Electrical characteristics of MOSFETs using low-pressurechemical-vapor-deposited oxide

The electrical characteristics of MOSFETs and MOS capacitors utilizing thin (80-230 Å) low-pressure chemical-vapor-deposited (LPCVD) oxide films deposited at 12 Å/min are presented. MOSFETs using CVD oxides show good electrical characteristics with 70-90% of the surface mobility of conventional MOSFETs. The CVD oxides exhibit the same low leakage current and high breakdown fields as the thermal oxides, and significantly lower trapping and trap generation rates than thermally grown oxides. Interface state densities of ⩽3×10¹⁰ cm^-2 eV^-1 are obtained from CVD devices by using a short annealing in oxygen ambient following the deposition. These results indicate that these LPCVD oxide films may be promising dielectrics for MOS device application     

Review of microwave distributed superconducting vortex-flow transistor amplifiers

The dynamics of fluxons in the vortex flow transistor (VFT) has been much studied. The fluxon transit time determines the fundamental speed limit of operation. Since fluxons can travel at the velocity of electromagnetic waves in the junction, the VFT has the potential for operating in high frequency systems ( ≈ 100 GHz). However, owing to the low input and output impedance of the VFT, use of the device in a conventional circuit would be quite limited. A distributed amplifier configuration consisting of many VFTs has been proposed to remedy the problems of low impedance levels. However, the realization of such an amplifier circuit at microwave or millimetre wave frequencies depends on obtaining a circuit model. In this paper, microwave superconducting VFT distributed amplifiers using the balanced control technique is reviewed. This kind of amplifier has the advantage that the capacitive feedthrough effect is decreased to a negligible extent. This is the major limiting factor for high frequency applications. The self-field effect which makes the current step inclined is reduced by injecting bias current only around the region of one end of the junction thus obtaining steeper I- V characteristics. With the asymmetric geometry, the slope of the current step is about one hundred times steeper than those obtained with the conventional overlap geometry. Owing to the diamagnetic behaviour of the Josephson junction, a little of the magnetic field induced by the current in the control line is allowed to penetrate the junction. Thus, the transresistance r_m of a VFT is very small. Some methods for maximizing r_m and minimizing the output impedance r₀ also appear in this review. The feasibility of fabricating VFT distributed amplifiers using low-T_c superconducting material has been demonstrated. The power gain of the amplifier can be as high as 15 dB with a flat frequency response.     

Highly Crystalline Soluble Acene Crystal Arrays for Organic Transistors: Mechanism of Crystal Growth During Dip‐Coating

The preparation of uniform large‐area highly crystalline organic semiconductor thin films that show outstanding carrier mobilities remains a challenge in the field of organic electronics, including organic field‐effect transistors. Quantitative control over the drying speed during dip‐coating permits optimization of the organic semiconductor film formation, although the kinetics of crystallization at the air–solution–substrate contact line are still not well understood. Here, we report the facile one‐step growth of self‐aligning, highly crystalline soluble acene crystal arrays that exhibit excellent field‐effect mobilities (up to 1.5 cm V^?1 s^?1) via an optimized dip‐coating process. We discover that optimized acene crystals grew at a particular substrate lifting‐rate in the presence of low boiling point solvents, such as dichloromethane (b.p. of 40.0 °C) or chloroform (b.p. of 60.4 °C). Variable‐temperature dip‐coating experiments using various solvents and lift rates are performed to elucidate the crystallization behavior. This bottom‐up study of soluble acene crystal growth during dip‐coating provides conditions under which one may obtain uniform organic semiconductor crystal arrays with high crystallinity and mobilities over large substrate areas, regardless of the substrate geometry (wafer substrates or cylinder‐shaped substrates).     

Permalloy loaded transmission lines for high-speed interconnect applications

The mutual inductance and self-inductance of global interconnects are important but difficult to extract and model in deep submicrometer very large scale integration (VLSI) designs. The absence of effective mutual magnetic field shielding limits the maximum unbuffered interconnect line length. In this paper, we propose and demonstrate that permalloy-loaded transmission lines can be used for high-speed interconnect applications to overcome these limitations. Permalloy films were incorporated into planar transmission lines using a CMOS-compatible process. The line characteristics show that eddy-current effects are the limiting factors for the high-frequency permalloy applications when ferromagnetic resonance are restrained through geometry design. Patterning permalloy films effectively extends their application to above 20 GHz. The line characteristic impedances are about /spl sim/90 /spl Omega/. Under 50 mA dc current biases, the line parameters did not change much. Moreover, the patterned permalloy reduces the magnetic field coupling between two adjacent transmission lines by about 10 dB in our design. The demonstrated operation frequency range, current carrying capability and magnetic field shielding properties indicate that the permalloy loaded lines are suitable for high-speed interconnect applications in CMOS technologies.     

Thin film Y-Ba-Cu-O/Ag composites for fluxonic devices

We have investigated thin film composites of YBa₂Cu₃O₇ (YBCO) with Ag for fluxonic device applications. YBCO/Ag composite films are produced by first depositing a layer of Ag onto a substrate and then heating the film to the YBCO deposition temperature of 680°C or higher. YBCO is deposited by off-axis sputtering onto the Ag-coated substrate. The resulting YBCO/Ag film is a composite of YBCO with well-defined Ag regions several microns in size. Scanning electron micrograph images of the films' surfaces show a background of smooth YBCO grains dotted with Ag clusters. For a wide range of increasing Ag composition, the transition temperatures of the composite films on SrTiO₃ remain high, while the critical current densities have been reduced as much as 65 times. On MgO substrates, critical current density has been reduced by more than four orders of magnitude. Also on MgO, significant voltage response is seen in external magnetic fields of less than 1 mT. These measurements suggest that the films may be arrays of superconductor-normal-superconductor (SNS) junctions formed by weakly coupled YBCO grains with Ag in the grain boundaries. The field responsivity and low critical current densities of these composites make them potentially useful for fabrication of fluxonic devices     

Modeling of the optical characteristics for twin-channel laser (TCL) structures

The twin-channel laser (TCL) structure was the first laser design which incorporated the use of optical gain in the regions between the elements of an array-type device. In this paper, we describe the important parameters affecting the performance of TCL devices and extend our concepts to multielement (n > 2) laser arrays. Our calculations indicate that the presence of a uniform gain distribution over the width of the array is necessary for the excitation of the fundamental array mode and to achieve a single lobe far field. Secondly, lateral array mode stability is drastically reduced for arrays having many elements (n > 2) and will be difficult to achieve in practice. Lastly, we find that the near-field intensity in laser array structures is more spatially sensitive to asymetric perturbations induced by either current or geometry nonuniformities than single-element devices. We believe that some of these problems can possibly be minimized by the use of a new laser array geometry which incorporates an unequal number of array elements along the cavity length in order to spatially filter the unwanted array modes.     

Nonlithographic nano-wire arrays: fabrication, physics, and deviceapplications

A novel system of nanostructures is described consisting of nonlithographically produced arrays of nano-wires directly electrodeposited into porous anodic aluminum oxide templates. Using this method regular and uniform arrays of metal or semiconductor nano-wires or nano-dots can be created with diameters ranging from ~5 nm to several hundred nanometers and with areal pore densities in the ~10⁹-10¹¹ cm^-2 range. We report on the present state of their fabrication, properties, and prospective device applications. Results of X-ray diffraction, Raman and magnetic measurements on metal (Ni, Fe) and semiconductor (CdS, CdSe, CdS_x Se_1-x, Cd_xZn_1-xS and GaAs) wires are presented. The I-V characteristics of two terminal devices made from the nano-arrays are found to exhibit room temperature periodic conductance oscillations and Coulomb-blockade like current staircases. These observations are likely associated with the ultra-small tunnel junctions that are formed naturally in the arrays. Single-electron tunneling (SET) In the presence of interwire coupling in these arrays is shown to lead to the spontaneous electrostatic polarization of the wires. Possible device applications such as magnetic memory or sensors, electroluminescent flat-panel displays, and nanoelectronic and single-electronic devices are also discussed     

A very-high impedance, high-swing cascode stage for sub-micronanalog VLSI

In this article we describe a novel sub-micron analog VLSI circuit which can be used as a cascode current source, current mirror, or transresistance stage amplifier with maximum output swing capability. The described circuit, which is also viewed as a “super-MOS” device, displays both gain-boosting and optimum biasing of the cascode stage for low-voltage applications     

Field response of ultra-thin type II superconducting transmissionlines

This work describes the effect of a tangential magnetic field on the superconducting penetration depth, λ, as modeled by the theory of Ginzburg and Landau. In particular, an increasing magnetic field decreases the magnitude of the order parameter Ψ. Consequently, the London equations have been modified to include field dependent values of the penetration depth, λ(T,H) and complex conductivity, σ(T,H). The analysis assumes that the superconducting films are free of vortices. For Type II films, this criteria can only be met when the thickness of the conductors is less than 1.8ξ_GL. The G-L theory is used to find closed form solutions that describe the electromagnetic characteristics of a kinetic inductance transmission line including phase velocity, impedance, and energy loss, as functions of power, temperature and magnetic fields. Limitations of power are also discussed. To validate these concepts, experiments verifying the effects of temperature and bias field were implemented using niobium superconductors 500 Å thick separated by 1000 Å of Al₂O₃. Potential device applications include variable phase-shifters, tunable filters, and extremely sensitive bolometers     

Influence of Electric Field on Microstructures of Pentacene Thin‐Films in Field‐Effect Transistors

We report on electric‐field‐induced irreversible structural modifications in pentacene thin films after long‐term operation of organic field‐effect transistor (OFET) devices. Micro‐Raman spectroscopy allows for the analysis of the microstructural modifications of pentacene in the small active channel of OFET during device operation. The results suggest that the herringbone packing of pentacene molecules in a solid film is affected by an external electric field, particularly the source‐to‐drain field that parallels the a–b lattice plane. The analysis of vibrational frequency and Davydov splitting in the Raman spectra reveals a singular behavior suggesting a reduced separation distance between pentacene molecules after long‐term operations and, thus, large intermolecular interactions. These results provide evidence for improved OFET performance after long‐term operation, related to the microstructures of organic semiconductors. It is known that the application of large electric fields alters the semiconductor properties of the material owing to the generation of defects and the trapping of charges. However, we first suggest that large electric fields may alter the molecular geometry and further induce structural phase transitions in the pentacene films. These results provide a basis for understanding the improved electronic properties in test devices after long‐term operations, including enhanced field‐effect mobility, improved on/off current ratio, sharp sub‐threshold swing, and a slower decay rate in the output drain current. In addition, the effects of source‐to‐drain electric field, gate electric field, current and charge carriers, and thermal annealing on the pentacene films during OFET operations are discussed.     

Analysis of current voltage characteristics oflow-temperature-processed polysilicon thin-film transistors

The relationship between device performance and trap state density in polysilicon films was investigated. The density in the silicon energy gap was obtained by fitting the calculated on-state current versus gate voltage curve to the measured one for low-temperature (⩽600°C) processed polysilicon TFTs fabricated under various conditions, such as different deposition temperatures and annealing methods for crystallization. On-state current was markedly improved by reducing the density near band edges in the gap, and the improvement was realized by depositing the films at around 500°C in an LPCVD system or employing laser annealing instead of thermal annealing at 600°C. Off-state current was reduced to a great extent by reduction of the density around the midgap by using a plasma-hydrogenation technique     

Hierarchical Directed Self‐Assembly of Diblock Copolymers for Modified Pattern Symmetry

Block copolymer lithography exploiting diblock copolymer thin films is promising for scalable manufacture of device‐oriented nanostructures. Nonetheless, its intrinsic limitation in the degree of freedom for pattern symmetry within hexagonal dot or parallel line array greatly diminishes the potential application fields. Here, we report multi‐level hierarchical self‐assembled nanopatterning of diblock copolymers for modified pattern symmetry. Sequential hierarchical integration of two layers of diblock copolymer films with judiciously chosen molecular weights and chemical composition creates nanopatterned morphology with modified pattern symmetry, including sparse linear cylinder or lamellar arrays. Internal structure of the hierarchically patterned morphology is characterized by grazing‐incidence small‐angle X‐ray scattering throughout the film thickness. Pattern transfer of the modified nanopattern generates linear metal nanodot array with uniform size and regular spacing as a typical example of functional nanopatterned structures.     

Pulsars as probes of the interstellar medium

Pulsar observations offer an opportunity to determine the distributions of the magnetic field, electron density, and neutral atomic hydrogen density in the interstellar space. Pulse polarization and time of arrival measurements indicate a relatively uniform magnetic field of about 3 µG, whereas new distance measurements through 21-cm line absorption indicate an electron density of about 0.03 cm^-3which is uniform when averaged over large distances (a kiloparsec or more).     

Characteristic modes for a slot in a conducting plane, TM case

Consider an infinitely long slot in a conducting plane in an unbounded medium illuminated by a uniform transverse magnetic (TM) (to the slot axis) plane wave. The theory of characteristic modes for apertures is applied to solve the problem. For a narrow slot, analytic expressions for the first two characteristic currents and the equivalent magnetic current are given. As computed by the method of moments, numerical results for the characteristic currents and fields, the equivalent magnetic current, and the transmitted field pattern are presented for the slot whose width is one wavelength.     

Near fields of the constant current thin circular loop antenna ofarbitrary radius

Assuming a known (constant) current distribution on the thin circular loop antenna of arbitrary radius in free space, an exact integration of the vector potential is performed without recourse to approximations. The only restrictions on the solution variables are that the observation point distance must be greater than the loop radius and that the polar angle must run between 0 and π. The resulting vector potential infinite series solution possesses a real part composed of linear combinations of complete elliptic integrals of the first and second kind and an imaginary part composed of elementary functions. Thus, it is possible to obtain an exact solution which is valid everywhere that r>a and 0⩽&thetas;⩽π. The electromagnetic field components of the constant current circular loop antenna are then determined by direct series differentiation. These solutions are valid in the near and induction fields, converging rapidly there, and are also valid in the far field, although many terms of the series are needed for convergence     

Geometry considerations for use of Bi-2223/Ag tapes and wires withdifferent models of Jc(B)

In typical power applications, Bi-2223 conductors carrying AC current will be subjected to external magnetic fields whose orientation and conductor's geometry are of major significance for the AC loss magnitude. This paper investigates the influence of the geometry and aspect ratio of nontwisted Bi-2223 conductors in reducing the AC loss for such applications. A numerical model of high-T_c materials has been used in finite-element-method (FEM) simulations. The model incorporates power-law E-J characteristics with J_c and n defined by both parallel and perpendicular local magnetic field components. It allows computations of field and current distributions with transport current and/or applied field of any orientation. Monofilamentary tapes of rectangular and elliptical geometry with anisotropic J_c(B), as well as square and round wires with isotropic J_c(B) have been used for simulations under various operating conditions. A comparison between AC losses, magnetic field, and current distributions in the tapes and wires is presented     

3D Helmholtz coil system setup for thermal conductivity measurements of magnetic nanofluids

This study aims to design a mechatronic system that involves a 3D Helmholtz coil system implemented with the 3ω method to measure the thermal conductivity of magnetic nanofluids under uniform and rotating magnetic fields. For this purpose, a 3D Helmholtz coil system was designed and manufactured to generate a uniform and rotating magnetic field up to 400 G. First, the uniformity and rotation abilities of the magnetic field generated by the system were investigated numerically and experimentally. The investigations pointed out that the 3D Helmholtz coil system can generate a uniform magnetic field in 1D, 2D, and 3D with a maximum non-uniformity factor of 0.0016. After that, the thermal conductivity of Fe₃O₄ – water magnetic nanofluid samples with 1, 2, 3, 4, and 4.8 vol.% were measured under 1D, 2D, and 3D uniform magnetic field application. The magnetic field was applied at different direction angles between X, Y, and Z axes in the Cartesian coordinate system. The results pointed out that the thermal conductivity of the samples increases as the magnetic field and particle concentration increase. The maximum thermal conductivity enhancement was observed as ∼9.1% and the minimum thermal conductivity was observed as ∼1.9% when the magnetic field is applied in parallel and perpendicular directions, respectively. The measurement results also pointed out that under the external uniform magnetic field application at 2D and 3D, thermal conductivity enhancement is less affected by the particle concentration increment.     

Prediction of magnetically induced electric fields in biologicaltissue

There are many potential medical applications in which it is desirable to noninvasively induce electric fields. One such application that serves as the backdrop of this work is that of stimulating neurons in the brain. The magnetic fields necessary must be quite high in magnitude, and fluctuate rapidly in time to induce the internal electric fields necessary for stimulation. Attention is focused on the calculation of the induced electric fields commensurate with rapidly changing magnetic fields in biological tissue. The problem is not a true eddy current problem in that the magnetic fields induced do not influence the source fields. Two techniques are introduced for numerically predicting the fields, each employing a different gauge for the potentials used to represent the electric field. The first method employs a current vector potential (analogous to A in classical magnetic field theory where DEL x A = B) and is best suited to two-dimensional (2-D) models. The second represents the electric field as the sum of a vector plus the gradient of a scalar field; because the vector can be determined quickly using Biot Savart (which for circular coils degenerates to an efficient evaluation employing elliptic integrals), the numerical model is a scalar problem even in the most complicated three dimensional geometry. These two models are solved for the case of a circular current carrying coil near a conducting body with sharp corners.     

A BiCMOS front-end system with binary delay line for capacitivedetector read-out

As part of the entire readout chip, a low-power high-gain transresistance amplifier has been developed, followed by a high-speed, low-power small offset comparator and a binary delay line. The amplifier is balanced, fully differential in circuit topology, and symmetrical in layout, making it radiation tolerant and relatively insensitive to varying magnetic fields. Also, the comparator is fully symmetrical with a balanced input stage. Before irradiation (pre-rad) the transresistance amplifier has a measured differential gain of 110 mV/4 fC, an average 10/90% rise time (t_10/90%) of 20 to 50 ns depending on the bias conditions, a noise figure of 433⊕93.(C_t)^1.08 (where the symbol ⊕ stands for √(()²+() ²)) electrons (e^-), and a power consumption of 750 μW. The comparator uses bipolar transistors in the regenerative stage resulting in a small offset, a sensitivity <1.5 mV, and a power consumption of ≈350 μW at 40 MHz. The maximum pre-rad frequency at which the comparator is still functioning correctly is ≈100 MHz. Pre-rad, the binary delay line has a delay of 2.1 μs at 40 MHz and a power consumption of ≈450 μW/channel for a four-channel design. The complete readout channel-amplifier, comparator, and binary delay line-consumes ≈1.5 mW. The entire readout system was implemented in the radiation-hard 0.8-μm SOI-SIMOX BiCMOS-PJFET technology of DMILL     

Characteristic modes for slots in a conducting plane, TE case

Consider an infinitely long slot in a perfectly conducting plane in an unbounded medium illuminated by a uniform transverse electric (TE) (to the slot axis) plane wave. The theory of characteristic modes for apertures is applied to solve the problem. For a narrow slot, analytic expressions for the first two characteristic currents and the equivalent magnetic current are given. As computed by the method of moments, numerical results for the characteristic currents and fields, the equivalent magnetic current, and the transmitted field pattern are presented for the slot whose width is one wavelength.