Low Energy Communication: NanoPhotonic & Electrical. Prof. Eli Yablonovitch EECS Dept. UC Berkeley

Size: px

Start display at page:

Download "Low Energy Communication: NanoPhotonic & Electrical. Prof. Eli Yablonovitch EECS Dept. UC Berkeley"

Cynthia Holland
5 years ago
Views:

1 Low Energy Communication: NanoPhotonic & Electrical Prof. Eli Yablonovitch EECS Dept. UC Berkeley

2 What is the energy cost of reading out your flash memory?

3 Read the current going through a resistor, in the presence of noise: Required ( i) ( i) voltage 2 2 V = 2q i f...shot Noise 4kT = f...johnson Noise R = ir >> 2kT / q ~ 50mVolts Signal to Noise Ratio i Required power iv With a safety margin: Energy Consumed = > > ~ 2q f 2q 40 i 2q i f f kt per = 2kT q bit i 2q f = 4kT f processed

4 Units: ~40kT/bit of information atto-joules/bit of information 0.16 nano-watts/gbit/second This is about 10 6 times better than we are doing today!

5 There are many type of memory possible: 1. Flash 2. SRAM 3. Dram 4. Magnetic Spin 5. Nano-Electro-Chemical Cells 6. Nano-Electro-Mechanical NEMS 7. Moletronic 8. Chalcogenide glass (phase change) 9. Carbon Nanotubes Similarly there are many ways to do logic. But there are not many ways to communicate: 1. Microwaves (electrical) 2. Optical 5

6 What will be the energy cost, per bit processed? 1. Logic energy cost ~40kT per bit processed 2. Storage energy cost ~40kT per bit processed 3. Communications currently >100,000kT per bit processed. 6

7 What is the energy cost for electrical communication? V V 2 noise 2 noise R = = 4kT 4kT R f f Signal Energy Noise Power per bit = 4kT per bit All information processing costs ~ 40kT per bit. (for good Signal-to-Noise Ratio) Great! So what s the problem?

8 The natural voltage range for wired communication is rather low: V V V V V 2 noise 2 noise 2 noise 2 noise noise = 4kT R f 1 = 4kT R RC 1 = 4kT C 4kT q = q C = 4kT / q mVolts V 1 mvolt q { / C 10µ Volts 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 = 10 6 The wire wants 1000 electrons at 1mVolt each. (to fulfill the signal-to-noise requirement >1eV of energy) The thermally activated device wants at least one electron at ~1Volt.

9 The other Moore's Law, for energy per bit function 10 8 Critical Dimension 10µm 1µm 100nm 10nm Gates including wires Gates only Technology Gap Energy per Bit function (kt) 1 Transistor Measurements by Robert Chau, Intel Year Shoorideh and Yablonovitch, UCLA 2006

10 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."

11 l l Repeater Repeater Repeater l l Resistance, R = ρ 2 a Capacitance, C ε r ε o l 2 l RC time RC εrεoρ a RC time = (clock period) /2π 2 2 = ε r ε o ρ l a a l a = clock period 2πε ε ρ r o aspect l ratio of wire a sec F/cm Ωcm

12 aspect l ratio of wire a 4800 Resistance, R l = ρ a 2 = ρ a l a = 45nm = 2000Ω ρ = 4800 a 6 Ωcm Capacitance, C ε r ε o l = ε r ε o a l a C = ε r ε o a 4800 C 7 femto-farads <V 2 > = 4kT R f V signal = 0.56 milli-volts <I 2 > = 4(kT/R) f I signal = 0.25 µamps

13 Johnson Noise: What about very short wires? V V V V 2 noise 2 noise 2 noise 2 noise = = = = 4kT 4kT R f R 4kT C 4kT q 1 RC q C If 4kT q q C, then the signals could be large enough to be efficiently amplified. If C 2 q kt The Coulomb 10 atto-farads, Blockade Capacitance. For wires less than 1µm, a conventional transistor amplifier configuration may be adequate.

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

15 in 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! ln{i} Current steeper sub-threshold slope Gate Voltage V g correlated electron motion?

16 Diffusion current The Zener Diode: E Fv E Fc Bias Voltage Current band to band tunneling

17 Diffusion current band to band tunneling E F Sharp Step Bias Voltage Current The Esaki Diode:

18 The Backward Diode as a Switch: The Backward Diode: These have been routinely made in Ge homo-junctions, since the 1960's. E Fv E Fc Sharp Step Current Diffusion current Bias Voltage band to band tunneling

19 The Backward Diode as a Switch: The sub-threshold slope for tunneling depends on the steepness of the band-edges:

21 I: Steepness, Low Operating Voltage II: On/Off Ratio >10 5 III: On-State Current Drive Capability, ma/µm or better: ms/µm With tunneling alone, you can pick any two out of the three above, but you can't get all three!

22 Engineer Bands to Move Together Un-Engineered Engineered

23 Consider Thermal Vibrations k x k x k z k z k y Uniaxial Strain ε xx k y k x k x k z k z Shear Strain k y ε xy k y Silicon Germanium

24 Electro-Chemically Driven Metallic Switch: 1nm Cu solid electrolyte Cu - V G +

A low-voltage technology, or an impedance matching device, needs to be invented/discovered at the Nano-scale: transistor amplifier with steeper sub-threshold slope hν

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

26 That was Electrical Communication. For longer distances, Optical Communication is needed; How efficient can that be? How many photons are needed? This is mainly determined by the photodetectors.

27 Two-Dimensional Thin Film Photonic Crystals Cross- Section: Si SiO 2 SiO 2 Si 1µm SOI Si/SiO 2 SOI

transistors are constructed side-by-side

28 Monolithic Integration of Optics and Electronics 8 RF amplifier 1x2 optical switch Optical devices and transistors are constructed side-by-side monolithically in the silicon Luxtera, Inc. Approved for Public Release

29 CMOS Optical Modulator Performance Rolloff >20GHz 10G 10G Eye Luxtera, Inc. Approved for Public Release

Blazar LUX5010 Multirate 4x10G Optical Active Cable Typical

Ethernet 2 / 4 / 8 / 10 Gbps Fibre Channel Proprietary Cluster

30 Blazar LUX5010 Multirate 4x10G Optical Active Cable Typical Applications InfiniBand Connectivity SDR/DDR/QDR 10/40 Gigabit Ethernet 2 / 4 / 8 / 10 Gbps Fibre Channel Proprietary Cluster Interconnect Ethernet Local Area Network (LAN) Storage Area Network (SAN) 2.5G / 5G PCI-Express Extension Storage Arrays Optical Backplanes Rack-to-Rack, Shelf-to-Shelf Interconnect Test Equipment

31 The problem is that optical communications As it is currently practiced uses >10,000 photons/bit of information communicated This is mainly determined by the photodetectors.

32 Ultra-Low Capacitance, Nano-Photodetectors "the photo-detector without the wire" small size small dark current small size small capacitance small capacitance more voltage/photo-electron, less pre-amp noise eliminate capacitance of connecting wire electrically floating no wire Lowest Possible Capacitance A floating island of Ge acting as a gate on a Si- FET, integrates photodetector and pre-amp in one. For example a 65nm island would have a capacitance ~6.5atto-Farads, a single photo-electron would produce a 25milli-volt gating signal, easily amplified.

33 Photo-hetero-JFET concept: Source n-si Floating Ge island h + h + e - e - Photons modulate depletion width Drain SiO 2 Silicon This is one of many types of photo-transistors!

34 Design Principle: Transistor with a Floating Gate p-ge e e e e h h+ h + + h + V F n-si Excess photo-holes attract electrons in the Silicon and change its conductance Open Ge/Si junction gets forward biased by an amount dependant on the light intensity

35 Reflectors integrated with SOI waveguides to create a transverse cavity n-si channel Ge island Source n-si n-si Drain incident light SiO 2 Silicon

36 The problem is that optical communications As it is currently practiced uses >10,000 photons/bit of information communicated If the photodetector is small enough, and preamp is sensitive enough, we can anticipate getting this down to: ~15 photons/bit of information communicated For a ~10 3 times improvement

37 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."

38 Conclusions: 1. Communications is a big bottleneck for reducing powering consumption in information processing. 2. The powering voltage for circuits is close to 1Volt, but it could be reduced to ~1milli-Volt, for a 10 6 reduction in power. 3. Optical communications uses >10,000 photons/bit of information, but with more nanoscopic photodetectors this could be reduced to ~15photons/bit for a 10 3 reduction in power.

39 Reversible and Adiabatic Computing: Energy/bit function <<kt Charles Bennett, IBM

40 The other Moore's Law, for energy per bit function 10 8 Critical Dimension 10µm 1µm 100nm 10nm Gates including wires Gates only Technology Gap Energy per Bit function (kt) 1 Transistor Measurements by Robert Chau, Intel Year Shoorideh and Yablonovitch, UCLA 2006

Lecture 4 INTEGRATED PHOTONICS

Lecture 4 INTEGRATED PHOTONICS What is photonics? Photonic applications use the photon in the same way that electronic applications use the electron. Devices that run on light have a number of advantages