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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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2 Talk Outline Background Motivation: Power Consumption FinFETs for Low Power Design V th Control through Multiple V dd s (TCMS) Extension of TCMS to Logic Circuits Conclusions

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3 Why Double-gate Transistors ? Non-Si nano devicesBulk CMOS Feature size32 nm 10 nm DG-FETs Gap 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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4 Different Types of DG-FETs Source: ( Hollis, Boston University)

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5 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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6 FinFET 3-D Structure Source: (Ananthan, 2004) Earliest FinFET processes: both gates inherently connected

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7 Independent-gate FinFETs 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 Back Gate Oxide insulation

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8 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, ISQED05

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9 Talk Outline Background Motivation: Power Consumption FinFETs for Low Power Design V th Control through Multiple V dd s (TCMS) Extension of TCMS to Logic Circuits Conclusions

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10 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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11 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 V dd /V th –Respect circuit operations and timing constraints –Can be used to reduce active mode leakage What opportunities do FinFETs provide us ?

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12 Talk Outline Background Motivation: Power Consumption FinFETs for Low Power Design V th Control through Multiple V dd s (TCMS) Extension of TCMS to Logic Circuits Conclusions

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13 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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14 Logic Styles: NAND Gates SG-mode NAND IG-mode NAND LP-mode NAND IG/LP-mode NAND pull up bias voltage pull down bias voltage IG-mode pull up LP-mode pull down

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15 Comparing Logic Styles Design ModeAdvantagesDisadvantages SGFastest under all load conditions High leakage (1μA) LPVery low leakage (85nA), low switched capacitance Slowest, especially under load. Area overhead (routing) IGLow area and switched capacitance Unmatched pull-up and pull-down delays. High leakage (772nA) IG/LPLow leakage (337nA), area and switched capacitance Almost as slow as LP mode Average leakage current for two-input NAND gate (V dd = 1.0V)

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16 FinFET Characteristics Simulated I d Vs. V gs characteristics for FinFETs at varying back-gate reverse biases LP-mode leakage is 10 times lower than SG-mode LP-mode delay ( 1/I on ) is twice that of SG-mode IG-mode I on is not much better than LP-mode I off is a strong function of back-gate reverse bias but I on is not

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17 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 = V dd +.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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18 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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19 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 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 32 nm PTM FinFET models Delay/power characterization in SPICE LP IG/LP IG SG Synopsys libraries 32 nm PTM inFET models 32nm PTM FinFET models Logic gate designs Logic gate designs D. Chinnery and K. Keutzer, Linear programming for sizing, V dd and V t assignment, in Proc. ISLPED, 2005.

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20 Power Consumption of Optimized Circuits 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%) Estimated total power consumption for ISCAS85 benchmarks V dd = 1.0V, α = 0.1, 32nm FinFETs Available modes

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21 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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22 Area Requirements for Optimized Circuits +18.8% +18.0%

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23 Talk Outline Background Motivation: Power Consumption FinFETs for Low Power Design V th Control through Multiple V dd s (TCMS) Extension of TCMS to Logic Circuits Conclusions

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24 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

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25 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

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26 Dual-V dd FinFET Circuits Conventional low- power principle: –1.0V V dd for critical logic, 0.7V for off-critical paths Our proposal: overdriven gates –Overdriven FinFET gates leak a lot less! 1.08V 1V Leakage current Reverse bias V gs =+0.08V Overdriven inverter Higher V th Vin

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TCMS Using only two V dd s saves leakage only in P-type FinFETs, but not in N-type FinFETs Solution –Use a negative ground voltage (V H ss ) to symmetrically save leakage in N-type FinFETs – V dd H > V dd L – V ss H < V ss L TCMS buffer V dd H V ss H Symmetric threshold control for P and N V dd L V ss L V dd H 1.08V V dd L 1.0V V ss H -0.08V V ss L 0.0V

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28 Voltage Level Conversion Static leakage in multiple-V dd designs –Low-V dd inputs must be up-converted to high- V dd before being used to drive high-V dd 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-V dd inverters through the use of high-V t FinFETs [1]. K. H. Tam and L. He, Power-optimal Dual-V dd Buffered Tree Considering Buffer Stations and Blockages, DAC 2005

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

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30 Link Design SPICE simulation to minimize power consumption in TCMS link while remaining within 1% of the delay of the single V dd link Parameter Single V dd TCMSChange Link length(l opt ) 0.199m m 0 Inverter widths (s1, s2) 42, 8430, % Delay (ps) % Power (μW) %

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31 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-V dd 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

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32 Power Savings Benchmarks are nets extracted from real layouts and scaled to 32nm rtion.html Power component Savings Dynamic power -29.8% Leakage power 57.9% Total power50.4%

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

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34 Talk Outline Background Motivation: Power Consumption FinFETs for Low Power Design V th Control through Multiple V dd s (TCMS) Extension of TCMS to Logic Circuits Conclusions

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35 Traditional Dual-V dd Dual-V th Schemes Logic gates on the critical path driven with high-V dd and low-V th ; those on the non- critical path with low-V dd and high-V th Exponential increase in leakage current Overhead of level converter delay and power

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36 TCMS Extension =1.08V = -0.08V =1.0V =Gnd Overdriven gates are faster Overdriven gates leak less

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37 Logic Library Design FinFETs connected to input-a follow TCMS FinFETs connected to input-b cannot exploit TCMS FinFETs connected to input-a cannot exploit TCMS FinFETs connected to input-b have high static leakage

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38 Logic Library Design (Contd.) Level conversion may be used to restore signal to V dd H Level converters not an attractive option in TCMS Level conversion can be built into logic gates through the use of high-V th FinFET

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39 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 – V th for FinFETs connected to input-a – V th for FinFETs connected to input-b

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40 Logic Library Design (Contd.) 32 NAND gate modes possible Certain combinations not allowed (High-V dd gate with low-V th 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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41 Optimization Flow Shorted-gate library Delay-minimized netlist from Design Compiler Combinational gate level Verilog netlist Phase I: Divide into alternate levels of high (odd) and low (even) V dd gates Phase II: Linear programming formulation TTmax P Optimized netlist TCMS library yes no

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42 Experimental Setup Switching activity at primary inputs set to 0.1 Temperature: 75 o C Technology node: 32nm Nominal-V dd : (1.0V,0V), High-V dd : (1.08V,-0.08V) Nominal-V th : (0.29V,-0.25V), High-V th : (0.45V,-0.40V) Cell libraries characterized using HSPICE based on PTM 1 in Synopsys-compatible format Interconnect delay and load modeled 5 sizes for logic gates: X1, X2, X4, X8 and X16 1

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43 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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44 Multi-V dd Multi-V th (1.3Tmin)

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45 Multi-V dd Single-V th (1.3Tmin)

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46 Fin-count Savings (1.3Tmin)

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47 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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