Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of...

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Semiconductor Semiconductor Technology Basics Technology Basics

Transcript of Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of...

Page 1: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Semiconductor Technology Semiconductor Technology BasicsBasics

Page 2: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Why Semiconductors?Why Semiconductors?• ConductorsConductors always have a high concentration always have a high concentration

of electrons in of electrons in conduction bandsconduction bands– states that are free to move through the materialstates that are free to move through the material

• InsulatorsInsulators always have virtually zero electrons always have virtually zero electrons in such bandsin such bands– conduction band energy is too highconduction band energy is too high– all the electrons are stuck in all the electrons are stuck in valance bandsvalance bands

• localized to particular atoms/molecules in the materiallocalized to particular atoms/molecules in the material

• SemiconductorsSemiconductors have a conduction band whose have a conduction band whose electron population is easily manipulatedelectron population is easily manipulated– Sensitive to dopants, applied potentials, temperatureSensitive to dopants, applied potentials, temperature

Page 3: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Electronic Structure of SiliconElectronic Structure of Silicon• Silicon, atomic number: 14Silicon, atomic number: 14

– s+p orbitals of shell 3 are (together) half fulls+p orbitals of shell 3 are (together) half full

– Like in Carbon (element 6), s,p orbitals can rearrange Like in Carbon (element 6), s,p orbitals can rearrange to form four spto form four sp33 hybrid orbitals w. tetrahedral hybrid orbitals w. tetrahedral symmetry:symmetry:

– Each Si can share electrons with 4 neighboring Si’s Each Si can share electrons with 4 neighboring Si’s to fill all the 3sp orbitals... Stable tetrahedral lattice, to fill all the 3sp orbitals... Stable tetrahedral lattice, like diamondlike diamond

1s 2s 3s2p 3p

Page 4: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

• At normal temperatures, At normal temperatures, – a small percentage ofa small percentage of

shell-3 electrons will be shell-3 electrons will be free of the bond orbitalsfree of the bond orbitals

• wandering thru the lattice…wandering thru the lattice…– leaving a “hole” in the lattice point they leftleaving a “hole” in the lattice point they left

• a hole acts like a positively charged particlea hole acts like a positively charged particle

• Once created, holes can “move,” too…Once created, holes can “move,” too…– by a nearby electron hopping over to fill themby a nearby electron hopping over to fill them– however, hole mobility is usually lower than that of however, hole mobility is usually lower than that of

electronselectrons

Electrons & HolesElectrons & Holes

Page 5: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Donor & Acceptor DopantsDonor & Acceptor Dopants• Boron (element 5) is one electron shy of having a Boron (element 5) is one electron shy of having a

half-empty shell 2 that would fit Si latticehalf-empty shell 2 that would fit Si lattice

– Boron atoms readily Boron atoms readily acceptaccept extra mobile electrons and extra mobile electrons and lock them in place, forming a negative Block them in place, forming a negative B-- ion ion• Reduces free-electron concentration, increases hole concentration Reduces free-electron concentration, increases hole concentration

when implanted into siliconwhen implanted into silicon

• Phosphorus (element 15) has one Phosphorus (element 15) has one too manytoo many shell-3 shell-3 electrons to fit in Si latticeelectrons to fit in Si lattice– DonatesDonates the extra electron the extra electron

readily to conduction bandreadily to conduction band• Increases free-electron conc., decreases hole conc.Increases free-electron conc., decreases hole conc.

1s 2s 3s2p 3p

1s 2s 3s2p 3p

Forms P+ ion

Page 6: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

pp-type vs. -type vs. nn-type Silicon-type Silicon• Pure silicon:Pure silicon:

– Has an equal number of positive & negative charge Has an equal number of positive & negative charge carriers (holes & electrons, resp.)carriers (holes & electrons, resp.)

• Acceptor-doped (Acceptor-doped (e.g.e.g., boron-doped) silicon:, boron-doped) silicon:– Has a charge-carrier concentration heavily dominated Has a charge-carrier concentration heavily dominated

by by ppositive charge carriers (holes, hositive charge carriers (holes, h++))• Balanced by negative, immobile ions of acceptor atomBalanced by negative, immobile ions of acceptor atom

– We call it a “We call it a “p-typep-type” semiconductor.” semiconductor.

• Donor-doped (Donor-doped (e.g.e.g., phosphorus-doped) silicon, phosphorus-doped) silicon– Has charge-carrier concentration heavily dominated by Has charge-carrier concentration heavily dominated by

nnegative charge carriers (electrons, eegative charge carriers (electrons, e--))• Balanced by positive, immobile ions of donor atomBalanced by positive, immobile ions of donor atom

– Call it “Call it “n-typen-type” semiconductor ” semiconductor

Page 7: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

pnpn junctions junctions• What happens when you put What happens when you put pp-type and -type and nn-type -type

silicon in silicon in direct direct contact with each other?contact with each other?– Near the junction, electrons from the Near the junction, electrons from the nn and holes and holes

from the from the pp diffuse into & annihilate each other! diffuse into & annihilate each other!– Forms a Forms a depletion regiondepletion region free of charge carriers free of charge carriers

B-

B-

B-

B-

B-

B-

B-

B-B-

B-

B-

B-

B-

B-

B-

B-B-

B-

B-

B-

B-

B-

P+

P+

P+P+ P+

P+

P+

P+

P+P+

P+

P+

P+

P+

P+

P+

P+

P+

P+

P+

P+

P+

P+

P+B-

B-

B-

B-

h+

h+

h+

h+h+

h+h+

h+h+

h+

h+

h+

h+

h+

h+h+

h+

h+

e-

e-

e-

e-e- e- e-

e-

e-

e-e-

e-

e-

e-

e-

e-

e-

Depletion regionp-type n-type

Page 8: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

pnpn junction electrostatics junction electrostatics

B-

B-

B-

B-

B-

B-

B-

B-B-

B-

B-

B-

B-

B-

B-

B-B-

B-

B-

B-

B-

B-

P+

P+

P+P+ P+

P+

P+

P+

P+P+

P+

P+

P+

P+

P+

P+

P+

P+

P+

P+

P+

P+

P+

P+B-

B-

B-

B-

h+

h+

h+

h+h+

h+h+

h+h+

h+

h+

h+

h+

h+

h+h+

h+

h+

e-

e-

e-

e-e- e- e-

e-

e-

e-e-

e-

e-

e-

e-

e-

e-

Depletion region

+ Charge density

Electric field

Electrostatic potentialBuilt-involtage

cf. Pierret ‘96

Page 9: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

npnnpn MOSFET ( MOSFET (nn-FET)-FET)

pn n

gateelectrode

Potential as seenby electrons

When Vbias > 0

Gate voltage > Vt

e e e e e e

e e e

e e e

e e e

e e e

e

ee

p+ p+ p+ p+

p+p+ p+p+

Vbias

Metal-Oxide-SemiconductorField-EffectTransistor

Electronpotentialenergy

(negative ofelectric

potential)

Page 10: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

CMOS InvertersCMOS Inverters

(a) CMOS inverter structure. (b) Transition curves.

Page 11: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Semiconductor Technology Semiconductor Technology ScalingScaling

Page 12: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

ITRS Feature Size Projections

0.1

1

10

100

1000

10000

100000

1000000

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050

Year of First Product Shipment

Fe

atu

re S

ize

(n

an

om

ete

rs)

uP chan L

DRAM 1/2 p

min Tox

max Tox

Atom

We are here

Bacterium

Virus

Proteinmolecule

DNA moleculethickness

Eukaryoticcell

Human hairthickness

(Sources: 1994-1999 SIA/ITRS roadmaps, 1997 lecture by Gordon Moore)

Page 13: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

(Source: ITRS 2000 Update)

Page 14: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

(source: ITRS ‘01 roadmap)

NOW

.08 m already available

Intel has verified20 nm transistors

in the lab

Page 15: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Technology Scaling: NotationTechnology Scaling: Notation• Historically, device feature length scales have Historically, device feature length scales have

decreased by ~12%/year.decreased by ~12%/year.– So:So: feature length feature length 0.88 0.88yearyear : : – 1/1/ (1/0.88) (1/0.88)yearyear 1.14 1.14 year year : :

• up 14%/yearup 14%/year

• Meanwhile, typical CPU die diameters have Meanwhile, typical CPU die diameters have increased by ~2.3%/year. (Less stable trend.)increased by ~2.3%/year. (Less stable trend.)– Diameter Diameter 1.0231.023yearyear : : – 1/Diameter 1/Diameter 0.9780.978yearyear : :

• Quantities that are constant over time are written as Quantities that are constant over time are written as 1 1 : :

Page 16: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Some 1st-order Some 1st-order Semiconductor Scaling LawsSemiconductor Scaling Laws

• Voltages Voltages VV (due to (due to e.g.e.g. punch-through ) punch-through )• Long-term: temperature Long-term: temperature TT (prevents leakage) (prevents leakage)• Resistance:Resistance:

– Fixed-shape wire: Fixed-shape wire: R R //wtwt // = = – Thin cross-chip wire: Thin cross-chip wire: RR // = =

• Capacitance:Capacitance:– Fixed-shape structure: Fixed-shape structure: C C ww//s s // = = – Per unit wire length: Per unit wire length: C C (constant) (constant)– Cross-chip wire: Cross-chip wire: C C – Per unit area:Per unit area: C C 1/ 1/s s

Page 17: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Why Voltage Scaling?Why Voltage Scaling?• For many years, logic voltages were maintained at For many years, logic voltages were maintained at

standard levels as transistors shrunkstandard levels as transistors shrunk– TTL 5V logic - standard for many yearsTTL 5V logic - standard for many years– later 3.3 V, now: ~1V within leading-edge CPUslater 3.3 V, now: ~1V within leading-edge CPUs

• No longer possible, due to various effects:No longer possible, due to various effects:– Punch-throughPunch-through– Device degradation from hot carriersDevice degradation from hot carriers– Gate-insulator failureGate-insulator failure– Carrier velocity saturationCarrier velocity saturation

• Things break down at high field strengthsThings break down at high field strengths– constant-field scaling may be preferredconstant-field scaling may be preferred

Page 18: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Punch-ThroughPunch-Through

Moderate bias

e e e e e e

e e e

e e e

p+ p+ p+ p+

pn n

gateelectrode

Vbias

e e e

e e e

Strong bias

e e e

e e e

e

e

Very strong bias

Zero bias

Page 19: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Need for Voltage ScalingNeed for Voltage Scaling

pn n

gateelectrode

Vbias

e e e e e e

e e e

e e e

p+ p+ p+ p+

e e e

e e e

e e e

e e e

e

e

pn n

eee eee

eee

eee

p+p+p+p+

eee

eee e

e

e

Smaller size & same voltage higher electric field strengths

easier punch-through

Page 20: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Long-term Temperature Scaling?Long-term Temperature Scaling?• May be needed in the long term.May be needed in the long term.• Standby power dissipation across off transistors is based Standby power dissipation across off transistors is based

on the on the leakage current density leakage current density exp( exp(−−VVt t / / TT))

– VVtt is the is the threshold voltagethreshold voltage• Must scale down with Must scale down with VVdddd, or else transistor can’t turn on!, or else transistor can’t turn on!

TT is the is the thermal voltagethermal voltage at temperature at temperature TT• Equal to Equal to kkBBT/qT/q, where , where qq is electron charge magnitude is electron charge magnitude• Voltage spread of individual electrons fr. thermal noiseVoltage spread of individual electrons fr. thermal noise

• As voltages decrease,As voltages decrease,– leakage power will dominateleakage power will dominate– devices will become unable to store chargedevices will become unable to store charge

• Unless (eventually), Unless (eventually), TT V V • Only alternative: Scaling halts!Only alternative: Scaling halts!

– Probably what must happen, because low temps.Probably what must happen, because low temps.imply slow rate of quantum evolution.imply slow rate of quantum evolution.

Unfortunately,lower T fewercharge carriers!

Page 21: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Resistance ScalingResistance Scaling• Fixed-shape wire (any shape):Fixed-shape wire (any shape):

R R //wtwt // = = – All dimensions scalingAll dimensions scaling

equally.equally.– E.g.E.g. a local interconnect a local interconnect

in a small scaled logicin a small scaled logicblock / functional unitblock / functional unit

• Constant-length thin wire: Constant-length thin wire: RR // = = • Thin cross-chip wire: Thin cross-chip wire: RR // = = !!

– Up 33%/year!Up 33%/year!– Long-distance wires have to be extra thick to be fastLong-distance wires have to be extra thick to be fast

• But, fewer thick wires can fit!But, fewer thick wires can fit!

Current flow

w

t

Page 22: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

• Fixed-shape structure (any):Fixed-shape structure (any):C C ww//s s // = =

– E.g. scaled devices/wiresE.g. scaled devices/wires

• Per unit wire length:Per unit wire length:– C C ww//s s // (constant) (constant)

• Cross-chip thin wire: Cross-chip thin wire: C C • Per unit area: Per unit area: C C //s s

– E.g.E.g., total on-chip cap./cm, total on-chip cap./cm22

Capacitance ScalingCapacitance Scaling

w

s

Page 23: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Delay ScalingDelay Scaling• Charging time delay Charging time delay t t RCRC : :

– Through fixed shape conductor: Through fixed shape conductor: RC RC = = – Thin constant-length wire: Thin constant-length wire: RCRC – Via cross-die thin wire: Via cross-die thin wire: RC RC ·· = up 36%/yr! = up 36%/yr!– Through a transistor: Through a transistor: RC RC ·· = =

• Implications:Implications:– Transistors increasingly faster than long thin wires.Transistors increasingly faster than long thin wires.– Even becoming faster than fixed-shape wires!Even becoming faster than fixed-shape wires!– LocalLocal communication among chip elements is communication among chip elements is

becoming increasingly favored!becoming increasingly favored!

Page 24: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Performance scalingPerformance scaling• Performance characteristics:Performance characteristics:

– Clock frequency for small, transistor-delay-dominated Clock frequency for small, transistor-delay-dominated local structures: local structures: ff 1/ 1/t t (up 14%/yr) (up 14%/yr)

– Transistor density (per area): Transistor density (per area): dd = 1/ = 1/ = = – Perf. density Perf. density RRA A = = fd fd = = ;; chip area: chip area: AA – Total raw performance (local transitions / chip / time): Total raw performance (local transitions / chip / time): RR = =

fd A = fd A = = 1.55 = 1.55yearyear • Up 55%/year!Up 55%/year!• Nearly doubles every 18 months (Moore’s Law).Nearly doubles every 18 months (Moore’s Law).• Raw perf. has so far been harnessed for performance Raw perf. has so far been harnessed for performance

improvements in serial microprocessor performance.improvements in serial microprocessor performance.• Future architectures may need to move to more parallel Future architectures may need to move to more parallel

programming models to fully use further improvements.programming models to fully use further improvements.

Page 25: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Charges & CurrentsCharges & Currents• Charges & fields:Charges & fields:

– Charge on a structure: Charge on a structure: Q = CV Q = CV – Surface charge density: Surface charge density: QQ//AA – Electric field strengths: Electric field strengths: E E = = VV//

• Currents:Currents:– Peak current densities: Peak current densities: J = EJ = E// – Peak current in a wire: Peak current in a wire: I I = = JA JA – Channel-crossing times: Channel-crossing times: t t = = //vv

• Due to constant Due to constant ee saturation velocity saturation velocity v v 200 kmph 200 kmph– Current in an on-transistor: Current in an on-transistor: I I = = QQ//t t // = = – Effective trans. on-resistance: Effective trans. on-resistance: R R = = VV//I I // = =

Resistivity: Constant

Page 26: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Interconnect ScalingInterconnect Scaling• Since transistor delay Since transistor delay ddtt scales as scales as ,,

• And wire delay And wire delay ddww (w. scaled cross-section size) for a wire of length (w. scaled cross-section size) for a wire of length

scales as scales as RCRC ( (//wtwt)()(ww//ss) = ) = 22//stst 22// = = 22,,

• Then to keep Then to keep ddww < < ddtt (1-cycle access) requires: (1-cycle access) requires:

22 < < 22 < < // = = < < 3/23/2

• So wire length So wire length in units of transistor length in units of transistor length tt is is

//tt < < 3/23/2// = = 1/21/2 (down 6%/year) (down 6%/year)

• So So number of devices accessible within a constant × dnumber of devices accessible within a constant × d tt in 2-D goes as in 2-D goes as

((1/21/2))22 = = , in 3-D as , in 3-D as ((1/21/2))33 = = 3/23/2..– Circuits must be increasingly Circuits must be increasingly locallocal..

Page 27: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Energy and PowerEnergy and Power• Energy:Energy:

– Energy on a structure: Energy on a structure: EE QVQV CVCV2 2 22 = = 33

– Energy per-area: Energy per-area: EEAA CVCV22//AA 33//22 = = – Energy densities: Energy densities: EE//33 33//33 (not a problem) (not a problem)

• Power levels:Power levels:– Per-area power: Per-area power: PPAA = = EEAAf f = = (not a problem) (not a problem)– Power per die: Power per die: P = PP = PAAA A (up ~5%/year) (up ~5%/year)

• Power-per-performance: Power-per-performance: PPAA/R/RAA = = // = = • But, if constant-field scaling is not used (and it has not been, very But, if constant-field scaling is not used (and it has not been, very

much), all the above scaling rates get increased by the much), all the above scaling rates get increased by the squaresquare of of the field strength (the field strength (FF) scaling rate.) scaling rate.– Since Since V V F F··, and , and EE and and PP scale with scale with VV22..

Page 28: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

3-D Scalability?3-D Scalability?• Consider stacking circuits in 3-D within a constant Consider stacking circuits in 3-D within a constant

volume.volume.• # of layers # of layers nn: : //thicknessthickness // • Total power: Total power: PPTT = = PP(flat chip)×(flat chip)×nn = = • Enclosing surface area Enclosing surface area AAEE: : • Power flux (if not recycled): Power flux (if not recycled): PPTT//AAEE = = // = =

– For this to be possible, coolant velocity, &/or thermal For this to be possible, coolant velocity, &/or thermal conductivity must also increase as conductivity must also increase as ! !

– Probably not feasible.Probably not feasible.

• Power recycling is needed to scale in 3-D!Power recycling is needed to scale in 3-D!

Page 29: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Semiconductor Technology Semiconductor Technology LimitsLimits

Page 30: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Types of LimitsTypes of Limits• Meindl ‘95 identifies several kinds of limits on Meindl ‘95 identifies several kinds of limits on

VLSI (from most to least fundamental):VLSI (from most to least fundamental):– Theoretical limits (focus on energy & delay)Theoretical limits (focus on energy & delay)

• Fundamental limits (such as we already discussed)Fundamental limits (such as we already discussed)• Material limits (dependent on materials used)Material limits (dependent on materials used)• Device limits (dependent on structure & geometry)Device limits (dependent on structure & geometry)• Circuit limits (dependent on circuit styles used)Circuit limits (dependent on circuit styles used)• System limits (dependent on architecture & packaging)System limits (dependent on architecture & packaging)

– Practical limitsPractical limits• Design limitsDesign limits• Manufacturing limitsManufacturing limits

Page 31: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Fundamental LimitsFundamental Limits• Thermodynamic limitsThermodynamic limits

– Minimum dissipation per bit erasureMinimum dissipation per bit erasure• kTkT ln 2 ln 2 limit. More stringent limits for reliability coming up. limit. More stringent limits for reliability coming up.

– Subthreshold conduction leakage currentsSubthreshold conduction leakage currents• IIonon//IIoffoff exp( exp(VVdd dd / / TT))

• Quantum mechanical limitsQuantum mechanical limits– Tunnelling leakage currents (Tunnelling leakage currents (cf.cf. Mead ’94, next slide) Mead ’94, next slide)– Energy-time uncertainty principle Energy-time uncertainty principle E E hh//tt

• Related to Margolus-Levitin bound Related to Margolus-Levitin bound ttnop nop ≥ ½≥ ½hh/(/(E−EE−E00))

• Electromagnetic limitsElectromagnetic limits– Speed-of-light lower bound on delay for an Speed-of-light lower bound on delay for an

interconnect of a given length, interconnect of a given length, tt ≥ ≥ //cc..

Page 32: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Tunneling Limit on Device SizeTunneling Limit on Device Size• This graph plots the de Broglie wavelength This graph plots the de Broglie wavelength

λλ = = hh(2(2mEmE))−1/2−1/2 of electrons of effective mass of electrons of effective mass mm having having kinetic energy equal to barrier height kinetic energy equal to barrier height EE..

• This is This is alsoalso the min. barrier the min. barrier width neededwidth neededto prevent to prevent electron electron tunneling with tunneling with probability probability greater than greater than 3.53.5×10×10−6−6..

Page 33: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Material LimitsMaterial Limits• Carrier mobility (carrier velocity/field strength)Carrier mobility (carrier velocity/field strength)

– Affects carrier velocity, on-current, transition timeAffects carrier velocity, on-current, transition time– 6x higher in GaAs than in Si, but only at low field6x higher in GaAs than in Si, but only at low field

• Carrier saturation velocity (max velocity)Carrier saturation velocity (max velocity)– Nearly equal for Si and GaAs.Nearly equal for Si and GaAs.– Velocity maxes out @ ~100 nm/psVelocity maxes out @ ~100 nm/ps– Occurs @ ~1-10 V/Occurs @ ~1-10 V/m in Si (depends on doping)m in Si (depends on doping)

• Breakdown field strength Breakdown field strength EEcc

– 33% higher in GaAs than Si33% higher in GaAs than Si

• Thermal conductivity – next slideThermal conductivity – next slide• Dielectric constants – slide afterDielectric constants – slide after

Page 34: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Thermal ConductivityThermal Conductivity• For a given device structure, For a given device structure, PP K K TT

– PP - rate of heat removal (power) - rate of heat removal (power)– KK - thermal conductivity of materials used - thermal conductivity of materials used T - T - how much hotter is device than its surroundingshow much hotter is device than its surroundings

• 3x lower for GaAs than for Si3x lower for GaAs than for Si– Implies GaAs is 3x slower when speed is limited by conductive Implies GaAs is 3x slower when speed is limited by conductive

cooling through substrate (often true)!cooling through substrate (often true)!• Highest known Highest known KK: Diamond!: Diamond!

– KK=2 mW/=2 mW/m·K, 14 times higher than Silicon!m·K, 14 times higher than Silicon!– Can be a semiconductor if Boron-doped, or an insulator if not.Can be a semiconductor if Boron-doped, or an insulator if not.

• Also has high mobility, high breakdown voltage, & good tolerance for Also has high mobility, high breakdown voltage, & good tolerance for high-temperature operation.high-temperature operation.

– NTT recently demonstrated a diamond semiconductor capable NTT recently demonstrated a diamond semiconductor capable of 81 GHz frequencies in analog applications.of 81 GHz frequencies in analog applications.

• Apollo Diamond in MA is developing a cheap manufacturing capability Apollo Diamond in MA is developing a cheap manufacturing capability for single-crystal diamond wafers using CVD.for single-crystal diamond wafers using CVD.

Page 35: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Dielectric ConstantsDielectric Constants• Dielectric constants Dielectric constants = = //00 = = CC//CC00. . SiO2SiO2 4 4

– Want Want highhigh in thin gate dielectrics, in thin gate dielectrics, • To maximize channel surface-charge density, & thus on-To maximize channel surface-charge density, & thus on-

current, for given current, for given VVG,onG,on, , • But avoid very low thickness w. high tunneling leakage.But avoid very low thickness w. high tunneling leakage.• Material must also be an insulator! (Material must also be an insulator! (SrTiSrTi = 310!) = 310!)

– Want Want lowlow for thick interconnect insulators for thick interconnect insulators• To minimize parasitic To minimize parasitic CC and delay of interconnects and delay of interconnects• Lowest Lowest possible is that of vacuum (1). Air is close. possible is that of vacuum (1). Air is close.

Page 36: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Some Device LimitsSome Device Limits• MOSFET channel lengthMOSFET channel length

– Generally, the lower, the better!Generally, the lower, the better!• Reduces load capacitance & thus load charging time.Reduces load capacitance & thus load charging time.

– But, lengths are lower-bounded by the following:But, lengths are lower-bounded by the following:• Manufacturing limits, such as lithography wavelengths.Manufacturing limits, such as lithography wavelengths.• Supply voltage lower-limits to keep a decent Supply voltage lower-limits to keep a decent IIonon//IIoffoff..• Depletion region thickness due to dopant density limits.Depletion region thickness due to dopant density limits.• Yield, in the face of threshold variation due to statistical Yield, in the face of threshold variation due to statistical

fluctuation in dopant concentrations.fluctuation in dopant concentrations.• Source-to-drain tunneling.Source-to-drain tunneling.

• Distributed Distributed RCRC network response time network response time– Limited by:Limited by:

of wires (of wires (e.g.e.g. the recent shift from Al to Cu) the recent shift from Al to Cu) of insulators (at most, 4x less than SiOof insulators (at most, 4x less than SiO22 is possible) is possible) • Widths, lengths of wires: limited by basic geometryWidths, lengths of wires: limited by basic geometry

Page 37: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Circuit LimitsCircuit Limits• Power supply voltage limits (later)Power supply voltage limits (later)• Switching energy limits (later)Switching energy limits (later)• Gate delays:Gate delays:

– Fundamentally limited by transistor characteristics, Fundamentally limited by transistor characteristics, RCRC network charging timesnetwork charging times

• each of which are limited as per previous slideeach of which are limited as per previous slide– There is a fastest possible logic gate in any given There is a fastest possible logic gate in any given

device technologydevice technology• esp. considering it has to be switched by similar gatesesp. considering it has to be switched by similar gates

– Static CMOS & its close relatives (precharged domino, Static CMOS & its close relatives (precharged domino, NORA) are probably close to the fastest-possible gates NORA) are probably close to the fastest-possible gates using CMOS transistors in a given tech. generation.using CMOS transistors in a given tech. generation.

Page 38: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

System LimitsSystem LimitsWe’ll discuss these more later in the course…We’ll discuss these more later in the course…

• Architectural limitsArchitectural limits• Power dissipationPower dissipation• Heat removal capability of packagingHeat removal capability of packaging• Cycle time requirementsCycle time requirements• Physical sizePhysical size

Page 39: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Design & Design-Verification LimitsDesign & Design-Verification Limits

• Increasing complexity (# of devices/chip) leads Increasing complexity (# of devices/chip) leads to continual new challenges in:to continual new challenges in:– Design organizationDesign organization

• modularity modularity vs.vs. efficiency efficiency– Automatic circuit synthesis & layoutAutomatic circuit synthesis & layout

• circuit optimizationcircuit optimization– Design verificationDesign verification

• layout-vs-schematiclayout-vs-schematic• logic-level simulationlogic-level simulation• analog (analog (e.g.e.g. SPICE) modeling SPICE) modeling

– Testing and design-for-testabilityTesting and design-for-testability• test coveragetest coverage

Page 40: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Manufacturing LimitsManufacturing LimitsSee the ITRS ‘01 roadmap for these.See the ITRS ‘01 roadmap for these.• Lithography resolution, toolsLithography resolution, tools• Dopant implantation techniquesDopant implantation techniques• Process changes for new device structuresProcess changes for new device structures• Assembly & packagingAssembly & packaging• Yield enhancementYield enhancement• Environmental / safety / health considerationsEnvironmental / safety / health considerations• Metrology (measurement)Metrology (measurement)• Product cost & factory costProduct cost & factory cost

“Red brick wall” could be reached as early as 2003! --ITRS ‘01

Page 41: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Possible Endpoints for ElectronicsPossible Endpoints for Electronics• Merkle’s minimal “quantum FET”Merkle’s minimal “quantum FET”• Mesoscale nanoelectronic devices based on Mesoscale nanoelectronic devices based on

metal or semiconductor “islands”metal or semiconductor “islands”– E.g.E.g. Single-electron transistors, quantum dots, Single-electron transistors, quantum dots,

resonant tunnelling transistors.resonant tunnelling transistors.

• Organic molecular electronic devicesOrganic molecular electronic devices– diodes, transistorsdiodes, transistors

• Inorganic atomic-scale devicesInorganic atomic-scale devices– 1-atom-wide chains of conductor/semiconductor 1-atom-wide chains of conductor/semiconductor

atoms precisely positioned on/in substratesatoms precisely positioned on/in substrates

• Also discuss: Superconducting devicesAlso discuss: Superconducting devices

Page 42: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Energy Limits in ElectronicsEnergy Limits in Electronics• Origin of Origin of CVCV22/2 switching energy dissipation/2 switching energy dissipation• Thermal reliability bounds on Thermal reliability bounds on CVCV22 scaling scaling

– Voltage limitsVoltage limits– Capacitance limitsCapacitance limits

• Leakage trends in MOSFETsLeakage trends in MOSFETs

Page 43: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.
Page 44: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Limit on Switching EnergyLimit on Switching Energy• Consider temporarily connecting a single Consider temporarily connecting a single

unknown bit to ground.unknown bit to ground.– Average Average dissipation is 1/4 dissipation is 1/4 CVCV22..– At least At least TT log 2 dissipation required to erase bit by log 2 dissipation required to erase bit by

Landauer’s principle.Landauer’s principle.– Therefore, Therefore, CVCV22 4 4TT log 2 = 4 log 2 = 4kkBBTT ln 2. ln 2.

0/1? 0 0 CV2/4

Entropy:log 2

Entropy:log 1 = 0

Page 45: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Reliability w. Thermal NoiseReliability w. Thermal Noise• Consider Consider NN logic nodes, 1 of which is high. logic nodes, 1 of which is high.

– Don’t know which: Entropy = log Don’t know which: Entropy = log N.N.

• Then, connect them all to ground temporarily.Then, connect them all to ground temporarily.– Want them all to be 0, with high probability.Want them all to be 0, with high probability.– Logical entropy is now 0.Logical entropy is now 0.

• Log Log NN entropy must be exported elsewhere. entropy must be exported elsewhere.• Requires Requires TT log log NN expenditure of energy. expenditure of energy.

– But, only But, only ½½CVCV22 energy was dissipated! energy was dissipated!

• So, to reliably do So, to reliably do NN arbitrary irreversible bit arbitrary irreversible bit operations requires operations requires at leastat least ½½CVCV22 TT log log N = N = kkBBTT ln ln NN energy energy per logic node.per logic node.

Page 46: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Illustration of ScenarioIllustration of Scenario0

0

1

0

0

0

N

0

0

0

0

0

0

CV2/2

0

0

0

0

0

0

Entropy:log N

Entropy:0

½CV2 T log N

Page 47: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

On/Off Ratio from State CountOn/Off Ratio from State Count• The theoretical maximum FET on/off ratio isThe theoretical maximum FET on/off ratio is

IIonon//IIoffoff ≤ ≤ RRmaxmax = exp( = exp(VV//φφTT))..• However, note that we can simplify:However, note that we can simplify:

• That is, the maximum on/off ratio equals the number That is, the maximum on/off ratio equals the number NNelecelec of distinct single-electron states between the of distinct single-electron states between the Fermi levels for on and off gate states!Fermi levels for on and off gate states!– IfIf this analysis is correct, it would mean there is a minimum this analysis is correct, it would mean there is a minimum

entropy generation for FET based switching of 1 bit!entropy generation for FET based switching of 1 bit!

eleclog

e e)/log(log)(logelog/log

//

/)//()/(/

elece

eleceelec

elecelec

elecelecelecelec

e

ee

ee

eee

NN

NN

kIkTTI

kTEqkTqEV T

V = gate voltage swing

Eelec = potential energy difference per electron

Ielec = change in gate info.content per electron

Nelec = number of single-electronstates between 1 and 0 levels

Page 48: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Thermal CapacitanceThermal Capacitance• What is the minimum entropy generation for a What is the minimum entropy generation for a

structure of given capacitance structure of given capacitance CC??– Consider minimal node voltage Consider minimal node voltage VV = (ln = (ln RR))φφTT

• Needed to get desired on/off ratio of Needed to get desired on/off ratio of RR..

• Let the Let the thermal capacitancethermal capacitance CCTT : :≡≡ qqee//TT..– At room temperature At room temperature CCTT = 6 aF = 6 aF..

• Then we can derive an expression for minimum Then we can derive an expression for minimum entropy generation for our structure:entropy generation for our structure:

S S ½½(log (log NN) ) CC//CCTT

• This implies that This implies that C C 2(ln 2(ln NN) ) CCTT at minimum at minimum VV..

Page 49: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

Voltage Bounds for ReliabilityVoltage Bounds for Reliability• Suppose we are stuck with a given Suppose we are stuck with a given CC. Then the . Then the

minimum voltage that we can tolerate isminimum voltage that we can tolerate is

– One implication: If some nodes have One implication: If some nodes have CC less than less than thermal capacitance, then voltages cannot actually thermal capacitance, then voltages cannot actually approach the thermal voltage.approach the thermal voltage.

• Other lower bounds on node voltages:Other lower bounds on node voltages:VV TT - to switch FETs strongly on & off - to switch FETs strongly on & off

VV >> >> VTVT - to avoid defects due to threshold variation - to avoid defects due to threshold variation

C

CNV T

T ln2

Page 50: Semiconductor Technology Basics. Why Semiconductors? Conductors always have a high concentration of electrons in conduction bandsConductors always have.

In Particular GenerationsIn Particular Generations• Year 2001 technology, aggressive low-power:Year 2001 technology, aggressive low-power:

– 9 knats per transistor-switching op9 knats per transistor-switching op

• Year 2012 projection:Year 2012 projection:– 2 knats2 knats– 30x what’s needed for 1e27 reliability (ln N=60)30x what’s needed for 1e27 reliability (ln N=60)

• 1e9 nodes lasting 1e9 seconds at 1e9 hertz w/o error1e9 nodes lasting 1e9 seconds at 1e9 hertz w/o error