Memory Designing Using Josephson Gates Susmit Biswas 02/07/2006

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  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  High Density and low power  but Slower than SRAM

DRAM Performance (August 2005)

Need For New Technology  Memory is the main bottleneck now  Multiprocessor system suffers most  SIMD and MIMD architecture  Data hungry

Josephson Memory: Previous Work  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  1962:  1962: Josephson predicted that a sandwich of S-I-S will 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 I J is exceeded   The magnitude of I J, depends sensitively on magnetic fields. A voltage V dc, 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   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   Bound pairs electrons all lie near the Fermi energy E F of the normal metal; the resulting pairs are in an energy state lower than E F by an amount Δ (binding energy of the pair (per electron)   As T becomes less than T c, 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 Effect: 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 insulating barrier placed between two superconductors, the whole is called Josephson junction.

Josephson Effect : Summary  DC Josephson effect: If no voltage is applied to the junction terminals, a direct current - a current of Cooper pairs I j, - flows through the junction up to a critical value I c, which depends on the geometry, temperature and magnetic field.

Josephson Effect : Summary  AC Josephson effect: If a direct voltage is applied to 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 Planck constant h) : f = 2eV/h Conversely, if an AC voltage of frequency is applied to the junction terminals by microwave irradiation, the current of Cooper pairs tends to synchronize with this frequency (and its harmonics) and a direct voltage appears at the junction terminals.

Josephson Junction Characteristics  Control currents I c,  Josephson threshold I m.  Gate current I g, I-V CurveThreshold Curve

Josephson Junction As Memory  Consists of a loop with three Josephson junctions in series that encloses a magnetic flux Ф driven by an external magnet.  The loop may have multiple stable persistent current states when the enclosed magnetic flux is close to half a superconducting flux quantum Ф Ф = h / 2e  System has two stable states  ׀ 0 › and ׀ 1 › with opposite circulating persistent currents

Josephson Junction As Memory ( cont.)  Operated by resonant microwave modulation of the 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 external magnetic flux generated by the circulating currents on the surrounded loops:  0 if magnetic field is < 1/2 Ф  1 if magnetic field is > 1/2 Ф  The state of the system is the superposition of all the states generated by the circulating current in each loop.

Josephson Junction As Memory ( cont.)  Combining several junctions results in different gates e.g. inverter  Can be designed in two ways  coupling two superconductive loops directly through magnetic interference  Coupling two loops through a superconductive flux transporter

Josephson Junction As Memory ( cont.)  Stronger interaction between the PC loops and better coupling to each other with the facilitation of transporter  But!  Coupling between neighboring loops makes it difficult for long-range communication  Solution  Transporter: fast data propagation

Josephson Junction As Memory ( cont.) NMV Gate can serve as NAND, NOR and NOT gate by setting instruction bits. Not Majority Vote (NMV) Gate

Memory Designing Using Josephson Gate

Memory Designing Using Josephson Gate (cont.)  A memory cell can not be refreshed by either a row or a column addressing line independently  the addressing lines are designed in such a way that 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.

Performance Evaluation  Pros:  Speed: 750GHz for single asynchronous cells and up to 320GHz for LSI devices  Low power consumption 0.2nanowatt/GHz per pulse and 0.1mW for LSI devices  Simple fabrication technology : lithography  Cons:  Low density  Operational temperature <20K

Performance Evaluation (cont.) Comparison of projected 2.5μm technology Josephson NDRO and DRO chip designs with advanced silicon memories having comparable line widths.

Conclusion  Josephson memory can become more and more popular because of its speed and low power characteristics  Designing larger memory is difficult  Low density  Limitation of fabrication technology

References 1. “Novel Computing Architecture on Arrays of Josephson Persistent Current Bits” : Jie Han, Pieter Jonker [Proc. MSM 2002 ] 2. “Memory-Cell Design in Josephson Technology” : Hans H. Zmpe [IEEE Transactions On Electron Devices, VOL. ED-27, NO. 10, OCTOBER 1980] 3. “570-ps 13-mW Josephson 1kbit NDRO RAM” : Shuichi Nagasawa et. al. [IEEE Journal of Solid-State Circuits, Vol 24, No 5, October, 1989]

References 4. “Design Of A 16-kbit Variable Threshold Josephson RAM”: I. Kurosawa, [IEEE Transactions On Applied Superconductivity, Vol. 3, No.l, March1993] 5. “Josephson Type Superconductive Tunnel Junctions and Applications” : Juri Matisoo [ 5. “Josephson Type Superconductive Tunnel Junctions and Applications” : Juri Matisoo [IEEE TRANSACTIONS ON XAGNETICS, DECEMBER 1969] 6. hson_effect_ej.shtml hson_effect_ej.shtml hson_effect_ej.shtml