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Unformatted text preview: Memory Designing Using Josephson Gates Josephson Susmit Biswas 02/07/2006 Outline Outline Refreshing Memory Memory Circuits CMOS Memory Circuits Need For New Memory Technology Josephson PC Memory Previous Work Josephson Junction Memory Designing Using Josephson Gate Performance Evaluation Conclusion Standard Memory Technology Standard The Memory Hierarchy CPU Registers L1 Cache (SRAM) L2 Cache (SRAM) Main Memory SRAM DRAM FPM DRAM (Fast Page Mode DRAM) EDORAM (Extended Data Out DRAM ) SDRAM (Synchronous DRAM) DDR DRAM (Double Data Rate DRAM) DRAM DRAM High Density and low power but Slower than SRAM DRAM Performance DRAM (August 2005) Need For New Technology Need Memory is the main bottleneck now Multiprocessor system suffers most SIMD and MIMD architecture Data hungry Josephson Memory: Previous Work Josephson Josephson Junction: Discovered and Demonstrated in early 60’s IBM till 1983 Nearly functional 1kBit memory using lead-alloy 1980s : ETL, NTT using Nb/Al0x/Nb 1993 : UC Berkeley designed a 4 kBit RAM 1997 : NEC developed a 4 kBit Memory 2002 : Hybrid Josephson memory Looking Back Looking 1962: Josephson predicted that a sandwich of S-I-S will 1962: show remarkable properties when the insulator is sufficiently thin ~ 10Å or so Current can flow through the junction with no voltage appearing across the junction until a critical current IJ is exceeded The magnitude of IJ, depends sensitively on magnetic fields. A voltage Vdc, impressed across the junction leads to an oscillating supercurrent whose frequency is proportional to the voltage. The frequency is very high for even modest voltages (483 MHz/μV). Josephson Effect Josephson Two-fluid model of Superconductor: One of the fluids is the normal fluid, the other the superfluid. Superfluid consists of paired electrons (Cooper pairs) of equal but opposite momentum and spin Josephson Effect Josephson Bound pairs electrons all lie near the Fermi energy EF of the normal metal; the resulting pairs are in an energy state lower than EF by an amount Δ (binding energy of the pair (per electron) As T becomes less than Tc, pairs begin to form and condense into the superconducting state At V = 2 Δ /e the tunneling current increases sharply (with +∞ slope) For V >> 2 Δ/e the current increases linearly with V Josephson Junction Josephson Josephson Effect: Josephson In superconducting state of certain metals, electrons are attracted by each other and form bound pairs, called Cooper pairs. When these pairs of electrons tunnel through a thin Cooper When insulating barrier placed between two superconductors, the whole is called Josephson junction. Josephson Josephson Effect : Summary Josephson DC Josephson effect: If no voltage is applied to the If junction terminals, a direct current - a current of Cooper pairs Ij, - flows through the junction up to a critical value pairs flows Ic, which depends on the geometry, temperature and which magnetic field. magnetic Josephson Effect : Summary Josephson AC Josephson effect: If a direct voltage is applied to If the junction terminals, the current of the electron pairs crossing the junction oscillates at a frequency which depends solely on the applied voltage V and fundamental constants (the electron charge e and the fundamental Planck constant h) : f = 2eV/h Conversely, if an AC voltage of frequency is applied to Conversely, the junction terminals by microwave irradiation, the current of Cooper pairs tends to synchronize with this synchronize frequency (and its harmonics) and a direct voltage appears at the junction terminals. appears Josephson Junction Characteristics Josephson Control currents Ic, Josephson threshold Im. Josephson Gate current Ig, Gate I-V Curve Threshold Curve Josephson Junction As Memory Josephson Consists of a loop with three Josephson junctions in Consists series that encloses a magnetic flux Ф driven by an external magnet. external The loop may have multiple stable persistent current The states when the enclosed magnetic flux is close to half a superconducting flux quantum Ф Ф = h / 2e System has two stable states System 0‫ ›׀‬and 1‫ ›׀‬with opposite with circulating persistent currents Josephson Junction As Memory ( cont.) cont Operated by resonant microwave modulation of the Operated enclosed magnetic flux by a superconducting control line on top of the qubit, separated by a thin insulator. The state of a bit (0 or 1) depends on the sum of the The external magnetic flux generated by the circulating currents on the surrounded loops: 0 iif magnetic field is < 1/2 Ф f 1 iif magnetic field is > 1/2 Ф f The state of the system is the superposition of all the The states generated by the circulating current in each loop. states Josephson Junction As Memory ( cont.) cont Combining several junctions results in different gates Combining e.g. inverter e.g. Can be designed in two ways coupling two superconductive loops directly through coupling magnetic interference magnetic Coupling two loops through a superconductive flux Coupling transporter transporter Josephson Junction As Memory ( cont.) cont Stronger interaction between the PC loops and better coupling to Stronger each other with the facilitation of transporter each But! Coupling between neighboring loops makes it difficult for long-range Coupling communication communication Solution Transporter: fast data propagation Josephson Junction As Memory ( cont.) cont NMV Gate can serve as NAND, NOR and NOT gate by setting NMV instruction bits. instruction Not Majority Vote (NMV) Gate Memory Designing Using Josephson Gate Josephson Memory Designing Using Josephson Gate (cont.) Josephson A memory cell can not be refreshed by either a row or a memory column addressing line independently column the addressing lines are designed in such a way that the the states of other cells in the same column are suppressed during reading, the selected one gets the bit from its adjacent memory cell, without interacting with its neighbors in the same column. neighbors Performance Evaluation Performance Pros: Pros: Speed: 750GHz for single asynchronous cells and up to 320GHz for LSI 750GHz devices devices Low power consumption 0.2nanowatt/GHz per pulse and 0.1mW for LSI devices for Simple fabrication technology : lithography Cons: Cons: Low density Operational temperature <20K Performance Evaluation (cont.) Performance Comparison of projected 2.5μm technology Josephson NDRO and DRO chip designs with advanced silicon memories having comparable line widths. Conclusion Conclusion Josephson memory can become more and more Josephson popular because of its speed and low power characteristics characteristics Designing larger memory is difficult Low density Limitation of fabrication technology References References 1. “Novel Computing Architecture on Arrays of Josephson Novel Persistent Current Bits” : Jie Han, Pieter Jonker [Proc. Persistent Jie Proc. MSM 2002 ] MSM 2. “Memory-Cell Design in Josephson Technology” : Memory-Cell Hans H. Zmpe [IEEE Transactions On Electron Devices, VOL. ED-27, NO. 10, OCTOBER 1980] Devices, 3. “570-ps 13-mW Josephson 1kbit NDRO RAM” : 570-ps Shuichi Nagasawa et. al. [IEEE Journal of Solid-State Shuichi Circuits, Vol 24, No 5, October, 1989] References References 4. “Design Of A 16-kbit Variable Threshold Josephson Design RAM”: I. Kurosawa, [IEEE Transactions On Applied RAM”: Superconductivity, Vol. 3, No.l, March1993] Superconductivity, 5. “Josephson Type Superconductive Tunnel Junctions Josephson and Applications” : Juri Matisoo [IEEE and TRANSACTIONS ON XAGNETICS, DECEMBER 1969] 6. http ://www.lne.fr/en/r_and_d/electrical_metrology/josephson_eff ://www.lne.fr/en/r_and_d/electrical_metrology/josephson_ef ...
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