Recent advances of superconducting single-flux-quantum (SFQ) circuit technology make it attractive to investigate computing systems using SFQ circuits, where arithmetic circuits play important roles. In order to develop excellent SFQ arithmetic circuits, we have to design or select their underlying algorithms, called hardware algorithms, from different point of view than CMOS circuits, because SFQ circuits work by pulse logic while CMOS circuits work by level logic. In this paper, we compare implementations of hardware algorithms for addition by synchronous-clocking SFQ circuits. We show that a set of individual bit-serial adders and Kogge-Stone adder are superior to others.

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We demonstrated an automated passive-transmission-line routing tool for single-flux-quantum (SFQ) circuits. The tool is based on the A^* algorithm, which is widely used in CMOS LSI design, and tuned for microstrip/strip lines formed in the SRL 4-Nb layer structure. In large-scale SFQ circuits with 10000-20000 Josephson junctions, such as microprocessors, 80-90% of the wires can be automatically routed in about ten minutes. We verified correct operation above 40GHz for an automatically routed 4 × 4 switch circuit from on-chip high-speed tests. The resulting circuit size and operating frequency were comparable to those of a manually designed result. We believe that the tool is useful for large-scale SFQ circuit design using conventional fabrication processes.

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A single flux quantum (SFQ) logic cell library has been developed for the 10kA/cm² Nb multi-layer fabrication process to efficiently design large-scale SFQ digital circuits. In the new cell library, the critical current density of Josephson junctions is increased from 2.5kA/cm² to 10kA/cm² compared to our conventional cell library, and the McCumber-Stwart parameter of each Josephson junction is increased to 2 in order to increase the circuit operation speed. More than 300 cells have been designed, including fundamental logic cells and wiring cells for passive interconnects. We have measured all cells and confirmed they stably operate with wide operating margins. On-chip high-speed test of the toggle flip-flop (TFF) cell has been performed by measuring the input and output voltages. The TFF cell at the input frequency of up to 400GHz was confirmed to operate correctly. Also, several fundamental digital circuits, a 4-bit concurrent-flow shift register and a bit-serial adder have been designed using the new cell library, and the correct operations of the circuits have been demonstrated at high clock frequencies of more than 100GHz.

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Superconducting Quantum Interference Devices (SQUIDs) are known to be the most sensitive magnetometers, used in a wide range of applications like biomagnetism, geomagnetism, Non Destructive Evaluation (NDE), metrology or fundamental science. For all these applications, the SQUID sensor is used in analog mode and associated with a carefully designed room-temperature control and/or feedback electronics. Nevertheless, the use of SQUID sensors in digital mode is of high interest for several applications due to their quantum accuracy associated to high linearity, and their potentially very high slew rate and dynamic range. The concept and performances of a low-Tc digital magnetometer based on Single-Flux-Quantum (SFQ) logic, fabricated at the FLUXONICS Foundry located at IPHT Jena, Germany, are given after a presentation of the context of development of superconductive digital magnetometers. The sensitivity, limited to one magnetic single flux quantum, and a dynamic range of 76dB, that corresponds to an upper limit of the magnetic field amplitude higher than 5µT, have been measured along with overnight stability. The dynamic range of about 2800 magnetic flux quanta Φ₀ has been experimentally observed with an external magnetic field. First signatures of magnetic fields have been observed simultaneously with the ones of analog SQUIDs in the low noise environment of the Laboratoire Souterrain à Bas Bruit (LSBB) located in Rustrel, Provence, France.

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A physical random number generator, which generates truly random number trains by using the randomness of physical phenomena, is widely used in the field of cryptographic applications. We have developed an ultra high-speed superconductive physical random number generator that can generate random numbers at a frequency of more than 10GHz by utilizing the high-speed operation and high-sensitivity of superconductive integrated circuits. In this study, we have statistically evaluated the quality of the random number trains generated by the superconductive physical random number generator. The performances of the statistical tests were based on a test method provided by National Institute of Standards and Technology (NIST). These statistical tests comprised several fundamental tests that were performed to evaluate the random number trains for their utilization in practical cryptographic applications. We have generated 230 random number trains consisting of 20, 000-bits by using the superconductive physical random number generator fabricated by the SRL 2.5kA/cm² Nb standard process. The generated random number trains passed all the fundamental statistical tests. This result indicates that the superconductive random number generator can be sufficiently utilized in practical applications.

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We have been studying a superconducting quantum-computing system where superconducting qubits are controlled and read out by rapid single-flux- quantum (RSFQ) circuits. In this study, we designed and fabricated an RSFQ microwave chopper, which turns on and off an externally applied microwave to control qubit states with the time resolution of sub-nanosecond. The chopper is implemented in a microwave module and mounted in a dilution refrigerator. We tested the microwave chopper at 4.2K. The amplitude of the output microwave was approximately 100μV which is much larger than that of previously designed chopper. We also confirmed that the irradiation time can be controlled by RSFQ control circuits.

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We present a technology based on Nb/Nb_xSi_1-x/Nb junctions, with barriers near the metal-insulator transition, for applications in superconducting electronics (SCE) as an alternative to Nb/AlO_x/Nb tunnel junctions. Josephson junctions with co-sputtered amorphous Nb-Si barriers can be made with a wide variety of electrical properties: critical current density (J_c), capacitance (C), and normal resistance (R_n) can be reliably selected within wide ranges by choosing both the barrier thickness and Nb concentration. Nonhysteretic Nb/Nb_xSi_1-x/Nb junctions with I_cR_n products greater than 1mV, where I_c is the critical current, and J_c values near 100kA/cm² have been fabricated and are promising for superconductive digital electronics. These barriers have thicknesses of several nanometers; this improves fabrication reproducibility and junction uniformity, both of which are necessary for complex digital circuits. Recent improvements to our deposition system have allowed us to obtain better uniformity across the wafer.

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All MgB₂ Josephson junctions with amorphous boron barriers have been fabricated on C-plane sapphire substrates by using a co-evaporation method. The junctions showed Josephson currents and the nonlinear current-voltage characteristics which seem to reflect the superconducting energy gap. The critical current was observed when the thickness of the amorphous boron was in the range of 5nm to 20nm. The critical current density was estimated to be 0.4A/cm² to 450A/cm². By observing he temperature dependence of the critical current we found that the junction had a critical temperature of 10K and a normal layer in its barrier structure.

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Low noise terahertz (THz) receivers based on superconducting niobium nitride (NbN) hot electron bolometer (HEB) mixers have been designed, fabricated and measured for applications in astronomy and cosmology. The NbN HEB mixer consists of a planar antenna and an NbN bridge connecting across the antenna's inner terminals on a high-resistivity Si substrate. To eliminate the influence of direct detection and instability of the local oscillation (LO) power, a wire grid has been used to change the input LO power for compensating the shift of bias current during Y-factor measurement. The double sideband (DSB) receiver noise temperatures at 4.2K without corrections have been measured from 0.65 to 3.1THz. The excess quantum noise factor β of about 4 has been obtained, which agrees well with the calculated value. Allan variance of the HEB has been characterized, and Allan time T_A longer than 0.4s is obtained. We also estimated the temperature resolution of the HEB from the Allan variance and obtained the minimum temperature resolution of 1.1K using a Gunn oscillator with its multipliers at 0.65THz as an LO source.

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A design of the tunable superconducting power filter is described. The filter consists of superconducting microwave cavities with a mechanical tuning method. The electromagnetic simulations using niobium cavity suggested that there were conditions where the resonator with high-unloaded Q can realize a fractional center frequency change of more than ± 10% by using a Nb rod moving in the cavity. The simulations approximated the resonant frequency dependence of the rod moved by a cryogenic actuator in the tunable cavity experiment. In addition, the simulation of the power handling capability showed a feasibility of the value more than 50dBW.

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We present analytical expression for inductance of a superconducting stripline, a strip sandwiched by two superconducting ground planes. In our method, we utilize the analytical formula for a perfect-conducting stripline derived by Chang in 1976. To utilize Chang's formula, we first transform the structure of a superconducting stripline into that of a perfect-conducting stripline by reducing the thicknesses of the superconducting layers. The thickness reduction is “λcoth (t/λ)” for each (upper or lower) side, where λ and t are the field penetration depth and the layer thickness, respectively. Then, we apply Chang's formula to the transformed stripline model. The calculated results are in good agreement with the numerical and experimental results.

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Nonlinear distortions in power amplifiers (PAs) generate spectral regrowth at the output, which causes interference to adjacent channels and errors in digitally modulated signals. This paper presents a novel method to evaluate adjacent channel leakage power ratio (ACPR) and error vector magnitude (EVM) from the amplitude-to-amplitude (AM/AM) and amplitude-to-phase (AM/PM) characteristics. The transmitted signal is considered to be complex Gaussian distributed in orthogonal frequency-division multiplexing (OFDM) systems. We use the Mehler formula to derive closed-form expressions of the PAs output power spectral density (PSD), ACPR and EVM for memoryless PA and memory PA respectively. We inspect the derived relationships using an OFDM signal in the IEEE 802.11a WLAN standard. Simulation results show that the proposed method is appropriate to predict the ACPR and EVM values of the nonlinear PA output in OFDM systems, when the AM/AM and AM/PM characteristics are known.

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We propose a proof-of-concept of a switchable multi-frequency MMIC (monolithic microwave integrated circuit) oscillator device, operating in the 60GHz millimeter wave band, which is implemented in GaAs p-HEMT transistor technology. Oscillators that can switch between two frequencies have been designed, fabricated and evaluated. The oscillator uses a cross-coupled FET topology, combined with a bent asymmetric coplanar stripline for the resonator, and a switched-capacitor for the frequency switching components. The oscillator generates two oscillations at ƒ/2 and ƒ where ƒ is the target frequency of around 60GHz. The switchable oscillator has been demonstrated for the range of frequency from 44GHz to 68.9GHz. Moreover, the designed oscillator exhibits a wide-band negative resistance property that allows fabricating switchable oscillators covering the 50 to 75GHz V-band. An evaluated switchable oscillator delivers -17.09dBm and -13.72dBm output power at 62.45GHz and 64.78GHz, for a supplied power of 40.6mW and 39.1mW, respectively.

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This paper presents a real integration of a 5.8-GHz injection-locked quadrature local oscillator that includes two LC-tuned injection-locked frequency dividers (ILFDs) and a wide-tuning stacked-transformer feedback voltage-controlled oscillator (VCO) operated in double frequency. A symmetric differential stacked-transformer with a high coupling factor and a high quality factor is used as a feedback component for the wide-tuning VCO design. The wide tuning range, which is greater than three times the desired bandwidth, is achieved by selecting a greater tuning capacitance ratio available from high-voltage N-type accumulation-mode MOS varactors and a smaller self-inductance stacked-transformer. Since the quality factors of the LC-resonator components can sustain at a high enough level, the wide-tuning VCO does not suffer from the phase noise degradation too much. In addition, the tuning range of the local oscillator is extended simultaneously by utilizing switched capacitor arrays (SCAs) in the ILFDs. The circuit is implemented by TSMC's 0.18-µm RF CMOS technology. At a 1-V power supply, the whole integrated circuit dissipates 6.72mW (4.05mW for the VCO and 2.67mW for the two ILFDs). The total tuning range frequency is about 500MHz (from 5.54GHz to 6.04GHz) when the tuning voltage V_tune ranges from 0V to 1.8V. At around the output frequency of 5.77 GHz (at V_tune =0.5V), the measured phase noise of this local oscillator is −119.4dBc/Hz at a 1-MHz offset frequency. This work satisfies the specification requirement for IEEE 802.11a UNII-3 band application. The corresponding figure-of-merit (FOM) calculated is 186.3dB.

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A multiple-peak negative differential resistance (NDR) circuit made of standard Si-based metal-oxide-semiconductor field-effect-transistor (MOS) and SiGe-based heterojunction bipolar transistor (HBT) is demonstrated. We can obtain a three-peak I-V curve by connecting three cascoded MOS-HBT-NDR circuits by suitably designing the MOS parameters. This novel three-peak NDR circuit possesses the adjustable current-voltage characteristics and high peak-to-valley current ratio (PVCR). We can adjust the PVCR values to be as high as 11.5, 6.5, and 10.3 for three peaks, respectively. Because the NDR circuit is a very strong nonlinear element, we discuss the extrinsic hysteresis phenomena in this multiple-peak NDR circuit. The effect of series resistance on hysteresis phenomena is also investigated. Our design and fabrication of the NDR circuit is based on the standard 0.35μm SiGe BiCMOS process.

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Electrical contacts are an important part of electrical circuits and many reliability problems are related to electrical contact failure. It is important to investigate the relationship between load and contact resistance which is an important factor of contact reliability. In this study, the effect of plated material and plated thickness on contact resistance was examined. The samples were constructed of a copper alloy with tin or silver plating. Contact configuration was hemispherical-flat contact. The contact resistance was measured by using a four-probe method with a load up to 40N. The relation between indentation contact area (i.e. apparent contact area) and contact resistance was determined. As experimental results, the contact resistance depends on the indentation of the contact area. In the same contact area, tin-plated samples have higher resistance than those that are silver-plated due to their own resistivity. The constriction resistance of a plated layer, which depends on contact area, plated material and plated thickness, is analyzed by a theoretical solution, which is shown by R=Φρ/2a, using a surface resistance coefficient Φ. The theoretical results show almost good agreement with the experimental results. Thus, the indentation contact area (i.e. apparent contact area) is almost the same as the real contact area in this study.

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To achieve conductive and wear-durable carbon thin films by metal doping, we deposited Au-, Pt-, and Pd-doped carbon thin films by RF sputtering, and evaluated the dopant concentrations, resistivity, and scratch hardness. Among the doped films, the Pt-doped film with low Pt concentration was most suitable from a practical perspective.

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