Advanced Topics In Solid State Devices EE290B Will a New Milli-Volt Switch

1. Advanced Topics In Solid State Devices EE290B Will a New Milli-Volt Switch Replace the Transistor for Digital Applications? August 28, 2007 Prof. Eli Yablonovitch Electrical Engineering & Computer Sciences Dept. University of California, Berkeley, CA 94720 1

2. Read the current going through a resistor, in the presence of noise: With a safety margin:

3. What will be the energy cost, per bit processed? • Logic energy cost ~40kT per bit processed • Storage energy cost ~40kT per bit processed • Communications currently >100,000kT per bit processed • . 3

4. There are many type of memory possible: • Flash • SRAM • Dram • Magnetic Spin • Nano-Electro-Chemical Cells • Nano-Electro-Mechanical NEMS • Moletronic • Chalcogenide glass (phase change) • Carbon Nanotubes •  •  • Similarly there are many ways to do logic. • But there are not many ways to communicate: • Microwaves (electrical) • Optical 4

5. Signal Noise Power Energy per bit  = 4kT per bit What is the energy cost for electrical communication? All information processing costs ~ 40kT per bit. (for good Signal-to-Noise Ratio) Great! So what’s the problem?

6. The natural voltage range for wired communication is rather low: The natural voltage range for a thermally activated switch like transistors is >>kT/q, eg. ~ 40kT/q or about ~1Volt Voltage Matching Crisis at the nano-scale! If you ignore it the penalty will be (1Volt/1mVolt)2 = 106 The thermally activated device wants at least one electron at ~1Volt. The wire wants 1000 electrons at 1mVolt each. (to fulfill the signal-to-noise requirement >1eV of energy)

7. The other , for energy per bit function 108 107 Gates including wires 106 105 Energy per Bit function (kT) 104 Gates only Technology Gap 103 102 10 1 Transistor Measurements by Robert Chau, Intel 0.1 Shoorideh and Yablonovitch, UCLA 2006 Moore's Law 10μm 1μm Critical Dimension 100nm 10nm 1960 1980 2000 2020 2040 2060 Year

8. p. 114 "In addition, power is needed primarily to drive the various lines and capacitances associated with the system. As long as a function is confined to a small area on a wafer, the amount of capacitance which must be driven is distinctly limited. In fact, shrinking dimensions on an integrated structure makes it possible to operate the structure at higher speed for the same power per unit area."

9. A low-voltage technology, or an impedance matching device, needs to be invented/discovered at the Nano-scale: ~1eV nano-transformer transistor amplifier with steeper sub-threshold slope photo-diode - VG + + + + + MEM's switch Cu solid electrolyte giant magneto-resistance spintronics Cryo-Electronics kT/q~q/C Cu Electro-Chemical Switch

10. An amplifying transistor as a voltage matching device: Small voltage in Large voltage out out Amplification of weak signals has an energy cost! Amplification of weak signals has a speed penalty! in ln{I} steeper sub-threshold slope Current Vg Gate Voltage correlated electron motion?

11. Cryo-Electronics: RSFQ

12. Nano-Mechanical Switch: - VG + + + + I ~ exp(-3qVG/kT) for 3 charges on the MEM's tip After M. A. Baldo

13. giant magneto-resistance "spintronics" + These switches are made of metallic components and are of inherently low impedance

14. Transpinnor Structure: Isignal Gate Current Gate Insulator B Field B Field Source Ru 15nm Ta 5nm Ferromagnetic CoFeB 3nm Magnetization 6:1 Resistance Change in Tunnel Magnetoresistive (TMR) stack[1] MgO 1.5nm Tunnel Barrier Isignal Ferromagnetic CoFeB 3nm Ru 0.8nm Source CoFe 2nm Antiferromagnetic MnIr 8nm NiFe 5nm Ta 5nm Ru 50nm Ta 5nm Drain Si (001) Substrate Drain Device Area  1μm2 Current Gate [1] Ikeda et. al., Japanese Journal of Applied Physics, Vol. 44, No 48, pp. L1442-L1445

15. -V Complementary Transpinnor Logic +3mV +V 2.275kΩ or 500Ω 5μA input 5μA output Output Power = 1.6*10-8 W Total Power = 2.5*10-8 W Efficiency=65% 500Ω or 2.275kΩ -3mV • Problems: On/Off ratio is only about 5:1 Still takes too many Amps to switch

16. nh nh A Photo-Diode for impedance matching: • multi-photon millimeter-wave "optical" • plasmonic wires h ~1eV • Problems: • Ten photons are needed • to assure a good • Bit Error Ratio. • Source efficiency • may be only 10%. • Energy penalty 100 × 40kT h Miller DAB "Optics for Low-Energy Communication Inside Digital processors – Quantum Detectors, Sources, and Modulators as Efficient Impedance Converters Optics Letters 14 (2): 146-148 JAN 15 1989

17. - - - + + + - - - + + + - - - +++ - - ++ - - - + + + - - - + + + - - - + + + Nano-transformers are feasible using tapered plasmonic transmission lines: Normal Impedance High Impedance

18. Electro-Chemically Driven Metallic Switch: Cu - 1nm solid electrolyte VG + Cu

19. An amplifying transistor as a voltage matching device: Small voltage in Large voltage out out Amplification of weak signals has an energy cost! Amplification of weak signals has a speed penalty! in ln{I} steeper sub-threshold slope Current Vg Gate Voltage correlated electron motion?

20. The Esaki Diode as a Switch: The sub-threshold slope for tunneling depends on the steepness of the band-edges: EF Sharp Step Current band to band tunneling Diffusion current Bias Voltage

21. + The tunnelling FET, (TFET); a gate controlled Esaki Diode: Gate Gate p+ n+ p+ n+ n- n-

22. The Gate Controlled Esaki Diode: The sub-threshold slope for tunneling depends on the steepness of the band-edges: EF Sharp Step Current band to band tunneling Diffusion current Bias Voltage

23. The optical absorption coefficient,(h), of Si at 300K, in the vicinity of the band edge. The Urbach edge grows as: (h) ~ exp{(h-Eg)/Eo}, where the Eo parameter is a type of sub-threshold slope. Eo ~ 10meV for Silicon It's good, but it should be better. We need to search for materials with steeper band-edges! Tom Tiedje, Eli Yablonovitch, George D. Cody, and Bonnie G. Brooks IEEE Trans. On Elec. Dev., VOL. ED-31, NO. 5 , p. 711 (MAY 1984)

24. Deformation Potential Energy level fluctuations = (% strain)  Why aren't the band edges sharp? – thermal strains & Deformation Potentials E Ec Ev Energy level fluctuations = 0.5%  5eV  25meV

25. Surprisingly, Si & Ge have opposite sign Deformation Potentials: Deformation Potentials:    opposite sign Cancelling the effects of the thermal sound waves might be possible: Deformation Potential Engineering?

26. Construct the channel as a short-period Si-Ge superlattice: Gate p+ n+ n- Ge Si The challenge is to cancel the effects of: isotropic strain uni-axial strain shear strain

27. If Vdd is lower, then the current drive requirements go down too! Some of the tunneling transistors have a relatively low current drive capability: Current Density Requirement: For conventional transistors: 1 milli-Amp / micron If a lower operating voltage is achieved, then the current required to charge the wires would also be lower. A Figure-of-Merit that allows for this possibility would be expressed as: 1 milli-Siemen / micron A current density as low as 1 micro-Amp / micron would be acceptable, if the operating voltage were as low as 1 milli-Volt.

28. ~1eV A low-voltage technology, or an impedance matching device, needs to be invented/discovered at the Nano-scale: nano-transformer transistor amplifier with steeper sub-threshold slope photo-diode - VG + + + + + MEM's switch Cu solid electrolyte giant magneto-resistance spintronics Cryo-Electronics kT/q~q/C Cu Electro-Chemical Switch

29. p. 114 "In addition, power is needed primarily to drive the various lines and capacitances associated with the system. As long as a function is confined to a small area on a wafer, the amount of capacitance which must be driven is distinctly limited. In fact, shrinking dimensions on an integrated structure makes it possible to operate the structure at higher speed for the same power per unit area."

30. The other , for energy per bit function 108 107 Gates including wires 106 105 Energy per Bit function (kT) 104 Gates only Technology Gap 103 102 10 1 Transistor Measurements by Robert Chau, Intel 0.1 Shoorideh and Yablonovitch, UCLA 2006 Moore's Law 10μm 1μm Critical Dimension 100nm 10nm 1960 1980 2000 2020 2040 2060 Year

31. Recommendations: • Milli-Volt powering should be regarded as a Goal for future electronic switching devices. • There would be both an immediate benefit, as well as a benefit at the end of the roadmap. • Band edge steepness is poorly known, and should be investigated for a number of semiconductors and semi-metals. • The full range of technology options should be included.