedfromJ.O.Linde,
Ann.Physik5,p.219(1932);andC.A.WertandR.M.Thomson,Physicsof
Solids,2nded.,McGrawHillBookCompany,NewYork,1970.)
Why????
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Variationinelectricalresistivitywithcompositionforvariouscopperalloyswithsmalllevelsof
elementaladditions.AlldataareatfixedT(20C).
xo 1o resistivityofpuremetal
slope
x amtofalloy(impurity)addition
DataforPureSilicon:
increaseswithT
oppositetometalsundoped e
Egap /kT
Energy
PURESEMICONDUCTORS: CONDUCTIVITYVST
Arrheniuseq.!
filled
filledvalenceband
emptyband
filledstates
GAP?
electrons
cancross
gapat
higherT
e ec r ca con uc v y,
(Ohm-m)-1
100
101
102
103
104
pure
material
Si
Ge
GaP
CdS
bandgap(eV)
1.11
0.67
2.25
2.40
AdaptedfromFig.19.15,Callister5e. (Fig.19.15adapted
fromG.L.PearsonandJ.Bardeen,Phys.Rev.75,p.865,
1949.)
SelectedvaluesfromTable
18.2,Callister6e.
50 100 100010-2
10-1(undoped)
T(K)
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CONDUCTIONINTERMSOFELECTRONANDHOLEMIGRATION
Light(shown),heat,
ElectricalConductivitygivenby:3
toexcitee from
valencebandto
conductionband
Conductioncanbeeitherbynegativecarriers,
nee peh
#electrons/m 3 electronmobility
holemobility
electrons(ntype)and/orpositivecarriers,holes(ptype).
Electronsmovetowards(+)potentialandholesmoveto()potential
kT
Eg
oe 2
Arrheniusequation;Why2? Producetwo
chargecarriers electronandhole
mxBy
Tk
E
e
g
o
Tk
E
o
g
1
2lnln
1
2
Determiningtheactivationenergyforconduction
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Intrinsicsemiconductors: #ofthermallygeneratedelectrons=
#of
holes
(broken
bonds)
INTRINSICVSEXTRINSICCONDUCTION
x r ns csem con uc ors: mpur esa e o esemiconductorthatcontributetoexcesselectronsorholes.Doping=intentionalimpurities
Siisthemainmaterialinsemiconductors Largebandgap(1.1eV);allowsSitooperateatwarmertemperatures(150oC) Canformanativeoxide,SiO2,forinsulatingbarriers(importantinfabrication)
Sicanbemadeintolarge(12inchdia.),highpurity,singlecrystalingots. Czochralski method. Wafersarecutfromtheingot
DopingofSi Sihas4outershellelectrons ntype:Phosphorous,arsenic(groupV),donateextraelectron ptype:boron(groupIII)forSi
ntype
4valenceelectronsofAsallowittobondlikeSi,butthefifthelectronisleftorbitingAs
site theenergytoreleasethefifthelectronintotheCBissmall
nee kT
EEg
o
d
e)(
Eg isbottomofconductionband
Ed isthedonorlevelArrheniusequation,again!
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ptype
Boron
has
only
3
valance
electrons.
When
it
substitutes
for
a
Si
atoms,
one
of
its
bonds
haveamissingelectron(hole)
Holetunnelsaround,andcanbelibratedbythermalvibrationofSiatoms,fromtheB
siteintotheVB.
p eh
kTE
o
a
e
ptype
ntype
?type
?type
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Arrheniusplotofelectricalconductivityforanntype
semiconductoroverawide
temperaturerange.Atlowtemperatures(high1/T),thematerialisextrinsic.Athightemperatures(low1/T),thematerialisintrinsic.Inbetweenistheexhaustion
range,inwhichallextra
electronshavebeenpromoted
why?
why?
CompoundSemiconductors
GroupIIIVandIIVcompoundsnominallyhave
theZincBlendstructureandareintrinsic
semiconductors.Can
be
doped,
like
Si,
to
change
conduction(extrinsic)
MXCompounds:GroupIII3+valence,GroupV
5+valence avg.of4+valenceperatom//
GroupII2+valence,GroupVI6+valence avg.4
valenceperatom
Applications:
Solarcells
Lightemittingdiodes(occurswith
electronholerecombination)
Higheroperationspeeds,etc.
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Allowsflowofelectronsinonedirectiononly (e.g.,useful
toconvert
alternating
current
to
direct
current).
Processing: diffusePintoonesideofaBdopedcrystal.
Results: - -
ICDEVICES:PNRECTIFYINGJUNCTION
Noappliedpotential:
nonetcurrentflow.
Forwardbias: carrier
flowthroughptypeand
ntyperegions;holesand
electronsrecombineat
++
+
+
-
-
--
-
++
++
+
---
--
p-type n-type+ -
14
.
Reversebias: carrier
flowawayfrompnjunction;
carrierconc.greatlyreduced
atjunction;littlecurrentflow.
+++
+
+
--- --
p-type n-type- +
PN
Rectifying
Junction
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LightEmittingDiode(LED)
http://electronics.howstuffworks.com/led.htm/printable
http://spie.org/Images/Graphics/Newsroom/Importe
d/0695/0695_fig4.jpg
Bandgapdetermines
light,viaDeBoglie
equation=h/(mv)
TheTransistor
InventedbyShockley,Bardeen,
andBrattainin1948.Nobelprize
in1956.
Athreeterminaldevicethatacts
likeasimpleonoffswitch.
ThebasisofIntegratedCircuits(IC)
technology
Computers,cellphones,
automotivecontrol,etc.
Ifvoltage(potential)appliedto
the ate,currentflowsbetween
thesource
and
the
drain.
On/offswitch logicswitch,
go/nogo
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Howtodoweprocessthematerials?
Features?
Controlling
oxide
thickness?
MakingSintype?MakingSiptype?
FabricatingSingleCrystal,HighPuritySi
Czochralski method Zonerefining
http://www.mindfiesta.com/images/article/Metallurgy_clip_image003_0002.gif
ls CCK /K=segregationcoefficient
C=concentrationofimpuritiesinsolidand
liquidMoresoluterejected asTdecreases
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Cleanrooms
Featuresbeingmadeare
submicron
DUST
is
your
enemy! Candestroyentire
waferofmulti le rocessors
Cleanroomclassification
http://docs.engineeringtoolbox.com/documents/933/cleanroomclass
particlesizediagramISO.png
http://zedomax.com/blog/wp
content/uploads/2009/12/silico
nwafer.jpg
http://philip.greenspun.com/images/pcd0094/microdisplaycleanroom
31.3.jpg
Turbulentflow Laminarflowhttp://en.wikipedia.org/wiki/Cleanroom
Oxidation
Oxidation =growthofanoxidelayerbythereactionofoxygenwiththesubstrate
Providesdopant maskinganddeviceisolation
ICtechnologyuses
Thermalgrownoxidization(dry)
O2richenvironment,7501100oC,
largebatchprocess(150+wafers)
Si+O2 SiO2 Wetoxidation
Si+H O SiO +2H
Highergrowthrates,lowoxidedensity(lowerdielectricproperties)
Selectiveoxidation:Useofsiliconnitridetopreventoxidationinspecificregions
http://www.semi.org/cms/groups/pu
blic/documents/web_content/~expor
t/CTR_032685~1~000086~DC_SNIPPE
T_LAYOUT/866811.jpg
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Lithography
Lithography =processbywhich
geometricpatternsaretransferredfromamask(reticle)
toasurfaceofachiptoformthe
.
Mooreslaw(1965)
Linewidth =widthofsmallest
featureobtainableonSisurface
(dependent)
Photoresist (PR)=lightsensitivepolymermaterial Patternsareimprintedusingphotoresist
Positiveresist =the
portion
of
the
photoresist that
is
exposedtolightbecomessolubletothephotoresistdeveloperandtheportionofthephotoresist thatisunexposedremainsinsolubletothephotoresistdeveloper(exposed,removed)
Negativeresist =unexposed(uncrosslinked)portionofthephotoresist isdissolvedbythephotoresistdeveloper (exposed,staysput)
Whytheyellowlight?ICFeatureMaking
(1) PRspunontowaferandprebaked(driveoffsolvents)(2) Alignmask,usingastepper,overthesubstrate(PR).Maskis
steppedoverthewaferandexposedtoUV makingseveralpatterns(devices)onasinglewafer(registration)
(3) Postbake hardenedtheresistthatisexposedtoUV.Regionnotcoveredbyresistisimplantedoretchedaway.
(4) ResistisstrippedawaybyawetstripperorO2 plasma(ashing).Lithographypatternedmanytimeswithdifferentmasks.
http://www.anl.gov/Media_Center/ArgonneNow/Fall_2006
/Center_for_Nanoscale_Materials.html
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Etching
Wet:isotropic,undercut Dry:anisotropic,directional
(Cl andFbasedgasremoval)
IonImplantation(Doping)
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pnDiodeFab Steps
p
n
Diode
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Metallization
Metallization =growing
interconnectionsto
(a)
connec ev ces
Lowelectricalresistance,
goodadhesionto
dielectricinsulators
Electromigration =rocessb whichmetal
atomsdiffusebecauseof
apotential.
Not
good
a
materialsresearcharea!
FilmDeposition
Tobuildadevice,variousthinmetalorinsulatingfilmsaregrownontopofeachother
Evaporation
Sputtering
Evaporation
MBE
Sputtering
CVD(ALD)
CVD
http://www rpl.stanford.edu/user/files/www/ald1.gif
MBEhttp://www.sandia.gov/media/NewsRel/NR2000/images/jpg/MBE.jpg
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WireBondingandPackaging
(a) (b) (c)
LargelydeterminestheoverallcostofIC
Massproduced,notindividuallypackaged
Packages:polymers,metals,ceramics
Yieldand
Reliability
Yield=ratiooffunctionalchipstototal#ofchips ,
processing
ReliabilityNodevicehasinfinitelifetime.Statisticalmethodsto
predictexpectedlifetime
Failuremechanisms
Oxidelayers(dielectricbreakdown)
Lithography(unevenfeaturedefintion)
Metallayers(poorconnections)
Lossofpackagehermeticity
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Electricalconductivity andresistivity are:
material
parameters.
geometryindependent.
Electrical resistance is:
SUMMARY
ageometryandmaterialdependentparameter.
Conductors,semiconductors, andinsulators...
differentinwhetherthereareaccessibleenergy
statesforconductanceelectrons.
Formetals,conductivityisincreasedby
reducingdeformation
reducingimperfections
decreasingtemperature.
Forpuresemiconductors,conductivityisincreasedby
increasingtemperature
doping(e.g.,addingBtoSi(ptype)orPtoSi(ntype).