VTCMOS characteristics and its optimum conditionspredicted by a compact analytical model

Hyunsik Im, T. Inukai, H. Gomyo, T. Hiramoto, and T. Sakurai

Low Power GroupInstitute of Industrial Science, University of Tokyo

OBJECTIVE

√√√√ To develop a compact analytical model of VTCMOS

√√√√ To demonstrate the low power operation of VTCMOS

√√√√ To investigate the short channel effect on VTCMOS

√√√√ To find optimum device conditions

Contents

√√√√ Introductory part - Necessity of low power MOSFET technology- Variable Threshold-Voltage CMOS (VTCMOS)

√√√√ Previous work: Numerical simulations of VTCMOS

√√√√ Analytical model of VTCMOS

√ Short Channel Effect on VTCMOS

√ Optimum conditions of VTCMOS

√√√√ Conclusions

IntroductionCircuits’ speed and power dissipation

12

34

-0.400.40.8

00.2

0.4

0.6

0.8

1x 10- 4

Vth(V)

VDD(V)

Pow

er (W

)

A

B

12

34

-0.400.40.8

0

1

2

3

4

5x 10

-10

Del

ay (s

)

VDD(V)

AB

Vth(V)

,102dd

SVth

oddloadstaticactivetotal VIVf CPPP−−−−

++++= = = = ++++==== )( ααααthdd

dd

on

ddloadpd VV

VI

VCt−−−−

∝∝∝∝≈≈≈≈

IntroductionTradeoffs in Low Power Devices

ddleakddloadstaticactivetotalVIVf CPPP ++++====++++====

( ) 1.5thdd

dd

drive

ddloadpd VV

V

I

VCt

−−−−∝∝∝∝=

Strong demand for ultra-low power VLSI

Low Vdd for low ac power Low Vth for high speed

Vg

Log Ids

Vdd1Vth1

Ioff1

Vdd2Vth2

Ioff2

Ion

Explosion of stand-by power

Introduction Explosion of Stand-by Power

Necessity of low power VLSI technology VTCMOS

1999 2002 2005 2008 2011100f1p

10p100p

1n10n

100n1µ

10µ

(1mW )

(Tech. Node)

(50nm)(70nm)(100nm)(130nm)(180nm)

0.1V

0.2V

0.3V

0.4V

Vth = 0.4V

I IIIoff(A/ µm)

Year

Requirement

SubthresholdLeakage

Log Ids

VgVdd1Vth1 Vdd2Vth2

Variable Threshold-Voltage CMOS (VTCMOS)

What determines ∆∆∆∆Vth?

Log Ids

Vth,standby VddVth,active

Ioff, active

Ion, active

Ioff,standby

∆∆∆∆Vth

Vg

Vbs1

Vbs2Vbs1 > Vbs2

Vds

Vg

Vbs

Body effect factor γγγγ

Vbs

∆Vth

0

Vth0

Vth

∆∆∆∆Vbs

Slope

Then ∆∆∆∆Vth = γγγγ ×××× |∆|∆|∆|∆Vbs||||

γ ∝∝∝∝ CD ⁄⁄⁄⁄ COXCD: Depletion layer cap. COX: Gate oxide cap.

(irrespective of device structure)- Directly related to ∆∆∆∆Vth- Applicable to any devices(including SOI devices)

- Also related to SCE

γγγγ ranges from 0.1 to 0.2 in advanced MOSFET’s

bs

th

VV

∆∆∆∆∆∆∆∆

≡≡≡≡γγγγ

Contents

√√√√ Introductory part - Necessity of low power MOSFET technology- Variable Threshold-Voltage CMOS (VTCMOS)

√√√√ Previous work: Numerical simulations of VTCMOS

√√√√ Analytical model of VTCMOS

√ Short Channel Effect on VTCMOS

√ Optimum conditions of VTCMOS

√√√√ Conclusions

Numerical simulations of VTCMOS Koura et al, JJAP pp.2312 (2000)

0.2 0.4 0.6 0.810 -13

10 -12

10 -11

10 -10

10 -9

10 -8

10 -7

10 -6

10 -5

10 -4

Vbs=−0.3V

0V

-5V

γ =0

0.17

0.240.30.36

0.45

γ = 0.6

-0.7V

-0.8V

-1V -1.2V-1.5V -2V -3V

Ioff (

a)(A

/µm

)

Ion(a)(mA/µm)

0.0 0.2 0.4 0.60.0

0.5

1.0

1.5

2.0

2.5

4X10 -44.5X10 -45X10 -4

5.5X10 -4

6X10 -4

6.5X10-4

7X10 -4

Ion(a) =7.5X10 -4 (A/µm)

| ∆V

bs|(V

)

γ

Vo

Vg

Log Ids

10 −−−−13 A/µµµµm

Vdd =1.5V

Ion(a)

Ioff(a)

t

Na, p++

Ns, n or p-

Simulation procedure

Contents

√√√√ Introductory part - Necessity of low power MOSFET technology- Variable Threshold-Voltage CMOS (VTCMOS)

√√√√ Previous work: Numerical simulations of VTCMOS

√√√√ Analytical model of VTCMOS

√ Short Channel Effect on VTCMOS

√ Optimum conditions of VTCMOS

√√√√ Conclusions

Purpose of Analytical VTCMOS Model

√√√√ Compact and practicalonly includes γγγγ, S, and Vbs

√√√√ Applicable to any devicesincluding short channel VTCMOSs

In terms of γγγγ, S and Vbs,

Find the following relations:Log Ids Ion,active

Ioff,standby

Vth,standbyVddVth,active

Ioff,active

Vg

γγγγ Vbs

IOFF(s)

ION(a) IOFF(a)

Analytical VTCMOS Model (Ioff, active)IOFF (a) ⇔⇔⇔⇔ IOFF (s)

Using the definitions of sub-threshold slope (S) and threshold voltage (Vth):

Log Ids

Vth,standbyVddVth,active

Ioff,active

Ion,active

Ioff,standbyVg

(A)10when 7IVV Lg

Wg

dsgth−−−−

========

110 )log

( −−−−

∆∆∆∆

∆∆∆∆≅≅≅≅

g

ds

V

IS

SV

SV

SV

off

offbsthth

sIaI γ γ γ γ −−−−

====≅≅≅≅r ≡≡≡≡ 1010)()(

/

Analytical VTCMOS Model (Ion, active)ION (a) ⇔⇔⇔⇔ IOFF (s)

Using the αααα-power law model:

Ioff,standby

Log Ids

Vth,standbyVddVth,active

Ioff,active

Ion,active

Vg

Ion(a) = B (Vdd −−−−VTH)αααα

VTH = Vth,active + ΘΘΘΘ (Θ≤Θ≤Θ≤Θ≤ 5kT/e)

⇒⇒⇒⇒ [S(log10(Ioff(s)) −−−− ΩΩΩΩ/ ) −−−− γγγγ ||||Vbs||||]

γγγγ++++∝∝∝∝

++++∝∝∝∝

11

1

1

ox

dmCCB

Relation between S factor and γγγγ

If the SCE occurs

Vg

Log Ids

Ioff

Ioff

Large γγγγLarge S

Small γγγγSmall Sn+ n+

Gate

ldtox

Depletion layer

S ≅ ≅ ≅ ≅ dS/d γγγγ (1+ γγγγ)[mV/decade]

dS/d γ≥γ≥γ≥γ≥60

Long channel

New γγγγ - S relation

)3(

160

D

OX

OX

D

ox

D

lt

CC

VbsVth

CCS

≅≅≅≅∝∝∝∝∆∆∆∆∆∆∆∆

≡≡≡≡γγγγ

÷÷÷÷÷÷÷÷

++++====

Comparison of measured and modeled Ioff (s)

Ids -Vg curves as a function of Vbs

0.0 0.5 1.0 1.510-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

Vbs

+0.1V

-0.7V

Lg = 2µm Lg = 0.1µm

I ds (A

/ µm

)

0.0 0.5 1.0 1.510-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

Vg (V) Vg (V)

01

23

4

-13

-12

-11

-10

-9

-0.75-0.50

-0.250.00

0.25

Measurement Modeling

Log 10

(I off ) (

A/µ

m)

Vbs (V)Channel length (µm)

Ssar

Vbs

off

off 10)(I)(I

γγγγ

≅≅≅≅====

Verify

Relation between power and speed in VTCMOS

0.00 0.25 0.50 0.75 1.00 1.25 1.50

0.751.00

1.251.50 0.0

0.2

0.4

0.6

0.8

1.0

PT tpd

Vdd (V)

Nor

mal

ised

t pd a

nd P

T

Vbs (V)

0.10

0.20

0.300.40

0.50

0.600.70

0.80

0.500.30

0.20

0.10

0.075

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5tpd

PT=P

ac+P

off

V dd (V

)

Vbs

(V)

POWER SPEED

)( ααααthdd

dd

on

ddloadpd VV

V

I

VCt

−−−−∝∝∝∝≈≈≈≈

Trade-off in a normal MOSFET

BUT, VTCMOS Low power and high speed

102dd

SVth

oddloadstaticactivetotal VIVf CPPP−−−−

++++====++++====

Modeling of Ion(a) characteristics( Ioff (s) = 10 −−−−13 A/µµµµm and Vdd = 1.5V)

0.0 0.3 0.6 0.9 1.2 1.5

0.50

0.75

1.00

1.25

0.2

0.3

0.4

0.5

V0

Nor

mal

ised

(I on(a

))

γ value

Vbs

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.2

0.2

0.3

0.3

0.3

0.4

0.4

0.5

Enhanced Ion

(a)

V0

Vbs

(V)

γ val

ue

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

√√√√ Characteristic Vbs (=Vo) exists.√√√√ Ion(a) increases with increasing Vbs.√√√√ If Vbs <<<< Vo, smaller γγγγ should be exploited.

Interpretation of V0

- rarely dependent on γγγγ- Scalability with Vdd- Related with the SCE

-9-10

-11-12

-13

7065605550

0.9

1.0

1.1

1.2

Suppressed SCE

Worse SCE

log10

(Ioff (s)) (A/µm)

V 0 val

ue (V

)

dS/dγ value(mV/decade)

(a) = 0 = 0 = 0 = 0 d γγγγ

d Ion

Definition of Vo: Vbs where Ion(a) is given by a constant

γγγγ

Ion(a)

|∆∆∆∆Vbs| = Vo

|∆∆∆∆Vbs| <Vo

|∆∆∆∆|Vbs| > Vo

(Vdd=1.5V)

γγγγ−−−−αααα××××γγγγ++++−−−−γγγγααααγγγγ++++ΩΩΩΩ−−−−+ + + + ====

−−−−

)1(]/)1)[())((( log /1

10 SddSsIV Vo offdd

Contents

√√√√ Introductory part - Necessity of low power MOSFET technology- Variable Threshold-Voltage CMOS (VTCMOS)

√√√√ Previous work: Numerical simulations of VTCMOS

√√√√ Analytical model of VTCMOS

√ Short Channel Effect on VTCMOS

√ Optimum conditions of VTCMOS

√√√√ Conclusions

Short Channel Effect on the performance of VTCMOS

0.1 170

80

90

100

110

120

130

γ value S value γ

valu

e

S va

lue

(mV

/dec

ade)

Channel length (µm)

0.00

0.05

0.10

0.15

0.20

Short Channel Effect, γγγγ decreases and S increases.

γ

New γγγγ - S relation

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.760

70

80

90

100

110

NO SCE line

S v

alue

(mV

/dec

ade)

γ value ( at Vbs = 1V )

Long channel 0.2um 0.15um 0.12um 0.1um

(mV/decade)

115)8.0( ++++−−−−γγγγγγγγ====

ddSS

Short Channel Effect - Standby off current, Ioff(s)

-0.75 -0.50 -0.25 0.00

0.01

0.1

1

10

Mea

sure

d 1/

r

Vbs

(V)

0.1 µm 0.125 µm 0.15 µm 0.5 µm 1.0 µm

-0.75 -0.50 -0.25 0.00

0.01

0.1

1

10

0.1 µm 0.125 µm 0.15 µm 0.5 µm 1.0 µm

Vbs (V)

Mod

eled

1/

r

SCE Degradation of γγγγ ↓↓↓↓ and S ↑↑↑↑

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

∆Iofflong > ∆Ioff

short

∆Ioffshort

∆Iofflong

I ds (A

/ µ m

)

Vg(V)

L = 2 µm L = 0.1 µm

Larger 1/r (or Smaller Ioff(s))

Ssar

Vbs

off

off 10)(I)(I

γ γ γ γ

≅≅≅≅====

Short Channel Effect – Active on current, Ion(a)

Ion(a) = B (Vdd −−−−VTH)αααα

VTH = [S(log10(Ioff(s)) −−−− ΩΩΩΩ/ ) −−−− γγγγ ||||Vbs||||]

SCE dS/dγγγγ decreases

Decreasing Ion(a)Vth2

Log Ids

VgVdd

Ion(a)

Ioff(s)

Smaller S, larger γγγγ

Ion(a)

Larger S, smaller γγγγ

Vth1 Vth3

Vbs ×××× γ γ γ γ

Increasing VTH

Larger SCESmaller SCE

(mV/decade)

115)8.0( ++++−−−−γγγγγ γ γ γ = = = = d dSS

( γγγγ ↓↓↓↓, S ↑↑↑↑ )

Short Channel Effect – Ion(a) ModelingAt Ioff(s)=10−−−−13 A/µµµµm

0.0 0.3 0.6 0.9 1.2 1.5

0.4

0.6

0.8

1.0

1.2

0.20.3

0.40.5

dS/dγ = 45

dS/dγ = 70

Nor

mal

ised

I on(a

)

γ valueVbs (V)

0.800.88 0.960.72

1.00.64

1.1

0.560.72 0.80 0.880.64

0.960.56

1.00.48

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

V0

V bs (V

)

γ value

SCE 1. To make Ion(a) Smaller.

2. To shift Vo to a lower value.( γγγγ ↓↓↓↓, S ↑↑↑↑ )

Optimum condition

(1) |∆∆∆∆Vbs| should be set as large as the junction leakage permits.

(2) Appropriate selection of Vdd and Vbs must be made to satisfy both low Ioff(s) and high Ion(a).

(3) When the values of γ γ γ γ and Vth can bedesigned at a fixed |∆∆∆∆Vbs|, the optimum γγγγ depends on Vo.

−−−− γγγγ should be large (|∆∆∆∆Vbs| > Vo)−−−− γγγγ should be small (|∆∆∆∆Vbs| < Vo)

(4) To design a VTCMOS of high performance,The following parameters must be larger. (suppressed SCE).

Eoff ≡≡≡≡ ∆γ∆γ∆γ∆γ / γγγγ −−−− ∆∆∆∆S / S and Eon ≡≡≡≡ ∆γ∆γ∆γ∆γ ⋅⋅⋅⋅Vbs −−−− ∆∆∆∆S ⋅⋅⋅⋅ ( Ioff(s) −−−− ΩΩΩΩ/ )

γγγγ

Ion(a)

|∆∆∆∆Vbs| = Vo

|∆∆∆∆Vbs| < Vo

|∆∆∆∆Vbs| > Vo

Conclusions

√√√√ A very compact analytical VTCMOS model has been developed.

√√√√ VTCMOS can attain lower power than a normal CMOSat the same speed.

√√√√ Short Channel Effect degrades the VTCMOS’ performance. SCE should be suppressed.

√√√√ Optimum device conditions for VTCMOS are discussed.