Laboratory for Sub-100nm DesignLaboratory for Sub-100nm DesignDepartment of Electrical and Computer EngineeringDepartment of Electrical and Computer Engineering

Novel dual-Vth independent-gateFinFET circuits

Masoud Rostami and Kartik Mohanram

Department of Electrical and Computer Engineering

Rice University, Houston, TX

Outline

Introduction and motivation Background Dual-Vth independent-gate FinFETs

Logic design Simulation results Conclusions and future directions

2

Introduction

Technology scaling Process variations Leakage power Short channel effects

Planar double-gate FETs and FinFETs Compatibility with planar CMOS Scalability Suppression of short channel effects Low parametric variations High Ion/Ioff

3

Motivation

Conventional FinFETs Tied-gate devices

Independent-gate FinFETs Removing top oxide Electrically isolated, electrostatically coupled gates Muttreja, Agarwal, and Jha, ICCD, 2007 Caciki and Roy, IEEE TED, 2007 Tawfik and Kursun, Microelectronics Journal, 2009

Motivation

Conventional FinFETs Tied-gate devices

Independent-gate FinFETs Low-power logic gates

Disabled, reverse-biased back-gate

Independent-gate logic gatesMerge parallel transistors, compact logic

Motivation

Merge series transistors? Dual-Vth independent-gate FinFETs

Device design considerations

Motivation

Merge series transistors? Dual-Vth independent-gate FinFETs

Device design considerations Independent-gate dual-Vth FinFETs

Logic design opportunitiesCompact logic gatesNew family of logic gates

Delay+power+area evaluation

Background

FinFET Cross-sectional view

Front Gate

Back Gate

Sourcen+

Channel (undoped)Drain

n+

tox

tsi

L

Underlap

Background

FinFET models University of Florida double-gate (UFDG)

Physics-based, good agreement with manufactured devicesFin height, silicon thickness, S/D doping, underlap, gate

electrode work-function

Predictive technology model (PTM)Modeled as two parallel coupled SOI devices

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Background

Finfet threshold voltage VT

Φms and Cox tuning

Independent-gate FinFETs Electrically isolated, electrostatically coupled Vth-dependence model [Colinge 2008]

Vth-front = Vth-front0 – δ (Vgbs – Vth-back) if Vgbs < Vth-back

Vth-front = Vth-front0 in all other cases

Substrate-like effect in planar CMOS

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Dual-Vth FinFETs

Φms tuning

Two additional mask steps Cox tuning through tox

Asymmetric oxides [Masahara, IEEE EDL 2007] Device design using UFDG model

VGS (V)

I DS

(A)

VDS = 1V

Dual-Vth FinFETs

Dual-Vth FinFETs

TCAD simulations (2D Sentaurus) Same device geometry

2D Sentaurus UFDG

Dual-Vth FinFETs

2D Sentaurus

UFDG

Dual-Vth FinFETs

Good Vth separation

Good noise margin (approx. 0.5Vdd)

Leakage current in high-Vth device

pA for low-Vth devices

nA for high-Vth devices in disabled-gate modeComparable to 32nm CMOS

Dual-Vth logic gates

Rules for pull-down and pull-up network: Parallel structure ↔ series structure “Weak” AND-like high-Vth transistor ↔ “strong” OR-

like low-Vth transistor

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Dual-Vth logic gates

Novel dual-Vth logic gates

Novel logic gates Independent back-gate as independent input n-input gate with n, n+1, …, 2n inputs Example f = (a + b)(c + d) n = 2, 12 unique combinations Some competitive, some not

Exponential in n Occupancy problem Series-parallel graphs Functionally equivalent,

electrically different gates

Results

Technology libraries for n = 3 Basic library

Traditional INV, NAND, NOR, AOI, OAI

Previous work library = Basic library +Compact gate with parallel transistors mergedLow power gates with disabled back-gate

Merged series = Previous work library +Dual-Vth logic gates, with series transistors merged as

appropriate

Complete library = Merged series library +Novel dual-Vth logic gates

Results

ISCAS and OpenSPARC circuits Area (no. of fins), delay, total power

Improvements: Basic, Previous work, Merged series Area savings: 27%, 23%, 12% Delay improvement: 7%, 7%, 1% Total power: 24%, 21%, 15%

Static power10-100X higher, but net contribution negligible

Dynamic powerDominatesImprovements with proposed complete library significant

Conclusions

Dual-Vth FinFET design

Gate work-function Oxide thickness UFDG-based search and 2D TCAD validation

Compact merged series-parallel logic gates Novel dual-Vth logic gates

New opportunities for FinFET-based design Leakage power control

Underlap as a design parameter

Double gate devices

Reduction in relative strength of gate

Two gates bring more electrostatic stability

Double gate devices have: Less DIBL, GIDL and leakage power Better Ion/Ioff

Better subthreshold slope Fabrication issues (alignment, etc)

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FinFET

FinFETs are folded channel MOSFETs Easy manufacturing process Narrow vertical fin(s) stick up from the surface

[1] D. Hisamoto, et al, “FinFET—A Self-Aligned Double-Gate MOSFET Scalable to 20nm”, IEEE Tran on Electron Devices

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[1]

FinFET cross view

Can you see the underlap?

Channel engineering unfeasible Different strength for each gates is possible by tuning:

Work–function Oxide thickness

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Front Gate

Back Gate

Sourcen+

Channel-Undoped Drainp+

Tox

Tsi

Length

Outline

Introduction and motivation Device characteristics Circuit innovation with FinFET and results Future directions and conclusion

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I-V curves (current vs. drain voltage)

NMOS

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PMOS

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.810

-6

10-5

10-4

10-3

Vds

Ids

Tied-Gate

Vbg=0Vbg=-.4

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 010

-5

10-4

10-3

Vds

Ids

Tied-Gate

Vbg=0Vbg=-.4

Ids

Ids

Vds Vds

On-CurrentOff-current

Backgate and discrete “width”

Disabling the backgate: An order of magnitude less on-current Less static leakage Suitable for off-critical paths

The height cannot be changed across chip Stronger devices by adding parallel fins [1] Gate sizing will be a discrete problem W = n.Hfin n=1,2,…

[1] J. P. Colinge, “FinFET and other multi-gate transistors”, chapter 1 and 7, Springer, 08

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DC properties

No dopant in channel: No random dopant fluctuations No Coulomb scattering => Higher mobility

Higher concentration of traps Higher flicker noise and noise figure

Due to 3D structure Much higher heat transmission resistance Danger of thermal runaway [1] Performance degrades less in alleviated temperatures

[1] J. H. Choi, et al, “The Effect of Process Variation on Device Temperature in FinFET circuits”, 2007

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Analog devices

The unavoidable underlap Big source and drain resistance Less gm

Less FT gm/2π(Cgs+Cgd)

Also due to new fringing capacitances

Still better gm/gds

Good for gain of amplifiers

Not a very good SNR reported in ADCs and LCOs Due to high flicker noise and charge trapping

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Sample RF circuit

Fast coupling of two independent gates can be exploited for building a compact low-power mixer A mixer for down converting the RF signal

LO=1.8 GHzRF=1.6 GHzIF=1.8-1.6 =200Mhz

Very good THD

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FFT of Mixer Output

Outline

Introduction and motivation Device characteristics Circuit innovation with FinFET and results Future directions and conclusion

31

Innovative circuit techniques

Disabling the backgate Merging parallel transistors Merging series transistors A novel class of static logic

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Disabling the backgate

Disabling backgate increases the threshold voltage Less leakage and slower Suitable for non-critical paths New gate has less Cin, too

Because the driver; sees less ‘gates’

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Merging parallel transistors

If either of the gates is active; we still have a channel Works like an OR function [1]

Suitable for non-critical paths. Less static leakage and dynamic power (due to Cin)

Higher sensitivity to parametric variation

[1] V. Trivedi, et al, “Physics-Based Compact Modeling for Nonclassical CMOS”, 2005.

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Merging series transistors

Series transistors can be merged if the transistor acts like an AND gate [1] It has low resistance; only if both of the inputs are active

Best design parameters chosen by SPICE simulation Oxide thickness and gate work-function tuning

[1] M. Chiang, “High-Density Reduced Stack Logic Circuit Techniques Using Independent-Gate Controlled Double-Gate Devices”, IEEE Tran On Electron Devices, 2006

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Why not use these ideas concurrently? 36

The main contributions

Rules for pull-down and pull-up network: Parallel structure=> series structure and vice versa. “Weak” type (AND-like) transistor => “strong” type

(OR-like) and vice versa

No substrate effect in FinFET Transistors can be stacked in pull-up or pull-down

network more easily Many more complex gates are possible!

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New gate

Pull-down Boolean equation: PD = (a+b) * (c+d)

Pull-up Boolean equation: PU = (~a*~b) +

(~c*~d) PU and PD are

complement Static logic

24 different gates realized by just four transistors Just 3 Boolean

functions with CMOS

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Results

All the gates were simulated using UFDG model The logical effort [1] model of each model was

calculated Technology libraries were constructed ISCAS85 benchmarks were mapped Addition of the new gates showed

XX improvement in area YY improvement in power

[1] R.F. Sproull and D. Harris, Logical Effort: Designing Fast CMOS Circuits, Morgan Kaufmann, 1999.

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Outline

Introduction and motivation Device characteristics Circuit innovation with FinFET and results Future directions and conclusion

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Conclusions

FinFET devices has a huge potential for replacing the planar CMOS technology

They have: Better Ion/Ioff ratios Better SCE suppression Possibility of a second independent gate

Innovative circuits were designed exploiting independent gate of FinFET

Savings in area and power consumptions observed

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Future work

Near term: Completing the Synopsys chain Using backgate for performance tuning

Offline/online Clustering?

Long term: SRAM

Issue due to discrete width Design centering

FinFET based RF circuitsAmplifiers

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