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**Low-power FinFET Circuit Design**

Niraj K. Jha Dept. of Electrical Engineering Princeton University Joint work with: Anish Muttreja and Prateek Mishra

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**Talk Outline Background Motivation: Power Consumption**

FinFETs for Low Power Design Vth Control through Multiple Vdd’s (TCMS) Extension of TCMS to Logic Circuits Conclusions

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**Why Double-gate Transistors ?**

Feature size 32 nm 10 nm Bulk CMOS DG-FETs Gap Non-Si nano devices DG-FETs can be used to fill this gap DG-FETs are extensions of CMOS Manufacturing processes similar to CMOS Key limitations of CMOS scaling addressed through Better control of channel from transistor gates Reduced short-channel effects Better Ion/Ioff Improved sub-threshold slope No discrete dopant fluctuations

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**Different Types of DG-FETs**

Source: ( Hollis, Boston University)

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**What are FinFETs? Fin-type DG-FET**

A FinFET is like a FET, but the channel has been “turned on its edge” and made to stand up Si Fin

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FinFET 3-D Structure Earliest FinFET processes: both gates inherently connected Source: (Ananthan, 2004)

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**Independent-gate FinFETs**

Back Gate Oxide insulation Both the gates of a FET can be independently controlled Independent control Requires an extra process step Leads to a number of interesting analog and digital circuit structures

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**FinFET Width Quantization**

Electrical width of a FinFET with n fins: W = 2*n*h Channel width in a FinFET is quantized Width quantization is a design challenge if fine control of transistor drive strength is needed E.g., in ensuring stability of memory cells FinFET structure Ananthan, ISQED’05

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**Talk Outline Background Motivation: Power Consumption**

FinFETs for Low Power Design Vth Control through Multiple Vdd’s (TCMS) Extension of TCMS to Logic Circuits Conclusions

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**Motivation: Power Consumption**

Traditional view of CMOS power consumption Active mode: Dynamic power (switching + short circuit + glitching) Standby mode: Leakage power Problem: rising active leakage 40% of total active mode power consumption (70nm bulk CMOS) † †J. Kao, S. Narendra and A. Chandrakasan, “Subthreshold leakage modeling and reduction techniques,” in Proc. ICCAD, 2002.

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**Low-power Design Techniques**

Standby mode Examples: Sleep transistor insertion, clock gating, minimum leakage vector application Interfere with (disable/slow) circuit operation Do not address active mode leakage Active mode: Circuit optimization Examples: Gate sizing, Multiple Vdd/Vth Respect circuit operations and timing constraints Can be used to reduce active mode leakage What opportunities do FinFETs provide us ?

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**Talk Outline Background Motivation: Power Consumption**

FinFETs for Low Power Design Vth Control through Multiple Vdd’s (TCMS) Extension of TCMS to Logic Circuits Conclusions

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**FinFETs for Low-power Design**

FinFET device characteristics can be leveraged for low-power design Static threshold voltage control through back-gate bias Area-efficient design through merging of parallel transistors Independent control of FinFET gates also provides novel circuit design opportunities

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**Logic Styles: NAND Gates**

SG-mode NAND IG-mode NAND IG-mode pull up pull up bias voltage LP-mode NAND IG/LP-mode NAND LP-mode pull down pull down bias voltage

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**Comparing Logic Styles**

Design Mode Advantages Disadvantages SG Fastest under all load conditions High leakage† (1μA) LP Very low leakage (85nA), low switched capacitance Slowest, especially under load. Area overhead (routing) IG Low area and switched capacitance Unmatched pull-up and pull-down delays. High leakage (772nA) IG/LP Low leakage (337nA), area and switched capacitance Almost as slow as LP mode †Average leakage current for two-input NAND gate (Vdd = 1.0V)

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**FinFET Characteristics**

Simulated Id Vs. Vgs characteristics for FinFETs at varying back-gate reverse biases LP-mode leakage is 10 times lower than SG-mode LP-mode delay (∞ 1/Ion) is twice that of SG-mode IG-mode Ion is not much better than LP-mode Ioff is a strong function of back-gate reverse bias but Ion is not

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**Back-gate Bias Voltage**

Value of back-gate bias voltage affects speed and leakage Heuristic: compare LP-mode inverter delay and leakage Bias values Pull-down= -0.2 V Pull-up = Vdd + .18V (1.18V). Adjusted to match delays Delay and leakage power variation with back-gate bias voltage for LP-mode FinFET inverter

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**Technical Challenges in FinFET-based Circuit Design**

Wide variety of logic styles possible (can be used simultaneously) No comprehensive circuit-level comparisons available Circuit synthesis challenges Industry-standard standard cell-based synthesis is often suboptimal FinFET width quantization is based on solving a convex integer formulation† Complex Does not handle all logic styles †B. Swahn and S. Hassoun, ``Gate sizing: FinFETs vs 32nm bulk MOSFETs,” in Proc. DAC, 2006

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**Our Approach Construct FinFET-based Synopsys technology libraries**

Extend linear programming based cell selection† for FinFETs Use optimized netlists to compare logic styles at a range of delay constraints 32 nm PTM FinFET models Delay/power characterization in SPICE LP IG/LP IG SG Synopsys libraries inFET models 32nm PTM Logic gate designs Benchmark Minimum-delay synthesis in Design Compiler SG-mode netlist Power-optimized mixed-mode netlists SG+ IG/LP SG+IG SG+LP Linear programming based cell selection †D. Chinnery and K. Keutzer, “Linear programming for sizing, Vdd and Vt assignment,” in Proc. ISLPED, 2005.

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**Power Consumption of Optimized Circuits**

Estimated total power consumption for ISCAS’85 benchmarks Vdd = 1.0V, α = 0.1, 32nm FinFETs Available modes Leakage power savings 120% a.t. (68.5%) 200% a.t. (80.3%) Total power savings 110% arrival time (a.t.) (34%) 200% a.t. ( 47.5%)

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**Optimized Circuit Constitution**

Fraction of cells in different FinFET modes in power-optimized FinFET circuits Available modes SG-mode cells are largely replaced by cells in other modes SG-mode cells only needed on critical paths Utilization of IG/LP-mode cells is higher than IG cells Result of unmatched delay and higher leakage of IG-mode cells compared to IG/LP-mode cells

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**Area Requirements for Optimized Circuits**

+18.8% +18.0%

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**Talk Outline Background Motivation: Power Consumption**

FinFETs for Low Power Design Vth Control through Multiple Vdd’s (TCMS) Extension of TCMS to Logic Circuits Conclusions

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**Future of Interconnect Power**

Interconnect power dissipation is projected to dominate both dynamic and static power Assorted projections from literature- Interconnect switched capacitance may be 65-80% of total on-chip switched capacitance at the 32nm node [1] In power-optimized buffered interconnects at 50nm, leakage power consumption may be > 80% of total interconnect power [2] [1] N. Magen et al., Interconnect Power Dissipation in a Microprocessor, System-level Interconnect Prediction, 2004 [2] K. Banerjee and A. Mehrotra, Power Dissipation Issues in Interconnect Performance Optimization for Sub-180 nm Designs, Symp. VLSI Circuits, 2002 24

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Gate Coupling Linear relationship between threshold voltage and back-gate voltage in the subthreshold region Stronger than the square root relationship between body bias and threshold effect observed in bulk-CMOS 25

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**Dual-Vdd FinFET Circuits**

Conventional low- power principle: 1.0V Vdd for critical logic, 0.7V for off-critical paths Our proposal: overdriven gates Overdriven FinFET gates leak a lot less! Reverse bias Vgs=+0.08V Higher Vth 1.08V 1V Leakage current Vin Overdriven inverter

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**threshold control for P and N**

TCMS Using only two Vdd’s saves leakage only in P-type FinFETs, but not in N-type FinFETs Solution Use a negative ground voltage (VHss) to symmetrically save leakage in N-type FinFETs VddH > VddL VssH < VssL VddH VddL Symmetric threshold control for P and N VddH 1.08V VddL 1.0V VssH -0.08V VssL 0.0V VssH VssL TCMS buffer

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**Voltage Level Conversion**

Static leakage in multiple-Vdd designs Low-Vdd inputs must be up-converted to high-Vdd before being used to drive high-Vdd inverters to avoid static leakage Dedicated level converters inserted between buffers must be sized prohibitively large in order to avoid delay penalties [1]. Level conversion is built into high-Vdd inverters through the use of high-Vt FinFETs [1]. K. H. Tam and L. He, Power-optimal Dual-Vdd Buffered Tree Considering Buffer Stations and Blockages, DAC 2005 28

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**Exploratory Buffer Design**

Size of high-Vdd inverters kept small to minimize leakage in them Wire capacitances not driven by high-Vdd inverters Output inverter in each buffer overdriven and its size (and switched capacitance) can be reduced High- and low-Vdd inverters alternate, providing maximum opportunities for power savings 29

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Link Design SPICE simulation to minimize power consumption in TCMS link while remaining within 1% of the delay of the single Vdd link Parameter Single Vdd TCMS Change Link length(lopt) 0.199mm Inverter widths (s1, s2) 42, 84 30, 50 -36.5% Delay (ps) 12.19 12.27 0.65% Power (μW) 1080 647 -40% 30

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**Interconnect Synthesis**

Problem: Insert buffers on a given wiring tree to meet a given delay bound while minimizing total power consumption Two types of buffers considered TCMS buffers Dual-Vdd buffering scheme† A van Ginneken-style dynamic programming buffer insertion algorithm developed †Y. Hu et al, Fast Dual-Vdd Buffering based on Interconnect Prediction and Sampling, SLIP 2007 31

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Power Savings Power component Savings Dynamic power -29.8% Leakage power 57.9% Total power 50.4% Benchmarks are nets extracted from real layouts and scaled to 32nm 32

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Fin-count Savings Transistor area is measured as the total number of fins required by all buffers TCMS can save 9% in transistor area 33

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**Talk Outline Background Motivation: Power Consumption**

FinFETs for Low Power Design Vth Control through Multiple Vdd’s (TCMS) Extension of TCMS to Logic Circuits Conclusions

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**Traditional Dual-Vdd Dual-Vth Schemes**

Logic gates on the critical path driven with high-Vdd and low-Vth; those on the non-critical path with low-Vdd and high-Vth Exponential increase in leakage current Overhead of level converter delay and power

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**TCMS Extension =1.08V = -0.08V =1.0V =Gnd Overdriven gates are faster**

Overdriven gates leak less

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**Logic Library Design FinFETs connected to input-a cannot**

Embedded in a large circuit FinFETs connected to input-a cannot exploit TCMS FinFETs connected to input-b have high static leakage FinFETs connected to input-a follow TCMS FinFETs connected to input-b cannot exploit TCMS

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**Logic Library Design (Contd.)**

Level conversion may be used to restore signal to VddH Level converters not an attractive option in TCMS Level conversion can be built into logic gates through the use of high-Vth FinFET

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**Logic Library Design (Contd.)**

Two-input NAND gate of a given size has five design variables: Supply voltage Gate input voltage for input-a Gate input voltage for input-b Vth for FinFETs connected to input-a Vth for FinFETs connected to input-b

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**Logic Library Design (Contd.)**

32 NAND gate modes possible Certain combinations not allowed (High-Vdd gate with low-Vth transistors cannot have high input voltage swings) 25 NAND and NOR gate modes 7 INV gate modes For each NAND, NOR and inverter mode: X1, X2, X4, X8 and X16 sizes

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**Optimization Flow Combinational gate level Verilog netlist**

Shorted-gate library Delay-minimized netlist from Design Compiler TCMS library Phase I: Divide into alternate levels of high (odd) and low (even) Vdd gates Phase II: Linear programming formulation T≤Tmax no yes yes P no Optimized netlist

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**Experimental Setup Switching activity at primary inputs set to 0.1**

Temperature: 75oC Technology node: 32nm Nominal-Vdd: (1.0V,0V), High-Vdd: (1.08V,-0.08V) Nominal-Vth: (0.29V,-0.25V), High-Vth: (0.45V,-0.40V) Cell libraries characterized using HSPICE based on PTM1 in Synopsys-compatible format Interconnect delay and load modeled 5 sizes for logic gates: X1, X2, X4, X8 and X16 1http://www.eas.asu.edu/~ptm/

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**Applying Methodology to c17**

Delay-minimized netlist Power : 283.6uW (leakage power: 10.3uW, dynamic power: 273.3uW) Area: 538 fins Power-optimized netlist Power : 149.9uW (leakage power: 2.0uW, dynamic power: 147.9uW) Area: 216 fins

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**Multi-Vdd Multi-Vth (1.3Tmin)**

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**Multi-Vdd Single-Vth (1.3Tmin)**

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**Fin-count Savings (1.3Tmin)**

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Conclusions FinFETs are a necessary step in the evolution of semiconductors because bulk CMOS has difficulties in scaling beyond 32 nm Use of the back gate leads to very interesting design opportunities Rich diversity of design styles, made possible by independent control of FinFET gates, can be used effectively to reduce total active power consumption IG/LP mode circuits provide an encouraging tradeoff between power and area TCMS able to reduce both delay and subthreshold leakage current in a logic circuit simultaneously

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