Magnetic tunnel junctions (MTJ), i.e., structures consisting of two ferromagnetic layers (FM1 and FM2), separated by a very thin insulator barrier (I), have recently attracted attention for their large tunneling magnetoresistance (TMR) which appears when the magnetization of the ferromagnets of FM1 and FM2 changes their relative orientation from parallel to antiparallel in an applied magnetic field. Using an ultrahigh vacuum magnetron sputtering system, a variety of MTJ structures have been explored. Double Hc magnetic tunnel junction, NiFe/Al2O3/Co and FeCo/Al2O3/Co, were fabricated directly using placement of successive contact mask. The tunnel barrier was prepared by in situ plasma oxidation of thin Al layers sputter deposited. For NiFe/Al2O3/Co junctions, the maximum TMR value reaches 5.0% at room temperature, the switching field can be less than 10 Oe and the relative step width is about 30 Oe. The junction resistance changes from hundreds of ohms to hundreds of kilo-ohms and TMR values decrease monotonously with the increase of applied junction voltage bias. For FeCo/Al2O3/Co junctions, TMR values exceeding 7% were obtained at room temperature. It is surprising that an inverse TMR of 4% was observed in FeCo/Al2O3/Co. The physics governing the spin polarization of tunneling electrons remains unclear. Structures, NiFe/FeMn/NiFe/Al2O3/NiFe, in which one of the FM layers is exchange biased with an antiferromagnetic FeMn layer, were also prepared by patterning using optical lithography techniques. Thus, the junctions exhibit two well-defined magnetic states in which the FM layers are either parallel or antiparallel to one another. TMR values of 16% at room temperature were obtained. The switching field is less than 10 Oe and step width is larger than 30 Oe.

Magnetic random access memory (MRAM) possesses the attractive properties of non-volatility, radiation hardness, nondestructive readout, low voltage, high access speed, unlimited read and write endurance and high density. MRAM technology is described for the devices using giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) materials in this paper. The TMR type MRAM architectures using ferromagnetic tunneling junctions (MTJ) are more attractive for mainstream RAM applications than the GMR type, because the signal of the TMR type is larger than that of the GMR type. A MRAM device with an MTJ plus MOS transistor switch architecture, which can provide large signal-to noise ratio, is detailed. A design of the MTJ element is discussed and the requirements for the junction resistance and the TMR needed for the memory device are demonstrated based on the simple signal voltage calculations. The TMR significantly decreases with increasing bias voltage, which leads to the reduction of the signal voltage for the actual MRAM. A ferromagnetic double tunneling junction is proposed for the high density MRAM application demanding large signal voltage, because of the smaller degradation of the TMR for the bias voltage, compared with that of the conventional single junctions. Recent trials of MRAM fabrication are introduced, which demonstrates high-speed access time. Finally, challenges for the higher bit density MRAM above Gb are discussed, and it is noticed that higher signal voltage, lower power consumption for writing and novel cell designs are needed for the achievement.

A complex coefficient filter obtained by directly exchanging several reactance elements included in a real coefficient BPF for imaginary valued resistors is described. By using the proposed method, we obtain four varieties of complex coefficient filters. The stability problem is examined.

The features of the negative resistance in common source and common gate FET configurations for wideband VCO are studied. They are also explained by the simplified three-capacitor model. A design procedure is then developed. The results are applied to a design of wide band oscillator at the several gigahertz region.

This paper describes 1.0 V operation power performance of a double doped AlGaAs/InGaAs/AlGaAs heterojunction FET for personal digital cellular phones. The developed FET with a multilayer cap consisting of a highly Si-doped GaAs, an undoped GaAs and a highly Si-doped AlGaAs exhibited an on-resistance of 1.3 Ωmm and a maximum drain current of 620 mA/mm. A 28 mm gate-width device, operating with a drain bias voltage of 1.0 V, demonstrated an output power of 1.0 W, a power-added efficiency of 59% and an associated gain of 13.7 dB at an adjacent channel leakage power at 50 kHz off-center frequency of -48 dBc with a 950 MHz π/4-shifted quadrature phase shift keying signal.

Some experimental results on the contact phenomena that have been observed with electrical contacts operated to break an inductive DC load current in nitrogen atmosphere and in air (laboratory atmosphere) are presented. When Ag, Pd and Cu contacts were operated to break an inductive DC load current in the range of 1.0 A to 3.0 A in nitrogen atmosphere, more stable contact resistance characteristics were obtained, as compared to the case where operated in air, or at least the occurrence of unstableness and increase in contact resistance was delayed. The arc duration in nitrogen atmosphere became shorter in general than in air, especially with the Pd and Cu contacts. Voltage waveforms of arc discharges in the Ag and Pd contacts operated in nitrogen atmosphere showed a relatively clear step-like transition from the metallic phase to the gaseous phase as compared to the case where operated in air, while the Cu arcs did not show such significant differences due to the surrounding atmosphere. Although any apparent differences on the contact surface conditions in connection with the surrounding atmosphere were not clearly observed after the switching operations, the anode mass change characteristics were found to be more significantly affected by the difference of surrounding atmospheres than the cathode mass changes. The obtained experimental results suggest that the difference in the surrounding atmosphere introduce some difference in the anode surface morphology, possibly through the deposition of arc products in different deposition patterns, which will then result in differences in the contact resistance characteristics.

The constriction resistance of an electric contact has frequently been obtained using a model of only one circular contact spot of radius a. However, cases of a single contact spot are extremely rare as the interface of the electrical contact actually consists of numerous micro-contact spots. A contact is therefore regarded as the aggregate of several micro-contact spots, which are referred to collectively as a cluster. The constriction resistance of the cluster can be calculated as the sum of the self-resistance and mutual resistance of individual micro-contact spots. In the present study, this model is expanded slightly for practical application by normalizing a previous theoretical formula. In order to obtain the constriction resistance for contacts between composite materials and mating metals, EPMA analysis is applied so as to determine real micro-contact spots. Theoretical calculations of the constriction resistance of multiple contact spots is shown to be reasonably consistent with experimental results. In addition, the contact of a composite material and a mating metal is shown to be made up of multispots. The current was recognized experimentally to flow more easily at micro-contact spots in the cluster periphery. These experimental findings coincide with simulation results obtained by theoretical calculations.

Silicone contamination due to SiO2 caused by decomposition of silicone vapor is recognized as an undesirable phenomenon in electrical contact applications. The effects of silicone vapor adsorbed on the contact surface were examined by using micro relay contacts. The amount of SiO2 formed by the decomposition of silicone vapor is expected to depend on the amount of silicone vapor adsorbed on the contact surface. Hence, first of all, an increase in the thickness of the film from the adsorbed silicone vapor as a function of exposure time was clarified for the static state of the surface. The thickness of the film of adsorbed silicone vapor increased in proportion to exposure time and saturated at a thin monolayer. Moreover, in this exposure period, the thickness was affected by the concentration of the silicone vapor. After the thickness of the molecular layer saturated, the thickness of the layer was not influenced by the concentration of the silicone vapor. Next, from these results obtained by examination of exposure in the static state, the following is deducible. The silicone molecule adsorbs easily on the contact surface during the opening period of making and breaking contacts as well as in the static state. As the time the contacts are open determines the exposure time, the amount of adsorbed silicone molecules depends on the switching rate (operation per second). Contact failure due to increases in contact resistance might be affected by the switching rate in a silicone environment. Accordingly, contact resistance characteristic was examined over a wide range of switching rates. It was found that number of operations up to contact failure was affected markedly by the switching rate. Namely, the number of operations up to contact failure decreases as the switching rate increases. However, once a very thin layer such as the monolayer has formed, the film thickness ceases to grow. Accordingly, after the very thin layer is formed, the occurrence of contact failure does not depend on the concentration of silicone and the switching rate.

Even in today's industries that are predominated by solid-state switching devices, electromechanical devices with electrical contacts are still widely used for switching and/or conveying electrical signals and power. In this paper, some interesting topics in the investigation of electrical contacts, which were selected mainly from those presented at recently held international conferences or submitted to the related Transactions of IEEE and IEICE, are introduced. Specifically, some topics related to investigation regarding contact materials, new techniques for evaluating electrical contact phenomena, new understanding of the contact phenomena, and new applications of electrical contacts are briefly explained.

An electronic model of the coronary vessel consisting of resistor, capacitor and Field Effect Transistor (FET) is proposed in order to perform a dynamic simulation of the left coronary circulation and to clarify its mechanisms. Based on this model, an equivalent circuit of the coronary circulation is constructed that is divided into subepicardial and subendocardial layers and consists of segments of artery, arteriole, capillary, venule and vein for both the layers. In this simulation, the observed flow waveform of the left main artery showed dominance of flow in diastole as compared to that in systole. In epicardium, inverse venous flow was observed in early systole. These simulated waveforms are similar to those in real left coronary circulation observed by physiological and clinical researchers. Among all the segments of intramyocardium, only the venules were found to possess a time-varying resistance characteristics. From the results of this study, it is considered that the combination of resistance and capacitance of the vessel acts as an integrator and a differentiator for blood pressure and intramyocardial pressure, respectively and that the effects of integration of blood pressure and differentiation of intramyocardial pressure play a very important role in determining the factors influencing the left coronary circulation.

In this paper, a new frequency transformation for complex analog filter design which is suitable for integration is discussed. Arbitrary specified passband and stopband edges are easily transformed into those of the normalized LPF by solving simultaneous equations with four unknowns. Different from previous methods, the proposed transformation provides better performance in active realization of complex filters.

Measurement of the viscosity of liquid using a piezoelectric disk is described. Experiments with a radial expansion mode of a piezoceramic disk were carried out for water-glycerin mixture samples. Resonant resistance has linearity to the square root of the product of density and viscosity of a liquid around 113 kHz.

Hawkins noise model is modified for HBT application. The non-ideal ideality factor of HBT is included in both dynamic resistance and noise figure equations. Emitter resistance is also included. The extraction method of noise resistance Rn is developed. Based on the method, a simple analytic equation of Rn is derived and experimentally verified. The effects of noise sources on minimum noise figure are analyzed. The dominant noise sources are the shot noises of emitter and collector currents. Generally, when the minimum noise figure is measured at various current levels, there exists an current level at which the slope of minimum noise figure curve is zero. The zero slope current level coincides with the current level at which the noise contribution of the emitter and collector shot noises including the cancellation by correlation of two sources is minimum. Parasitic resistance degrades output noise through the shot noise amplification with a minor effect of the thermal noise of itself.

We study ultrafast operation of multiple-valued quantizers composed of resonant-tunneling diodes (RTDs) and high electron mobility transistors (HEMTs). The operation principle of these quantizers is based on the monostable-multistable transition logic (MML) of series-connected RTDs. The quantizers are fabricated by monolithically integrating InP-based RTDs and 0.7-µm-gate-length HEMTs with a cutoff frequency of 40 GHz. To perform high-frequency experiments, an output buffer and termination resistors are attached to the quantizers, and the quantizers are designed to accommodate high-frequency input signals. Our experiments show that both ternary and quaternary quantizers can operate at clock frequencies of 10 GHz and at input frequencies of 3 GHz. This demonstrates the potential of applying RTD-based multiple-valued quantizers to high-frequency circuits.

A novel method of extraction of emitter, Re, and base, Rb, resistances of bipolar junction transistors, BJTs, is proposed. Re and Rb are obtained from static characteristics and noise power spectral density of low frequency, 1/f, fluctuations, measured in the base and collector currents of the devices. Measurements carried out on quasi self-aligned silicon BJTs show that Re and Rb values obtained by the proposed method scale correctly with transistor dimensions and match the values estimated from the device layout.

An advanced characterization method for sub-micron DRAM cell transistors has been proposed for the analysis of transistor test structures using memory cell patterns. When the actual memory cell layout is used as a test structure, the parasitic source and drain resistance of the test structure affected conventional transistor parameters such as threshold voltage. To solve this problem, reduced drain current measurement methods have been proposed to suppress the parasitic resistance voltage drop. In these measurements, two new transistor parameters, Vgoff and Vgsat, have been proposed which are related to off-leakage and full writing, respectively. These parameters are found to be good parameters for monitoring DRAM bit failures. A new threshold voltage measurement methodology has also been proposed for test structures with high parasitic resistance.

Contact resistance of nickel hardened gold electroplate (NiHG) deposited on nickel-underplated phosphor bronze disk coupons (substrate) after thermal aging was measured with a hard gold-plated beryllium copper alloy pin probe by means of a four-point probe technique, compared to that of cobalt-hardened gold electroplate (CoHG). Surface of NiHG plated coupons after aging was analyzed by X-ray photoelectron spectroscopy (XPS) to investigate the influence of the oxide film formation during thermal aging on contact resistance of NiHG electroplate, compared to that of CoHG. Initial contact resistance of the NiHG coupons was less than 10 mΩ at a contact forces more than 0.05 N, increased to 10 mΩ at a contact force of 0.05 N after 100 hours aging at 200. In contrast, contact resistance of the CoHG coupons progressively increased with increase in aging time, reached 1000 mΩ even at a contact force of 0.05 N after 52 hours aging. XPS analysis for the NiHG coupons demonstrated that nickel oxide film was formed on the NiHG surface in conformity with parabolic growth kinetics, as cobalt oxide film formed on CoHG surface. However, a thickness of the latter film was approximately 4-fold larger than that of former after 100 hours aging at 200. The small increase in contact resistance of NiHG coupons after aging suggested to be due to inhibitory of nickel oxide film growth on the surface. The cause of relatively low and steady contact resistance of NiHG during thermal aging was discussed.

The applicability of composite materials containing laminar solid lubricants to sliding contacts was studied. Performances of several composite materials prepared by incorporating solid lubricants with the basic alloys of the Cu-Nb system and Cu-Sn system were investigated to test the suitability of the composite materials as sliding contacts. As a result, it was clarified that the composite materials based on Cu-Sn alloy were superior to those based on Cu-Nb alloy and those containing only WS2 and not MoS2 were more effective in reducing both the contact resistance and the coefficient of friction. Based on the relationship between the contact resistance and the coefficient of friction obtained in this experimental study, the author proposed a new model for electric contact of composite materials.

Simple expressions for constriction resistance of multitude conducting spots were analytically formulated by Greenwood. These expressions, however, include some approximations. Nakamura presented that the constriction resistance of one circular spot computed using the BEM is closed to Maxwell's exact value. This relative error is only e=0. 00162 [%]. In this study, the constriction resistances of two, five and ten conducting spots are computed using the boundary element method (BEM), and compared with those obtained using Greenwood's expressions. As the conducting spots move close to each other, the numerical deviations between constriction resistances computed using Greenwood's expressions and the BEM increase. As a result, mutual resistance computed by the BEM is larger than that obtained from Greenwood's expressions. The numerical deviations between the total resistances computed by Greenwood's expressions and that by the BEM are small. Hence, Greenwood's expressions are valid for the total constriction resistance calculation and can be applied to problems where only the total resistance of two contact surfaces, such as a relay and a switch, is required. However, the numerical deviations between the partial resistances computed by Greenwood's expression and that by the BEM are very large. The partial resistance calculations of multitude conducting spots are beyond the applicable range of Greenwood's expression, since Greenwood's expression for constriction resistance of two conducting spots is obtained by assuming that the conducting spots are equal size. In particular, the deviation between resistances of conducting spots, which are close to each other, is very large. In the case of partial resistances which are significant in semiconductor devices, Greenwood's expressions cannot be used with high precision.

In this paper, we describe the implementation of the proposed single user type CDMA adaptive interference canceller (AIC) with RAKE structure in the developed testbed for the base station, and evaluate its performance in the multiuser and multipath fading environment. Laboratory experiment demonstrates that the AIC receiver is much more near-far resistant than the conventional matched filter (MF) receiver in the multiuser case. When the power of the other users is 6 dB larger than that of the desired user, the AIC receiver can achieve the BER of 10-3 at C/PG = 33. 3 % in the 2-path fading channel, while the MF receiver cannot achieve the BER at C/PG of more than 20. 8%. Furthermore, we evaluate the effect of transmission power reduction in the transmitter with transmission power control (TPC). The experimental result shows that the required transmission power can be greatly reduced by 3. 0 dB and 9. 2 dB with the AIC receiver at C/PG = 29. 2 % and 33. 3%, respectively.