Circuit Design with Alternative Energy-Efficient Devices

1. Circuit Design with Alternative Energy-Efficient Devices Elad Alon Collaborators: Hei Kam, Fred Chen (MIT), Tsu-Jae King-Liu, Vladimir Stojanovic (MIT), Dejan Markovic (UCLA), Mark Horowitz (Stanford) Dept. of EECS, UC Berkeley

2. CMOS is Scaling, Power Can Not 1000 Predictions(ca. 2000) Reality (Core 2) Itanium II Itanium 100 Pentium 4 Pentium III Pentium Pro Power (W) 10 Pentium Pentium II 486DX 80286 8086 8088 1 386DX 8080 8008 S. Borkar, Intel 4004 0.1 1970 1975 1980 1985 1990 1995 2000 2005 2010

3. Supply and Threshold Voltages kT/q doesn’t scale, so lowering Vth increases leakage Fixed Vth, Vdd power density doesn’t scale well Ed Nowak, IBM Drain Current Id Scaling Vth, Vdd Gate Voltage Vg

4. Alternative Devices to the Rescue? Many new devices withS-1<60mV/dec proposed But, many of these are slow (low Ion) And/or have other “weird”characteristics Can these devices reduce energy? If so, at what performance? Need to look at the circuits MOSFET Drain Current Id New Device Slope=S-1 Gate Voltage Vg

5. Outline Energy-Performance Analysis Circuit Design with Relays Conclusions

6. Processor Power Breakdown • Most components track performance vs. energy curves of logic • Control, Datapath, Clock • Use proxy circuit to examine tradeoffs

7. Proxy Circuit for Static Logic Vdd Vdd Output Input 0V Ld stages Switching activity factor = , Gate capacitance per stage = C • tdelay = LdCVdd/(2Ion) • Edyn+Eleak = αLdCVdd2 + LdIoffVddtdelay

8. Simple Optimization Rule Optimal Ion/Ioff Ld/α Derived in CMOS But holds for nearly all switching devices Pleak/Pdyn ~constant ~30-50% across wide range of parameters Nose and Sakurai

9. Using the Rule to Compare MOSFET • Match Ioff by adjusting “VT” • New device wins if: Ion,new(Vdd) > Ion,MOS(Vdd) “New Device” Energy “New Device” Drain Current Id MOSFET Vddx Vddx Performance Gate Voltage Vg

10. What Else Matters: Variability • Leakage: • E(Ioff) vs. E(Vth) • Delay: • Finite Ld • Cycle time set by worst-case

11. What Else Matters: Wires & Area Vdd Vdd Output Input 0V Cw Cw Cw Cw • Devices don’t drive just other devices • Need to look at extrinsic cap (wires) too • Especially if device has area overhead

12. Parallelism Serial: Perf. f Energy “New Device” Parallel: Perf.  2f, E/op ~const MOSFET Performance • If available, parallelism allows slower devices • Extends energy benefit to higher performance

13. Minimum Energy At low performance or high parallelism: Lowest Vdd for required Ion/Ioff wins Vdd,min Seff,Emin  Seff2 Drain Current Id Seff-1 Normalized Energy/cycle Lower Seff Gate Voltage Vg Vdd(V)

14. Example: Tunneling FET Band-to-band tunneling device Steep transition (<60mV/dec) at low current Low Ion(<~100μA) Assume work function can be tuned Gate N Drain Source Drain Current Id (A/mm) P Ion ≈A(Vgs+VT)exp[-B/(Vgs+VT)] [1] [1]J. Chen et al., IEEE Electron Device Lett., vol. EDL-8, no. 11, pp. 515–517, Nov. 1987. Gate Voltage Vg (V)

15. Energy-Performance Tradeoff 30 stages α=0.01 TFET Energy (J) MOSFET Performance (GHz) • Competitive with subthreshold CMOS • TFETs promising below ~100MHz

16. Outline Energy-Performance Analysis Circuit Design with Relays Conclusions

17. Nano-Electro-Mechanical Relay Based on mechanically making and breaking contact No leakage, perfectly abrupt transition Reliability is the key challenge Gon Conductance Vrl Vpi Gate Voltage Vg [V]

18. Circuit Design with Relays CMOS delay set by electrical time constant Distribute logical/electrical effort over many stages Relay: mechanical delay (~10ns) >> electrical t (~1ps) Implement logic as a single complex gate CMOS: Relay:

19. Relay Energy-Perf. Tradeoff Stack of 30 series relays No leakage Vdd,min set onlyby functionality(surface force) How about real logic circuits? TFET MOSFET Energy (J) Relay Performance (GHz)

20. Relay-Based Adder • Manchester carrychain • Ripple carry • Cascade full adder cells • N-bit adder still 1 mechanical delay

21. Adder Energy-Delay • Compare vs. optimalCMOS adder • ~10-40x slower • Low Rcont not critical • ~10-100x lower E/op • Lower Cg • Fewer devices, all minimum size • Lower Vdd,min

22. Parallelism and Area • If parallelism available, can trade area for throughput • Competing with sub-threshold CMOS • Area-overhead bounded

23. Power Breakdown Revisited • Better logic  “uncore” power dominant • Need to analyze (and leverage) devices for entire system… • Relay DRAM or NVM (not SRAM)? • Relay ADC/DACs?

24. Outline Simple Energy-Performance Analysis Circuit Design with Relays Conclusions

25. Summary • New devices need circuit level analysis • Ion/Ioff set by logic depth, activity factor • Don’t forget about variability, wires • Tailor circuit style to the device • If available, parallelism may allow slower (low Ion) devices • Don’t forget about the rest of the system

26. Good News/Bad News Today:Parallelism lowers E/op Future: Parallelism doesn’t help -1 • Parallelism still available in CMOS • But eventually limited by Emin • Opportunity for new devices… • At least in sub-100MHz applications

27. Acknowledgements • Berkeley Wireless Research Center • NSF • DARPA • FCRP