elec 5270/6270 fall 2007 low-power design of electronic circuits low voltage low-power devices

Copyright Agrawal, 2007Copyright Agrawal, 2007 ELEC6270 Fall 07, Lecture 5ELEC6270 Fall 07, Lecture 5 11

ELEC 5270/6270 Fall 2007ELEC 5270/6270 Fall 2007Low-Power Design of Electronic CircuitsLow-Power Design of Electronic Circuits

Low Voltage Low-Power DevicesLow Voltage Low-Power Devices

Vishwani D. AgrawalVishwani D. AgrawalJames J. Danaher ProfessorJames J. Danaher Professor

Dept. of Electrical and Computer EngineeringDept. of Electrical and Computer EngineeringAuburn University, Auburn, AL 36849Auburn University, Auburn, AL 36849

vagrawal@eng.auburn.eduhttp://www.eng.auburn.edu/~vagrawal/COURSE/E6270_Fall07/course.html

CapacitancesCapacitances

In Out

Source

Miller CapacitanceMiller Capacitance

In Out

Before TransitionBefore Transition

In = 0 Out = VDD

0 +VDD

After TransitionAfter Transition

In Out

Energy from supply = 2 CM VDD

Effective capacitance = 2 CM

from pull-updevices ofprevious gate

Capacitances in MOSFETCapacitances in MOSFET

Source Drain

Gate oxide

BulkCs Cd

CgdCgs

Bulk nMOSFETBulk nMOSFET

p-type body (bulk)

Thickness = tox

SourceDrain

Polysilicon

Gate CapacitanceGate Capacitance

Cg = Cox WL = C0 , intrinsic cap.

Cg = Cpermicron W

εoxCpermicron = Cox L= ── L

where εox = 3.9ε0 for Silicon dioxide

= 3.9×8.85×10-14 F/cm

Approx. Intrinsic CapacitancesApprox. Intrinsic Capacitances

CapacitanceCapacitanceRegion of operationRegion of operation

CutoffCutoff LinearLinear SaturationSaturation

CgbCgb CC00 00 00

CgsCgs 00 CC0 0 /2/2 2/32/3 C C00

CgdCgd 00 CC0 0 /2/2 00

Cg = Cg = Cgs+Cgd+CgbCgs+Cgd+Cgb

CC00 CC00 2/3 2/3 CC00

Weste and Harris, CMOS VLSI Design, Addison-Wesley, 2005, p. 78.

Low-Power TransistorsLow-Power Transistors

Device scaling to reduce capacitance and Device scaling to reduce capacitance and voltage.voltage.

Body bias to reduce threshold voltage and Body bias to reduce threshold voltage and leakage.leakage.

Multiple threshold CMOS (MTCMOS).Multiple threshold CMOS (MTCMOS).Silicon on insulator (SOI)Silicon on insulator (SOI)

Device ScalingDevice Scaling

Reduced dimensionsReduced dimensionsReduce supply voltageReduce supply voltageReduce capacitancesReduce capacitancesReduce delayReduce delay Increase leakage due to reduced Increase leakage due to reduced VVDD DD / V/ Vthth

A Simplistic ViewA Simplistic View

Assume:Assume:Dynamic power dominatesDynamic power dominatesPower reduces as square of supply voltage; should Power reduces as square of supply voltage; should

reduce with device scalingreduce with device scalingPower reduced linearly with capacitance; should Power reduced linearly with capacitance; should

reduce with device scalingreduce with device scalingDelay is proportional to Delay is proportional to RCRC time constant; time constant; RR is is

constant with scaling, constant with scaling, RCRC should reduce should reduce

Power reduces with scalingPower reduces with scaling

Simplistic View (Continued)Simplistic View (Continued)What if voltage is further reduced below the What if voltage is further reduced below the

constant electric field value?constant electric field value?Will power continue to reduce? Will power continue to reduce? YesYes..Since Since RCRC is independent of voltage, can clock rate is independent of voltage, can clock rate

remain unchanged?remain unchanged?

Answer to last question:Answer to last question:Yes, if threshold voltage was zero.Yes, if threshold voltage was zero.No, in reality. Because relatively higher threshold No, in reality. Because relatively higher threshold

voltage will delay the beginning of capacitor voltage will delay the beginning of capacitor charging/discharging.charging/discharging.

Consider Delay of InverterConsider Delay of Inverter

In Out

t B t B

Charging ofC begins

Idealized Input and OutputIdealized Input and Output t f

0.5VDD

time0.69CR

Gate delay

t B = t f Vth /VDD

0.5VDD

Gate DelayGate Delay

For VDD >Vth

Gate delay = (t fVth/VDD) + 0.69RC – 0.5 t

= t f (Vth/VDD – 0.5 ) + 0.69RC

For VDD ≤Vth

Gate delay = ∞

Approx. Gate Delay vs. Approx. Gate Delay vs. VVDDDD

0.69RC

0.5t f

0 1 2 3 4 5

VDD /Vth

Power - Delay vs. Power - Delay vs. VVDDDD

0.69RC

0.5t f

0 1 2 3 4 5

VDD /Vth

With leakage

~CVDD2

Optimum Threshold VoltageOptimum Threshold Voltage

VDD / Vth

0 1 2 3 4 5 6

Energy-delay product

Vth = 0.7V

Vth = 0.3V

J. M. Rabaey and M. Pedram, Low Power Design Methodologies,Boston, Springer, 1996, p. 26.

Vth can be changed by varying doping level, oxide thickness and body bias.

Low-Voltage InverterLow-Voltage Inverter Assumed always in saturation.Assumed always in saturation. Modeled as ideal current source.Modeled as ideal current source.

Vi = VDDVo

K(VDD –Vthn)

K(VDD+ Vthp)

Vi = 0 Vo

K(VDD –Vthn)

K(VDD+ Vthp)

VDDVDDVDD

Power Supply ScalingPower Supply ScalingV

0.00.0 0.2 0.4 0.6 0.8 1.0

I DD m

Vi Volts

VDD= 1V

VDD= 0.5V

Vth ≈ 0.35V

J. Segura and C. F. Hawkins, CMOS Electronics, How It Works,How It Fails, IEEE Press and Wiley-Interscience, 2004, p. 116.

Bulk nMOSFETBulk nMOSFET

p-type body (bulk)

Thickness = tox

SourceDrain

Polysilicon

Vgs Vgd

Transistor in Cut-Off StateTransistor in Cut-Off State

+- Vg < 0

- - - - - - - - - - - - - - - - - -

+ + + + + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + +

Polysilicon gateSiO2

p-type body

Threshold Voltage, Threshold Voltage, VVthth

+-0 < Vg < Vth

+ + + + + + + + + +

+ + + + + + + + + + + + +

Depletion region

p-type body

+-Vg > Vth

+ + + + + + + + + + + + +

- - - - - - - - - - - - - - - - - - -Depletion region

+ + + + + + + + + + + + ++ + + + + + + + + + + + +

p-type body

Vth is a function of:Dopant concentration,Thickness of oxide

Cutoff: Cutoff: IIdsds = 0 = 0

p-type body (bulk)

Thickness = tox

SourceDrain

Polysilicon

Vgs Vgd

Vds = 0

Linear: Linear: IIdsds = 0 = 0

p-type body (bulk)

Thickness = tox

SourceDrain

Polysilicon

Vgs Vgd

Vds = 0

+> Vth

- - - - - -

Linear: Linear: IIdsds Increases with Increases with VVdsds

p-type body (bulk)

Thickness = tox

SourceDrain

Polysilicon

Vgs Vgd

0 < Vds < Vgs-Vth

+> Vth

Vgs > Vgd > Vth

- - - - - -

Saturation: Saturation: IIdsds Independent of Independent of VVdsds

p-type body (bulk)

Thickness = tox

SourceDrain

Polysilicon

Vgs Vgd

0 < Vds > Vgs- Vth

+> Vth

Vgd < Vth

- - - - - -

αα-Power Law Model-Power Law ModelVgs > Vth and Vds > Vdsat = Vgs – Vth (Saturation region):

βIds = Pc ─ (Vgs – Vth)α

where β = μCoxW/L, μ = mobility

For fully ON transistor, Vgs = Vds = VDD:

βIdsat = Pc ─ (VDD – Vth)α

T. Sakurai and A. R. Newton, “Alpha-Power Law MOSFET Model and Its Applications to CMOS Inverter Delay and Other Formulas,”IEEE J. Solid State Circuits, vol. 25, no. 2, pp. 584-594, 1990.

αα-Power Law Model (Cont.)-Power Law Model (Cont.)

Vgs = 1.8V

Shockley

α-power law

Simulation

0 0.3 0.6 0.9 1.2 1.5 1.8

0 Vgs < Vth cutoff

Ids = Idsat × Vds/Vdsat Vds < Vdsat linear

Idsat Vds > Vdsat saturation

Vdsat = Pv (Vgs – Vth)α/2

αα = 2, for long channel devices or low = 2, for long channel devices or low VVDDDD

αα ~ ~ 1, for short channel devices1, for short channel devices

Power and DelayPower and Delay

Power = CVDD2

CVDD 1 1Inverter delay = ──── (─── + ─── )

4 Idsatn Idsatp

KVDD= ───────

(VDD – Vth)α

Power-Delay ProductPower-Delay Product

Power × Delay = constant × ─────── (VDD – Vth)α

0.6V 1.8V 3.0V VDD

Optimum Threshold VoltageOptimum Threshold Voltage

For minimum power-delay product:

3VthVDD = ───

3 – α

For long channel devices, α = 2, VDD = 3Vth

For very short channel devices, α = 1, VDD = 1.5Vth

LeakageLeakage

GroundVDD

Leakage Current ComponentsLeakage Current Components

Subthreshold conduction, Subthreshold conduction, IIsubsub

Reverse bias pn junction conduction, Reverse bias pn junction conduction, IIDD

Gate induced drain leakage, Gate induced drain leakage, IIGIDLGIDL due to due to

tunneling at the gate-drain overlaptunneling at the gate-drain overlap Drain source punchthrough, Drain source punchthrough, IIPTPT due to short due to short

channel and high drain-source voltagechannel and high drain-source voltage Gate tunneling, Gate tunneling, IIGG through thin oxidethrough thin oxide

Subthreshold LeakageSubthreshold LeakageVgs – Vth

Isub = I0 exp( ───── ), where vT = kT/q = 26 mV n vT at 300K

0 0.3 0.6 0.9 1.2 1.5 1.8 V Vgs

1mA100μA10μA1μA

100nA10nA1nA

100pA10pA

Saturation region

Normal CMOS InverterNormal CMOS Inverter

Polysilicon (input)SiO2

p+ n+ n+ p+ p+ n+

n-well p-substrate (bulk)

metal 1VDDGND output

input output

Leakage Reduction by Body BiasLeakage Reduction by Body Bias

Polysilicon (input)SiO2

p+ n+ n+ p+ p+ n+

n-well p-substrate (bulk)

metal 1VDDGND output

input output

GNDVBBn

VBBn VBBp

Body Bias, Body Bias, VVBBnBBn

+-0 < Vg < Vth

+ + + + + + + + + +

+ + + + + + + + + + + + +

Depletion region

p-type body

+-Vg < 0

- - - - - - - - - - - - - - - - - - + + + + + + + + + + + + ++ + + + + + + + + + + + ++ + + + + + + + + + + + ++ + + + + + + + + + + + +

p-type body

Vt is a function of:Dopant concentration,Thickness of oxide

Further on Body BiasFurther on Body Bias

Large body bias can increase gate Large body bias can increase gate leakage (leakage (IIGG) via tunneling through oxide.) via tunneling through oxide.

Body bias is kept less than 0.5V.Body bias is kept less than 0.5V.For For VVDDDD = 1.8V = 1.8V

VVBBnBBn = - 0.4V = - 0.4V

VVBBpBBp = 2.2V = 2.2V

SummarySummary Device scaling down reduces supply Device scaling down reduces supply

voltagevoltageReduced powerIncreases delay

Optimum power-delay product by scaling Optimum power-delay product by scaling down threshold voltagedown threshold voltage

Threshold voltage reduction increases subthreshold leakage power

Use body bias to reduce subthreshold leakage Body bias may increase gate leakage

elec 5270/6270 fall 2007 low-power design of electronic circuits low voltage low-power devices

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