8/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3 1 ELEC 5270-001/6270-001 (Fall 2006) Low-Power...
-
date post
19-Dec-2015 -
Category
Documents
-
view
219 -
download
2
Transcript of 8/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3 1 ELEC 5270-001/6270-001 (Fall 2006) Low-Power...
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 11
ELEC 5270-001/6270-001 (Fall 2006)ELEC 5270-001/6270-001 (Fall 2006)Low-Power Design of Electronic CircuitsLow-Power Design of Electronic Circuits
(ELEC 5970/6970)(ELEC 5970/6970)
Low Voltage Low Power Devices Low Voltage Low Power Devices
Vishwani D. AgrawalVishwani D. AgrawalJames J. Danaher ProfessorJames J. Danaher Professor
Department of Electrical and Computer Department of Electrical and Computer EngineeringEngineering
Auburn UniversityAuburn Universityhttp://www.eng.auburn.edu/~vagrawalhttp://www.eng.auburn.edu/~vagrawal
[email protected]@eng.auburn.edu
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 22
CapacitancesCapacitances
In Out
C1
C2
VDD
GND
CW
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 33
Miller CapacitanceMiller Capacitance
In Out
C1
C2
VDD
GND
CW
CM
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 44
Before TransitionBefore Transition
In Out
C1
C2
VDD
GND
CW
CM
0 +VDD
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 55
After TransitionAfter Transition
In Out
C1
C2
VDD
GND
CW
CM
0-VDD
Energy from supply = 2 CM VDD
2
Effective capacitance = 2 CM
from pullupdevices ofprevious gate
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 66
Capacitances in MOSFETCapacitances in MOSFET
Source Drain
Gate oxide
Gate
BulkCs Cd
Cg
CgdCgs
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 77
Bulk nMOSFETBulk nMOSFET
n+
p-type body (bulk)
n+
L
W
SiO2
Thickness = tox
Gate
SourceDrain
Polysilicon
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 88
Gate CapacitanceGate CapacitanceCg = Cox WL = C0, intrinsic cap.
Cg = Cpermicron W
εoxCpermicron = Cox L= ── L
tox
where εox = 3.9ε0 for Silicon dioxide
= 3.9×8.85×10-14 F/cm
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 99
Intrinsic CapacitancesIntrinsic Capacitances
CapacitanceCapacitanceRegion of operationRegion of operation
CutoffCutoff LinearLinear SaturatioSaturationn
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.
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 1010
Low-Power TransistorsLow-Power Transistors
Device scaling to reduce capacitance Device scaling to reduce capacitance and voltage.and voltage.
Body bias to reduce threshold Body bias to reduce threshold voltage and leakage.voltage and leakage.
Multiple threshold CMOS (MTCMOS).Multiple threshold CMOS (MTCMOS). Silicon on insulator (SOI)Silicon on insulator (SOI)
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 1111
Device ScalingDevice Scaling
Reduced dimensionsReduced dimensions Reduce supply voltageReduce supply voltage Reduce capacitancesReduce capacitances Reduce delayReduce delay Increase leakage due to reduced Increase leakage due to reduced VVDD DD / V/ Vthth
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 1212
A Simplistic ViewA Simplistic View
Assume:Assume: Dynamic power dominatesDynamic power dominates Power reduces as square of supply voltage; Power reduces as square of supply voltage;
should reduce with device scalingshould reduce with device scaling Power reduced linearly with capacitance; Power reduced linearly with capacitance;
should reduce with device scalingshould reduce with device scaling Delay is proportional to Delay is proportional to RCRC time constant; time constant; RR
is constant with scaling, is constant with scaling, RCRC should reduce should reduce
Power reduces with scalingPower reduces with scaling
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 1313
Simplistic View (Continued)Simplistic View (Continued) What if voltage is further reduced What if voltage is further reduced
below the constant electric field value?below the constant electric field value? Will power continue to reduce? Yes.Will power continue to reduce? Yes. Since RC is independent of voltage, can clock Since RC is independent of voltage, can clock
rate remain unchanged?rate 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 No, in reality. Because higher threshold voltagehigher threshold voltage
will delay the beginning of capacitor will delay the beginning of capacitor charging/discharging.charging/discharging.
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 1414
Consider Delay of InverterConsider Delay of Inverter
In Out
VDD
GND
C
R
t B t B
Charging ofC begins
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 1515
Idealized Input and OutputIdealized Input and Output t f
Vth
t B
0.5VDD
VDD
time0.69CR
INPU
TO
UTPU
T
Gate delay
t B = t f Vth /VDD
0.5VDD
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 1616
Gate DelayGate Delay
For VDD >Vth
Gate delay = (t fVth/VDD) + 0.69RC – 0.5 t
f
= t f (Vth/VDD – 0.5 ) + 0.69RC
For VDD ≤Vth
Gate delay = ∞
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 1717
Approx. Gate Delay vs. Approx. Gate Delay vs. VVDDDD
0.69RC
0.5t f
0.5t f
0 1 2 3 4 5
Gate
dela
y
VDD /Vth
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 1818
Power - Delay vs. Power - Delay vs. VVDDDD
0.69RC
0.5t f
0.5t f
0 1 2 3 4 5
Gate
dela
y
VDD /Vth
Pow
er
With leakage
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 1919
Optimum Threshold VoltageOptimum Threshold Voltage
VDD / Vth
0 1 2 3 4 5 6
Delay orEnergy-delayproduct
Delay
Energy-delay product
Vth = 0.7V
Vth = 0.3V
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 2020
Bulk nMOSFETBulk nMOSFET
n+
p-type body (bulk)
n+
L
W
SiO2
Thickness = tox
Gate
Source Drain
Polysilicon
Vgs Vgd
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 2121
Transistor in Cut-Off StateTransistor in Cut-Off State
+- Vg < 0
- - - - - - - - - - - - - - - - - -
+ + + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + +
Polysilicon gateSiO2
p-type body
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 2222
Threshold Voltage, Threshold Voltage, VVthth
+-0 < Vg < Vth
+ + + + + + + + + +
+ + + + + + + + + + + + +
+ + + + + + + + + + + + +
Depletion region
Polysilicon gateSiO2
p-type body
+-Vg > Vth
+ + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - - -Depletion region
+ + + + + + + + + + + + ++ + + + + + + + + + + + +
Polysilicon gateSiO2
p-type body
Vth is a function of:Dopant concentration,Thickness of oxide
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 2323
αα-Power Law Model-Power Law ModelVgs > Vth and Vds > Vdsat = Vgs – Vth (Saturation region):
βIds = Pc ─ (Vgs – Vth)α
2
where β = μCoxW/L, μ = mobility
For fully ON transistor, Vgs = Vds = VDD:
βIdsat = Pc ─ (VDD – Vth)α
2
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.
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 2424
αα-Power Law Model (Cont.)-Power Law Model (Cont.)
Vgs = 1.8V
Shockley
α-power law
Simulation
Vds
I ds
(μA
)
0 0.3 0.6 0.9 1.2 1.5 1.8
400
300
200
100
0
Idsat
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 2525
αα-Power Law Model (Cont.)-Power Law Model (Cont.)
0 Vgs < Vth cutoff
Ids = Idsat×Vds/Vdsat Vds < Vdsat linear
Idsat Vds > Vdsat saturation
Vdsat = Pv (Vgs – Vth)α/2
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 2626
αα-Power Law Model (Cont.)-Power Law Model (Cont.)
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
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 2727
Power and DelayPower and Delay
Power = CVDD2
CVDD 1 1Inverter delay = ──── (─── + ─── )
4 Idsatn Idsatp
KVDD= ───────
(VDD – Vth)α
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 2828
Power-Delay ProductPower-Delay Product VDD
3
Power × Delay = constant × ─────── (VDD – Vth)α
0.6V 1.8V 3.0V VDD
Power
Delay
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 2929
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
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 3030
LeakageLeakage
IG
ID
Isub
IPT
IGIDL
n+ n+
GroundVDD
R
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 3131
Leakage Current Leakage Current ComponentsComponents
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 due to
short channel and high drain-source short channel and high drain-source voltagevoltage
Gate tunneling, Gate tunneling, IIGG through thin oxidethrough thin oxide
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 3232
Subthreshold LeakageSubthreshold LeakageVgs – Vth
Isub = I0 exp( ───── ) nvth
0 0.3 0.6 0.9 1.2 1.5 1.8 V Vgs
Ids
1mA100μA10μA1μA
100nA10nA1nA
100pA10pA
Vth
Sub
thre
shol
dre
gion
Saturation region
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 3333
Normal CMOS InverterNormal CMOS Inverter
Polysilicon (input)SiO2
p+ n+ n+ p+ p+ n+
n-well p-substrate (bulk)
metal 1VDDGND output
input output
VDD
GND
o
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 3434
Leakage Reduction by Body Leakage Reduction by Body BiasBias
Polysilicon (input)SiO2
p+ n+ n+ p+ p+ n+
n-well p-substrate (bulk)
metal 1VDDGND output
input output
VBBp
VDD
GNDVBBn
VBBn VBBp
o
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 3535
Body Bias, Body Bias, VVBBnBBn
+-0 < Vg < Vth
+ + + + + + + + + +
+ + + + + + + + + + + + +
+ + + + + + + + + + + + +
Depletion region
Polysilicon gateSiO2
p-type body
+-Vg < 0
- - - - - - - - - - - - - - - - - - + + + + + + + + + + + + ++ + + + + + + + + + + + ++ + + + + + + + + + + + ++ + + + + + + + + + + + +
Polysilicon gateSiO2
p-type body
Vt is a function of:Dopant concentration,Thickness of oxide
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 3636
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 ) via tunneling through oxide.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
8/29/06 and 8/31/068/29/06 and 8/31/06 ELEC5270-001/6270-001 Lecture 3ELEC5270-001/6270-001 Lecture 3 3737
SummarySummary Device scaling down reduces supply Device scaling down reduces supply
voltagevoltage Reduced powerReduced power Increases delayIncreases delay
Optimum power-delay product by Optimum power-delay product by scaling down threshold voltagescaling down threshold voltage
Threshold voltage reduction increases Threshold voltage reduction increases subthreshold leakage powersubthreshold leakage power
Use body bias to reduce subthreshold leakageUse body bias to reduce subthreshold leakage Body bias may increase gate leakageBody bias may increase gate leakage