Showing papers on "Biasing published in 2008"

42.3: Transparent ZnO Thin Film Transistor for the Application of High Aperture Ratio Bottom Emission AM‐OLED Display

Abstract: We have fabricated 2.5″ QCIF+ bottom emission AM-OLED with aperture ratio of 59.6% using fully transparent ZnO-TFT array and highly conductive oxide/metal/oxide electrode for the first time. The bias stability of ZnO TFT was improved by optimizing ZnO deposition and first gate insulator process. Plasma free process for the gate insulator makes ZnO TFT very stable under electrical bias stress. The Vth shift was less than 0.3V after VDS=25 V and VGS=15 V application for 60 hours. Transparent ZnO TFT characteristics did not change noticeably under irradiation of visible light.

High-Performance Inversion-Type Enhancement-Mode InGaAs MOSFET With Maximum Drain Current Exceeding 1 A/mm

Abstract: High-performance inversion-type enhancement- mode (E-mode) n-channel In0.65Ga0.35As MOSFETs with atomic-layer-deposited Al2O3 as gate dielectric are demonstrated. A 0.4-mum gate-length MOSFET with an Al2O3 gate oxide thickness of 10 nm shows a gate leakage current that is less than 5 times 10-6 A/cm2 at 4.0-V gate bias, a threshold voltage of 0.4 V, a maximum drain current of 1.05 A/mm, and a transconductance of 350 mS/mm at drain voltage of 2.0 V. The maximum drain current and transconductance scale linearly from 40 mum to 0.7 mum. The peak effective mobility is ~1550 cm2/V ldr s at 0.3 MV/cm and decreases to ~650 cm2/V ldr s at 0.9 MV/cm. The obtained maximum drain current and transconductance are all record-high values in 40 years of E-mode III-V MOSFET research.

Critical Voltage for Electrical Degradation of GaN High-Electron Mobility Transistors

Abstract: We have found that there is a critical drain-to-gate voltage beyond which GaN high-electron mobility transistors start to degrade in electrical-stress experiments. The critical voltage depends on the detailed voltage biasing of the device during electrical stress. It is higher in the OFF state and high-power state than at VDS = 0. In addition, as |VGS| increases, the critical voltage decreases. We have also found that the stress current does not affect the critical voltage although soft degradation at low voltages takes place at high stress currents. All of our findings are consistent with a degradation mechanism based on crystallographic-defect formation due to the inverse piezoelectric effect. Hot-electron-based mechanisms seem to be in contradiction with our experimental results.

InP nanowire/polymer hybrid photodiode.

Abstract: A novel design is presented for a nanowire/polymer hybrid photodiode. n-InP nanowires are grown directly onto an indium tin oxide (ITO) electrode to increase carrier collection efficiency and to eliminate the need for an expensive substrate. Experiments show that an ohmic contact is achieved between the nanowires and the ITO electrode. The nanowires are then enveloped by a high hole mobility conjugated polymer, poly(3-hexylthiophene). Compared to the control polymer-only device, the inclusion of InP nanowires increases the forward bias current conduction by 6-7 orders of magnitude. A high rectification ratio of 155 is achieved in these photodiodes along with a low ideality factor of 1.31. The hybrid device produces a photoresponse with a fill factor of 0.44, thus showing promise as an alternative to current polymer solar cell designs.

Design and Measurement of Reconfigurable Millimeter Wave Reflectarray Cells With Nematic Liquid Crystal

Abstract: Numerical simulations are used to study the electromagnetic scattering from phase agile microstrip reflectarray cells which exploit the voltage controlled dielectric anisotropy property of nematic state liquid crystals (LCs). In the computer model two arrays of equal size elements constructed on a 15 mum thick tuneable LC layer were designed to operate at center frequencies of 102 GHz and 130 GHz. Micromachining processes based on the metallization of quartz/silicon wafers and an industry compatible LCD packaging technique were employed to fabricate the grounded periodic structures. The loss and the phase of the reflected signals were measured using a quasi-optical test bench with the reflectarray inserted at the beam waist of the imaged Gaussian beam, thus eliminating some of the major problems associated with traditional free-space characterization at these frequencies. By applying a low frequency AC bias voltage of 10 V, a 165deg phase shift with a loss 4.5-6.4 dB at 102 GHz and 130deg phase shift with a loss variation between 4.3-7 dB at 130 GHz was obtained. The experimental results are shown to be in close agreement with the computer model.

Sub-100 Nanometer Channel Length Ge/Si Nanowire Transistors with Potential for 2 THz Switching Speed

Abstract: Ge/Si core/shell nanowires (NWs) are attractive and flexible building blocks for nanoelectronics ranging from field-effect transistors (FETs) to low-temperature quantum devices. Here we report the first studies of the size-dependent performance limits of Ge/Si NWFETs in the sub-100 nm channel length regime. Metallic nanoscale electrical contacts were made and used to define sub-100 nm Ge/Si channels by controlled solid-state conversion of Ge/Si NWs to NiSixGey alloys. Electrical transport measurements and modeling studies demonstrate that the nanoscale metallic contacts overcome deleterious short-channel effects present in lithographically defined sub-100 nm channels. Data acquired on 70 and 40 nm channel length Ge/Si NWFETs with a drain−source bias of 0.5 V yield transconductance values of 78 and 91 µS, respectively, and maximum on-currents of 121 and 152 µA. The scaled transconductance and on-current values for a gate and bias voltage window of 0.5 V were 6.2 mS/µm and 2.1 mA/µm, respectively, for the 4...

Experimental investigation of dielectric barrier discharge plasma actuators driven by repetitive high-voltage nanosecond pulses with dc or low frequency sinusoidal bias

Abstract: Experimental studies were conducted of a flow induced in an initially quiescent room air by a single asymmetric dielectric barrier discharge driven by voltage waveforms consisting of repetitive nanosecond high-voltage pulses superimposed on dc or alternating sinusoidal or square-wave bias voltage. To characterize the pulses and to optimize their matching to the plasma, a numerical code for short pulse calculations with an arbitrary impedance load was developed. A new approach for nonintrusive diagnostics of plasma actuator induced flows in quiescent gas was proposed, consisting of three elements coupled together: the schlieren technique, burst mode of plasma actuator operation, and two-dimensional numerical fluid modeling. The force and heating rate calculated by a plasma model was used as an input to two-dimensional viscous flow solver to predict the time-dependent dielectric barrier discharge induced flow field. This approach allowed us to restore the entire two-dimensional unsteady plasma induced flow ...

Retarding field analyzer for ion energy distribution measurements at a radio-frequency biased electrode

Abstract: A retarding field energy analyzer designed to measure ion energy distributions impacting a radio-frequency biased electrode in a plasma discharge is examined. The analyzer is compact so that the need for differential pumping is avoided. The analyzer is designed to sit on the electrode surface, in place of the substrate, and the signal cables are fed out through the reactor side port. This prevents the need for modifications to the rf electrode—as is normally the case for analyzers built into such electrodes. The capabilities of the analyzer are demonstrated through experiments with various electrode bias conditions in an inductively coupled plasma reactor. The electrode is initially grounded and the measured distributions are validated with the Langmuir probe measurements of the plasma potential. Ion energy distributions are then given for various rf bias voltage levels, discharge pressures, rf bias frequencies—500kHzto30MHz, and rf bias waveforms—sinusoidal, square, and dual frequency.

NiO∕ZnO light emitting diodes by solution-based growth

Abstract: Heterojunction NiO∕ZnO light emitting diodes have been fabricated using low temperature solution-based growth methods. While negligible light emission has been obtained for the as-grown NiO film, devices with annealed NiO film exhibit room-temperature electroluminescence (EL), which was attributed to the detrimental effects of nickel oxide hydroxide in as-grown NiO layers. The device performance can be further modified by insertion of the organic layers between NiO and ZnO and the EL spectra exhibited dependence on the bias voltage. For higher bias voltages, strong UV-violet emission peak can be obtained in spite of the dominance of defect emission in the photoluminescence spectra.

Wide temperature range operation of micrometer-scale silicon electro-optic modulators.

Abstract: We demonstrate high bit rate electro-optic modulation in a resonant micrometer-scale silicon modulator over an ambient temperature range of 15 K. We show that low bit error rates can be achieved by varying the bias current through the device to thermally counteract the ambient temperature changes. Robustness in the presence of thermal variations can enable a wide variety of applications for dense on chip electronic photonic integration.

Low temperature tunneling magnetoresistance on (La,Sr)MnO3∕Co junctions with organic spacer layers

Abstract: This paper concerns with giant magnetoresistance (MR) effects in organic spin valves, which are realized as layered (La,Sr)MnO3 (LSMO)-based junctions with tris-(8, hydroxyquinoline) aluminum (Alq3)-spacer and ferromagnetic top layers. The experimental work was focused on the understanding of the transport behavior in this type of magnetic switching elements. The device preparation was carried out in an ultrahigh vacuum chamber equipped with a mask changer by evaporation and sputtering on SrTiO3 substrates with LSMO stripes deposited by pulsed laser technique. The field and temperature dependences of the MR of the prepared elements are studied. Spin-valve effects at 4.2K have been observed in a broad resistance interval from 50Ω to MΩ range, however, without systematic dependence on spacer layer thickness and device area. In some samples, the MR changes sign as a function of the bias voltage. The observed similarity in the bias voltages dependences of the MR in comparison with conventional magnetic tunnel junctions with oxide barriers suggests a description of the found effects within the classical tunneling concept. This assumption is also confirmed by a similar switching behavior observed on ferromagnetically contacted carbon nanotube devices. The proposed model implies the realization of the transport via local Co chains embedded in the Alq3 layer and spin dependent tunneling over barriers at the interface Co grains∕Alq3∕LSMO. The existence of conducting Co chains within the organics is supported by transmission electron microscopic∕electron energy loss spectroscopic studies on cross-sectional samples from analogous layer stacks.

Variable-load quenching circuit for single-photon avalanche diodes.

Abstract: We present a compact 50 µm×100 µm cell for single-photon detection, based on a new circuitry monolithically integrated together with a 20 µm-diameter CMOS Single-Photon Avalanche Diode (SPAD). The detector quenching relies on a novel mechanism based on starving the avalanche current till quenching through a variable-load (VLQC, Variable- Load Quenching Circuit). Fabricated in a standard 0.35 µm CMOS technology, the topology allows a SPAD bias voltage higher than the chip supply voltage to be used. Moreover it preserves the advantages of active quenching circuits, in terms of hold-off capability (from 40 ns to 2 µs) and fast reset (≤2 ns), while maintaining the low avalanche charge (≤1.6 pC/avalanche) and extremely small dimensions of passive quenching circuits. The cell enables the development of large-dimension dense arrays of SPADs, for two-dimensional imaging at the photon counting level with photon-timing jitter better than 40 ps.

Surface charge in dielectric barrier discharge plasma actuators

Abstract: Direct measurements of the dielectric surface potential and its dynamics in asymmetric dielectric barrier discharge (DBD) plasma actuators show that the charge builds up at the dielectric surface and extends far downstream of the plasma. The surface charge persists for a long time (tens of minutes) after the driving voltage has been turned off. For a sinusoidal voltage waveform, the dielectric surface charges positively. With the voltage waveform consisting of nanosecond pulses superimposed on a dc bias, the sign of the dielectric surface charge is the same as the sign (polarity) of the bias voltage. The surface charging significantly affects DBD plasma actuator performance.

An Active High-Impedance Surface for Low-Profile Tunable and Steerable Antennas

Abstract: In this letter, an approach for designing a tunable and steerable antenna is presented. The antenna model is based on a wideband bow-tie radiating element mounted above an active artificial magnetic conductor (AMC). The AMC geometry consists of a frequency selective surface (FSS) printed on a thin grounded dielectric slab in which some chip-set varactor diodes are placed between the metallic elements and the backing plane through vias. The resulting antenna can be tuned over the S-Band by simply changing all varactor capacitances through an appropriate biasing voltage. Moreover, this structure can operate a beam scanning over each working frequency by applying an appropriate biasing voltage to the active elements of the AMC surface in accordance to leaky radiation principles. The low-profile active antenna is characterized by an overall thickness of 5.32 mm, which corresponds to approximately lambda/24 at the center of the operating band.

An all-metallic logic gate based on current-driven domain wall motion.

Abstract: The walls of magnetic domains can become trapped in a ferromagnetic metallic point contact when the thickness of the film and the width of the contact are less than their critical values 1 . The discovery that domain walls can be moved from such constrictions by a sufficiently large current has attracted considerable attention from researchers working on both fundamental research and potential applications 2‐12 . Here we show that Invar nanocontacts fabricated on silica substrates exhibit a sharp drop in resistance with increasing bias voltage at room temperature in the absence of an applied magnetic field. Moreover, when two nanocontacts are combined in an all-metallic comparison circuit, it is possible to perform logical NOT operations. The use of electrical currents rather than applied magnetic fields to control the domain walls also reduces energy consumption and the risk of crosstalk in devices 13,14 . We fabricated nanocontact structures on an Invar alloy film

Perpendicular spin torques in magnetic tunnel junctions.

Abstract: We quantitatively determine a perpendicular spin torque in magnetic tunnel junctions by measuring the room-temperature critical switching current at various magnetic fields and current pulse widths. We find that the magnitude of the torque is proportional to the product of the current density and the bias voltage, and the direction of the torque reverses as the polarity of the voltage changes. By taking into account the energy-dependent inelastic scattering of tunnel electrons, we formulate the bias dependence of the perpendicular spin torque which is in qualitative agreement with the experimental results.

A biocompatible Parylene thermal flow sensing array

Abstract: A microelectromechanical systems (MEMS) thermal flow sensing array constructed of biocompatible materials has been designed, fabricated, and tested. In addition to the construction, the electronic biasing conditions were selected such that sensor operation was compatible with biological fluids. The device comprises several thin film platinum sensing elements sandwiched in a Parylene C membrane. The membrane is suspended over a bulk-micromachined silicon channel for improved thermal isolation. This sensing array layout permits, for the first time, operation in multiple flow sensing modes using a single device. Multi-mode testing was performed in hot-film, calorimetric, and time-of-flight modes at low overheat ratios. Furthermore, constant current (CC) and constant temperature (CT) biasing methods were explored in hot-film mode. The results of the various testing modes were compared and flow sensing down to 0.5 μL/min has been demonstrated.

Mechanical and tribological properties of multi-element (AlCrTaTiZr)N coatings

Abstract: Multi-element (AlCrTaTiZr)N coatings are deposited onto Si and cemented carbide substrates by reactive RF magnetron sputtering in an Ar + N2 mixture. The influence of substrate bias voltage, ranging from 0 to − 200 V, on the microstructural, mechanical and tribological properties of these nitride coatings is studied. A reduction in concentration of N and Al is observed with increasing substrate biases. The (AlCrTaTiZr)N coatings show the face-centered-cubic crystal structure (B1–NaCl type). The use of substrate bias changes the microstructure of the (AlCrTaTiZr)N coating from the columns with microvoids in boundaries to the dense and less identified columns. The compressive macrostress increases from − 0.9 GPa to − 3.6 GPa with an increase of substrate bias. The hardness and adhesion increase to peak values of 36.9 GPa and 60.7 N at the bias voltage of − 150 V, respectively. The tribological properties of the (AlCrTaTiZr)N coatings against 100Cr6 steel balls are evaluated by a ball-on-disc tribometer with a 10 N applied load. With an increase of substrate bias, the wear rate reduces while the friction coefficient almost keeps constant at 0.75. The lowest wear rate of 3.65 × 10− 6 mm3/Nm is obtained for the (AlCrTaTiZr)N coating deposited at the bias voltage of − 150 V.

Chemical Sensing Using Nanostructured Polythiophene Transistors

Abstract: We show that the chemical sensing responses of organic field-effect transistors based on nanostructured regioregular polythiophene are strongly dependent upon the gate biasing field. With different applied gate voltages, the source-drain current response can be different both in sign and magnitude for the same analyte. This implies that multiple competing sensing mechanisms exist at the same time. We propose that the sensing mechanisms for polycrystalline semiconducting polymer thin films are mainly an intragrain effect, which yields a positive response, and a grain boundary effect, which yields a negative response.

A Comparison of the Performance and Stability of ZnO-TFTs With Silicon Dioxide and Nitride as Gate Insulators

Abstract: The performance and stability of thin-film transistors with zinc oxide as the channel layer are investigated using gate bias stress. It is found that the effective channel mobility, ON/OFF ratio, and subthreshold slope of the devices that incorporate SiN are superior to those with SiO2 as the dielectric. The application of positive and negative stress results in the device transfer characteristics shifting in positive and negative directions, respectively. The devices also demonstrate a logarithmic time-dependent threshold voltage shift suggestive of charge trapping within the band gap and the band tails responsible for the deterioration of device parameters. It is postulated that this device instability is partly a consequence of the lattice mismatch at the channel/insulator interface. All stressed devices recover to near-original characteristics after a short period at room temperature without the need for any thermal or bias annealing.

An integrated superconductive magnetic nanosensor for high-sensitivity nanoscale applications.

Abstract: An integrated magnetic nanosensor based on a niobium dc SQUID (superconducting quantum interference device) for nanoscale applications is presented. The sensor, having a washer shape with a hole of 200 nm and two Josephson–Dayem nanobridges of 80 nm × 100 nm, consists of a Nb(30 nm)/Al(30 nm) bilayer patterned by electron beam lithography (EBL) and shaped by lift-off and reactive ion etch (RIE) processes. The presence of the niobium coils, integrated on-chip and tightly coupled to the SQUID, allows us to easily excite the sensor in order to get the voltage–flux characteristics and to flux bias the SQUID at its optimal point. The measurements were performed at liquid helium temperature. A voltage swing of 75 µV and a maximum voltage–flux transfer coefficient (responsivity) as high as 1 mV/Φ0 were directly measured from the voltage–flux characteristic. The noise measurements were performed in open loop mode, biasing the SQUID with a dc magnetic flux at its maximum responsivity point and using direct-coupled low-noise readout electronics. A white magnetic flux noise spectral density as low as 2.5 μΦ0 Hz−1/2 was achieved, corresponding to a magnetization or spin sensitivity in units of the Bohr magneton of 100 spin Hz−1/2. Possible applications of this nanosensor can be envisaged in magnetic detection of nanoparticles and small clusters of atoms and molecules, in the measurement of nanoobject magnetization, and in quantum computing.

All-Metal Current-Perpendicular-to-Plane Giant Magnetoresistance Sensors for Narrow-Track Magnetic Recording

Abstract: Read heads using current-perpendicular-to-plane (CPP) giant magnetoresistance sensors have been fabricated and tested under high-density recording conditions. A magnetoresistance of 5.5% and shield-to-shield spacing of 45 nm have been achieved by using an all-metal single-spin-valve with Heusler-alloy-based free and reference magnetic layers. Read heads with magnetic read widths ~45 nm were tested on perpendicular media, resulting in signals above 1 mV and signal-to-noise ratio ~30 dB. Linear densities in excess of 1050 kbpi were achieved with thermal fly-height control, compatible with recording areal densities of ~400 Gb/in2. Current-induced spin-torque effects in the recording head were observed to result in rapid performance degradation above a threshold bias voltage of about 75 mV, corresponding to current densities >108 A/cm2.

Spin-transfer torque in magnetic tunnel junctions: Scattering theory

Abstract: We study the bias-dependent spin-transfer torque in magnetic tunnel junctions in the Stoner model by scattering theory We show that the in-plane (Slonczewski type) torque vanishes and subsequently reverses its direction when the bias voltage becomes larger or the barrier wider than material and device-dependent critical values We are able to reproduce the magnitude and the bias dependence of measured in-plane and out-of-plane torques using realistic parameters The condition for the vanishing torque is summarized by a phase diagram depending on the applied bias and barrier width, which is explained in terms of an interface spin polarization and the electron focusing by the barrier Quantum size effects in the spin-transfer torque are predicted as a function of the thickness of a normal-metal layer inserted between the ferromagnet and tunnel barrier

Raman spectroscopic evidence for hot-phonon generation in electrically biased carbon nanotubes.

Abstract: We provide spectroscopic evidence for hot-phonon generation in biased single-walled carbon nanotubes by measuring simultaneously the Stokes and anti-Stokes Raman lines of the G mode and of the radial-breathing mode as a function of current bias. Using Bose-Einstein statistics we can directly calculate the phonon temperature from the intensity ratio of the anti-Stokes to Stokes lines. Upon nanotube biasing we observe (i) an increase of the G mode phonon temperature in contrast to the radial-breathing mode phonons that remain thermalized at room temperature, and (ii) no softening of the G mode. Based on these observations, we exclude current-induced thermal heating of the nanotube.

Influence of phonon scattering on the performance of p-i-n band-to-band tunneling transistors

Abstract: Power dissipation has become a major obstacle in performance scaling of modern integrated circuits and has spurred the search for devices operating at lower voltage swing. In this letter, we study p-i-n band-to-band tunneling field effect transistors taking semiconducting carbon nanotubes as the channel material. The on current of these devices is mainly limited by the tunneling barrier properties, and phonon-scattering has only a moderate effect. We show, however, that the off current is limited by phonon absorption assisted tunneling, and thus is strongly temperature dependent. Subthreshold swings below the 60mV∕decade conventional limit can be readily achieved even at room temperature. Interestingly, although subthreshold swing degrades due to the effects of phonon scattering, it remains low under practical biasing conditions.

Thermal Analysis of Degradation in Ga2O3–In2O3–ZnO Thin-Film Transistors

Abstract: Degradation of Ga2O3–In2O3–ZnO (GIZO) thin-film transistors (TFTs), which are promising for driving circuits of next-generation displays, was studied We found a degradation mode that was not observed in silicon TFTs A parallel shift without any change of the transfer curve was observed under gate voltage stress Judging from the bias voltage dependences we confirmed that the mode was mainly dominated by a vertical electric field Thermal distribution was measured to analysis the degradation mechanism Joule heating caused by drain current was observed; however, a marked acceleration of degradation by drain bias was not found Therefore, we concluded that Joule heating did not accelerate degradation Recovery of electrical properties independent of stress voltage were observed

Influence of frequency and bias voltage on dielectric properties and electrical conductivity of Al/TiO2/p-Si/p+ (MOS) structures

Abstract: In this study, the frequency and voltage dependence of the dielectric constant (e'), dielectric loss (e''), loss tangent (tan δ), electric modulus (M' and M'') and ac electrical conductivity (σac) of Al/TiO2/p-Si (MOS) structures has been investigated using the capacitance–voltage (C–V) and conductance–voltage (G/ω–V) characteristics. A TiO2 thin film was deposited on the p-type Si substrate by using the sol–gel dip coating method. These C–V and G/ω–V characteristics were measured by applying a small ac signal of 50 mV amplitude in the frequency range 5 kHz–1 MHz, while the dc bias voltage was swept from (−4 V) to (4 V) at room temperature. Experimental results show that e', e'', tan δ and σac are strongly frequency and voltage dependent. Accordingly, it has been found that as the frequency increases, e' and e'' values decrease while an increase is observed in σac and the electric modulus. The results can be concluded to imply that the interfacial polarization can more easily occur at low frequencies consequently contributing to the deviation of dielectric properties and ac electrical conductivity of Al/TiO2/p-Si/p+ (MOS) structures.

Multiple ZnO Nanowires Field-Effect Transistors

Abstract: We report on the multiple ZnO nanowires field-effect transistors (FETs), which were formed by assembling as-synthesized ZnO nanowires on a SiO2/Si substrate using an optimized alternating current (AC) dielectrophoresis (DEP) technique in three-probe back-gate geometry. The AC DEP was optimized with a bias voltage of 10 Vp-p at a frequency of 10 kHz. Our multiple ZnO nanowires FETs containing ca. 50∼65 nanowires in one device exhibited excellent electrical characteristics with a transconductance of 3∼11.5 μS at a drain voltage of 1∼5 V, a mobility of ∼30 cm2/V·s, and a carrier concentration of 9.4 × 1017 cm-3. For a comparison study, we also present conventional single ZnO nanowire FETs prepared by e-beam lithography with a back-gate structure.