Effect of the Second Sintering Temperature on the Microstructure and Electrical Properties of PbNb2O6-0.5 wt.%ZrO2 Obtained via a Two-Step Sintering Process

Lead metaniobate PbNb₂O₆ ceramics with addition of 0.5 wt.% ZrO₂ (PNZr) were processed via the conventional solid-state reaction method and a two-step sintering process. A lower second sintering temperature efficiently prevents volatilization of Pb²⁺. X-ray diffraction patterns indicate that the samples are of orthorhombic ferroelectric phase. Scanning electron microscopy shows that abnormal grain growth was restrained. The dielectric anomalies around 300°C caused by oxygen vacancies are effectively reduced.     

Formation of the Thermoelectric Candidate Chromium Silicide by Use of a Pack-Cementation Process

Transition-metal silicides are reported to be good candidates for thermoelectric applications because of their thermal and structural stability, high electrical conductivity, and generation of thermoelectric power at elevated temperatures. Chromium disilicide (CrSi₂) is a narrow-gap semiconductor and a potential p-type thermoelectric material up to 973 K with a band gap of 0.30 eV. In this work, CrSi₂ was formed from Si wafers by use of a two-step, pack-cementation, chemical diffusion method. Several deposition conditions were used to investigate the effect of temperature and donor concentration on the structure of the final products. Scanning electron microscopy and x-ray diffraction analysis were performed for phase identification, and thermal stability was evaluated by means of thermogravimetric measurements. The results showed that after the first step, chromizing, the structure of the products was a mixture of several Cr–Si phases, depending on the donor (Cr) concentration during the deposition process. After the second step, siliconizing, the pure CrSi₂ phase was formed as a result of Si enrichment of the initial Cr–Si phases. It was also revealed that this compound has thermoelectric properties similar to those reported elsewhere. Moreover, it was found to have exceptional chemical stability even at temperatures up to 1273 K.     

Submillimeter backward wave oscillators

NASA Lewis Research Center is engaged in a program to develop a series of backward wave oscillators (BWO's) for the frequency range 500 to 2000 GHz. Generically BWO's are electron beam traveling wave tubes operating in a dispersive regime in which the group velocity and the phase velocity of the induced electromagnetic wave are in opposite directions. The oscillation frequency of a BWO is controlled by the electron beam velocity (anode voltage). Such tubes because of their frequency tunability, phase locking capability, and large bandwidth are ideal local oscillators for heterdyne receiver/spectrometers. The design of the BWO's will be discussed with emphasis on the etched slow wave structure, zero compression electron beam, long life cathode, and moderate operating voltages.     

Effect of Adding Zinc Oxide Particles to the Anode Catalyst Layer on the Performance of a Proton-Exchange Membrane Fuel Cell

Commercial and home-made hygroscopic zinc oxide (ZnO) particles were added to the anode catalyst layer of a membrane electrode assembly (MEA) to improve its wettability, and thus the performance of a proton-exchange membrane fuel cell under low-humidity conditions. Scanning electron microscopy revealed that the size of the home-made ZnO particles calcined at 300°C ranged from 20 nm to 30 nm. Single-cell performance with different types of ZnO particle in the anode catalyst layer was investigated at anode humidifier temperatures of 25, 45, and 65°C; the cell and cathode humidifier temperatures were fixed at 65°C. MEA with the ZnO particles calcined at 300°C had maximum power densities of 0.26, 0.33, and 0.34 W/cm² at anode humidifier temperatures of 25, 45, and 65°C, respectively; these were 30, 37.5, and 36% higher, respectively, than for MEA without ZnO particles.     

Electrical Characteristics of Al/Poly(methyl methacrylate)/p-Si Schottky Device

Al/Poly(methyl methacrylate)(PMMA)/p-Si organic Schottky devices were fabricated on a p-Si semiconductor wafer by spin coating of PMMA solution. The capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics of Al/PMMA/p-Si structures have been investigated in the frequency range of 1 kHz–10 MHz at room temperature. The diode parameters such as ideality factor, series resistance and barrier height were calculated from the forward bias current–voltage (I–V) characteristics. In order to explain the electrical characteristics of metal–polymer–semiconductor (MPS) with a PMMA interface, the investigation of interface states density and series resistance from C–V and G–V characteristics in the MPS structures with thin interfacial insulator layer have been reported. The measurements of capacitance (C) and conductance (G) were found to be strongly dependent on bias voltage and frequency for Al/PMMA/p-Si structures. The values of interface state density (D _it) were calculated. These values of D _it and series resistance (R _s) were responsible for the non-ideal behavior of I–V and C–V characteristics.     

Electrical conduction in polyimide between 20 and 350° C

Although conduction in polyimides at elevated temperatures has been widely reported, measurements at ordinary device temperatures have been less well documented. Quantitatively reproducible low field conduction measurements on two device-grade polyimides (PMDA-ODA, BTDA-ODA/MPDA) in the temperature range of 20–350° C and under dry conditions are reported. Aluminum—polyimide—aluminum capacitors are prepared by spin coating an aluminized silicon wafer with between two and four coats of polyimide (prebake at 135° C for 10 min between coats). Samples are cured in dry nitrogen at 400° C for 45 min. Final thickness ranged between 3.3 and 6.6 μm. To permit rapid equilibration of moisture between the film and ambient, the upper electrode is patterned into multiple 25 μm stripes with 5 μ spaces for a total area of 5.1 cm². After a bake-out at 120° C under dry air and subsequent equilibration in a dry ambient at the test temperature, a voltage step is applied to the sample and the current versus time is recorded for 16,000 sec (the charging current). The sample is then shorted, and the discharging current is recorded. Below 100° C, both charging and discharging currents are dominated by a reversible polarization that follows a power law (approximately t^−0.8). Isochronal plots of the polarization current reveal a linear dependence on the applied voltage for fields in the range 10⁴–10⁵ V/cm. The polarization current is nearly independent of temperature and is well modeled by the Lewis molecular dipole theory of polarization. Above 150° C, the current is increasingly dominated by a relatively constant transport current, defined as the difference between charging and discharging currents. This current is ohmic over the field range examined, and shows a complex, activated temperature dependence. For PMDA-ODA the transport current has an activation energy (E _a ) of 0.5 eV below 175° C and 1.5 eV above that temperature. For BTDA-ODA/MPDA the E_a is 0.6 up to 250° C and 2.1 eV above. This corresponds to a resistivity of 9 × 10¹⁸ Ω-@#@ cm at 23° C and 3.5 × 10¹⁴ Ω-cm at 200° C for PMDA-ODA and 5 × 10¹⁹ Ω-cm at 23° C and 5.6 × 10¹³ Ω-cm at 300° C for BTDA-ODA/MPDA. This work demonstrates that the low temperature behavior of polyimide cannot be extrapolated from high temperature measurements. Work sponsored in part by E. I. DuPont de Nemours & Co., Inc.     

Effects of Partial Replacement of Iron with Tungsten on Microstructure,Electrical, Magnetic and Humidity Properties of Copper-Zinc Ferrite Material

This paper presents the analysis of the influence of partial replacement of iron with tungsten on the properties of copper-zinc spinel ferrite material. The samples of Cu_0.5Zn_0.5W _x Fe_2?x O₄ spinel powder ferrites were prepared by using a sol–gel self-combustion technology. The ferrite samples were treated for 30 min at 1000°C. The x-ray diffraction was used in order to establish the differences between the phase compositions of ferrites with different tungsten content. Scanning electron microscopy was used to highlight the influence of the tungsten content on the crystallites. All the samples of Cu_0.5Zn_0.5W _x Fe_2?x O₄ were subject to investigation of the influence of the substitution of tungsten upon their electrical and magnetic properties. The measurements of the electrical properties were performed in different humidity conditions, in order to highlight the effect of moisture conditions on the electrical properties of the material and to analyze the applicability of Cu_0.5Zn_0.5W _x Fe_2?x O₄ ferrite for resistive or capacitive humidity sensors.     

Optoelectronic Properties of ZnO Nanoparticle/Pentacene Heterojunction Photodiode

We fabricated photodiodes based on planar heterojunctions of zinc oxide (ZnO) nanoparticles (NPs, ～5 nm) and pentacene. The current density–voltage (J–V) characteristics of the photodiodes were investigated in the dark and under illumination. The photodiodes had good rectifying behavior in the dark and under illumination. A high rectification ratio (RR) of 878 at ±1.75 V and a low turn-on voltage of 1.3 V were achieved in the dark. Under 100 mW/cm² illumination, an RR of 55.3 was obtained at ±1.90 V. Furthermore, the photoresponsive mechanism of the device was explained in terms of the schematic band diagram and the transport of charge carriers in the device.     

Fault Diagnosis of Analog Circuit Based on High-Order Cumulants and Information Fusion

This paper proposes a method of analog circuit fault diagnosis by using high-order cumulants and information fusion. We extract the original voltage and current signals from output terminal of the circuit under test, and determine corresponding kurtosis and skewness as fault eigenvectors, which are then used to improve Error Back Propagation (BP) neural network for fault diagnosis. With respect to fault eigenvectors consider more about the information which are sometimes ignored by principal component analysis (PCA) using second order statistics. By employing information fusion to integrate voltage with current as fault eigenvectors, eigenvectors can be used to express fault information better. Diagnosis examples are used to illustrate that our fault eigenvectors own higher recognition rate and diagnosis accuracy.     

Isothermal current–voltage characteristics of high-voltage 4<Emphasis Type="Italic">H</Emphasis>-SiC junction barrier Schottky rectifiers

The forward-pulse isothermal current–voltage characteristics of 4H-SiC junction barrier Schottky rectifiers (JBSs) with a nominal blocking voltage of 1700 V are measured in the temperature range from–80 to +90°C (193–363 K) up to current densities j of ~5600 A/cm² at–80°C and 3000 A/cm² at +90°C. In these measurements, the overheating of the structures relative to the ambient temperature, ΔT, did not exceed several degrees. At higher current densities, the effective injection of minority carriers (holes) into the base of the structures is observed, which is accompanied by the appearance of an S-type differential resistance. The pulsed isothermal current–voltage characteristics are also measured at a temperature of 77 K.     

Fabrication and analysis of GaAs Schottky barrier diodes fabricated on thin membranes for terahertz applications

The GaAs Schottky diode is predominantly used as the critical mixer element in heterodyne receivers in the frequency range from 300 GHz to several THz[1]. At operating frequencies above one THz the skin effect adds significant parasitic resistance to the diode which degrades the receiver sensitivity. A novel diode structure called the Schottky barrier membrane diode is proposed to decrease the skin effect resistance by reducing the current path between the Schottky and ohmic contacts. This is accomplished by fabricating the diode on a very thin membrane of GaAs (about 1 μm thickness). A theoretical analysis has shown that this will reduce the substrate resistance by 60% at 3 THz. This reduction in resistance corresponds to a better frequency response which will improve the device's performance as a mixer element.     

Low Voltage Rf Mems Capacitive Shunt Switches

Micro-electro-mechanical-systems(MEMS) switches have low resistive loss, negligible power consumption, good isolation and high power handling capability compared with semiconductor switches. Lifetime of capacitive shunt switches strongly depends on the actuation voltage so low voltage switches is necessary to enhance its performance as well as to broaden its application area. This paper presents the design and simulation of low voltage capacitive shunt MEMS switches together with its RF performance for high frequency applications. The low voltage switches are realized by lowering the spring constant of the beam using serpentine spring designs together with large capacitive area so as to achieve the good RF performance as well. The pull-in voltage is analyzed with commercial CAD finite element analysis software CoventorWare. The electromagnetic performance in terms of scattering parameters, insertion loss, and isolation are analyzed with software Ansoft HFSS10. The switches achieved insertion loss \(<\) 0.47 dB in on state from 2 to 40 GHz; it provided better than 25 dB isolation in off state with a capacitance ratio of 94–96. The actuation voltage as low as 1.5 V with actuation area \(110\,\times \,100\,\upmu \mathrm{m^2}\) along with good RF performance is reported. The design parameter optimization including selection of appropriate number of meanders and its width found to be one of the most sensitive factors affecting the spring stiffness and actuation voltage.     

Nonpolar AlGaN/GaN Metal–Insulator–Semiconductor Heterojunction Field-Effect Transistors With a Normally Off Operation

Nonpolar AlGaN/GaN metal–insulator–semiconductor-heterojunction field-effect transistors (MIS-HFETs) with a complete normally off operation have been demonstrated for the first time. To realize the normally off operation of the MIS-HFETs, a 2-nm-thick SiN (as an insulator) and GaN-based nonpolar epitaxial layers that are free from polarization charges are employed. We have found that a thicker nonpolar AlN buffer layer achieves a GaN layer with a narrower full width at half maximum of the X-ray rocking curve and higher electron mobility. The fabricated MIS gate structure on the AlN buffer layer successfully decreases the gate leakage current and enables a more positive gate bias of up to $+$4 V, which is advantageous to achieve a higher drain current. Moreover, the n-GaN capping layer between gate and ohmic electrodes helps to reduce parasitic resistance and suppress current collapse. The fabricated $a$-plane MIS-HFET exhibits a threshold voltage of $+$ 1.3 V with a high drain current of 112 mA/mm. The presented MIS-HFETs will be desirable in next-generation radio-frequency and power switching application fields.      

Improved Hydrogen Capping Effect in n-Type Crystalline Silicon Solar Cells by SiN(Si-Rich)/SiN(N-Rich) Stacked Passivation

The effect of hydrogen capping of SiN(Si-rich)/SiN(N-rich) stacks for n-type c-Si solar cells was investigated. Use of a passivation layer consisting of Si-rich SiN with a refractive index (n) of 2.7 and N-rich SiN with a refractive index of 2.1 improved the thermal stability. A single SiN passivation layer with a refractive index of 2.05 resulted in an initial lifetime of 200 μs whereas the layer with a refractive index of 2.7 resulted in a high initial lifetime of 2 ms, but the layer degraded rapidly after firing. A stacked passivation layer with refractive indices of 2.1 and 2.7 had a stable lifetime of 1.5 ms with an implied open-circuit voltage (iV _oc) of 720 mV after firing. The thermally stable passivation mechanism with changing amounts of Si–N and Si–H bonding was analyzed by Fourier-transform infrared (FTIR) spectroscopy. Incorporation of the SiN _x stack layer (2.7 + 2.1) into the passivated rear of n-type Cz silicon screen-printed solar cells resulted in energy conversion efficiency of 19.69%. Improved internal quantum efficiency in the long-wavelength range above 900 nm, with V _oc of 630 mV, is mainly because of superior passivation of the rear surface compared with conventional solar cells.     

On current spreading in solar cells: a two-parameter tube model

The phenomenon of current spreading is essential for concentrator solar cells since it limits the conversion efficiency at high sunlight-concentration ratios. A model, which describes the regularities of the above phenomenon, is proposed and developed. The model uses a stylized representation of current lines and, respectively, of current tubes; it includes two resistive parameters accounting for the variable lateral (horizontal) component and the constant vertical component of the resistance of each tube. In the model the fact that the thickness of the spreading region is much less than the distance between the contact grid strips is taken into account. The calculated current—voltage (I–V) characteristics of a solar cell in the resistive and a nonresistive cases are obtained. The spreading-resistance I–V characteristic obtained by the voltage subtraction of these characteristics is nonlinear and depends on the photogenerated current. Thus, the equivalent electrical circuit of a solar cell includes a lumped nonlinear resistance, which depends parametrically on the photogenerated current. The comparison of experimental and calculated I–V characteristics by the example of Ge, GaAs, and GaInP solar cells is performed and both resistive parameters of the model are determined. The model describes correctly the regularities of spreading in single-junction solar cells and can be extended to multijunction solar cells.     

Current-voltage characteristics of electric contacts on p-type ZnSe

The current-voltage characteristics of electric contacts made of different materials on p-type ZnSe that form Schottky barriers from 0.3 to 1.2 eV are studied theoretically using the formula $$J = \frac{{A^* T}}{k}\int_0^\infty {T(E)[F(E) - F(E - eV)]dE,} $$ where T(E) is the energy-dependent quantum tunneling probability and F(E) is the Fermi distribution function. The contribution to the total current of both the thermionic emission and the tunneling are therefore included. The net doping concentrations under study range from 1.0×10¹⁷ cm^?3 to 1.0×10¹⁹ cm^?3. The reverse bias voltage across the barrier at a current density of 200 A/cm² is used to assess whether the barrier is reduced to an ohmic contact. A barrier of 0.3 eV is already an ohmic contact at doping concentration p=1.0×10¹⁷ cm^?3, while a barrier of 1.2 eV still behaves like a diode event at p=1.0×10¹⁹ cm^?3.     

Analysis of MBE grown AI(x)Ga(1-x)As-GaAs heteroepitaxial layers by rutherford backscattering

The Rutherford Backscattering (RBS) technique has been optimized for the measurement of thin (>85Å) alternating layers of A1(x)Ga(l-x)As and GaAs, which are at the limit of resolution of standard RBS measurements. RBS analysis of these structures provides both layer thicknesses and Al content. This information is useful for device processing since the layer thicknesses impact the FET threshold voltage and the Al content is important for proper selective chemical etching. Experimental conditions for the beam energy, detection angle, and sample rotation have been determined which allow measurement of the layer thicknesses to a precision of ±20-30Å (for layers >85A) and of the Al content,x, to a precision of ±.02. Comparison of the RBS data with Transmission Electron Microscopy (TEM), Cathodoluminescence (CL), and Reflection High Energy Electron Diffraction (RHEED) measurements on the same material showed very good agreement between the different techniques. The advantage of RBS is that it can measure much thinner layers than CL and it provides thickness and composition information at the same time whereas TEM only provides thickness for this particular material. RBS is currently used tocharacterize a monitor wafer from each day’s run to provide a processing baseline for the MBE growth.     

First experiment of gyrotron tube with quasi-optical cavity of special configuration

A new kind of quasi-optical cavity—Axisymmetrical Quasi-Optical Cavity of Oblique Rotation (AQCOR) and the new scheme for quasi-optical gyrotron with AQCOR had been proposed in [1], and the experimental tube was built in our institute. Now the initial test results of the tube are reported in this paper.     

Design and implementation of all MOS micro-power voltage reference circuit

In this work we are proposing the all MOST based reference voltage generating circuit, which utilizes the classical principle of addition of two voltages with opposite temperature coefficients. The targeted application of the proposed circuit is a low-dropout regulator which is used in a RF energy harvesting system. The proposed voltage reference circuit is implemented using a standard 0.18 μm CMOS technology. It generates the average reference voltage of 543.658 mV with an average temperature coefficient of 17.43 ppm/°C in the temperature range of ?40 to +85 °C, for the operating supply voltage ranging from 1.25 to 2 V. The maximum power consumption of the proposed architecture is ≈1.5 μW, including power dissipation in bias circuitry and the reference voltage generating core at 2 V supply voltage. The averaged measured line regulation is 1.642 mV/V. The measured power-supply rejection ratio without any filtering capacitor at 100 Hz and 1 MHz are ?62.24 and ?18.94 dB, respectively. Additionally, the measured noise density without any filtering capacitor at 10 Hz and 100 KHz is 20.54 and \(0.30\,\upmu \hbox {V}/\sqrt{\hbox{Hz}}\) , respectively. The proposed circuit has silicon area of ≈0.007 mm².