Low Current Applications Research Articles

Joining of 1050/5052 by resistance seam welding and materials evaluation of joints

Spot welding is extensively used for joining aluminium alloys, although it is necessary to have available a large-capacity power source with a maximum capacity of around 300 000 A for joining by resistance seam welding1, low-current applications not being very often encountered. Recently developed welders, however, allow welding to proceed with ease at low currents of 20 or so kA. Joining of 1050/5052 by resistance seam welding was therefore tried together with a materials evaluation of joints.

Slit scattering in a microbeam set-up

Slit scattering of beam particles in a microbeam set-up becomes significant for low current applications and thus limits its lateral resolution and application potential. In this work, the angular and energy distributions of beam particles interacting with cylindrically shaped slit-jaws of different radii, surface roughnesses, and materials are calculated numerically, employing the computer code SRIM. To quantitatively investigate their effect on the beam spot quality of a microbeam set-up, a quality factor is proposed as an index to evaluate the effect of slit-scattered particles on the beam spot. The results of the investigation reveal a high sensitivity of slit scattering in relation to the slit-jaw surface roughness.

Characterization of a new large area HPGe X-ray detector for low beam current application

In the new sub-micron beamline at the Lund Nuclear Microprobe facility a special large area HPGe X-ray detector has been installed. It consists of eight 100-mm 2-size detector elements arranged in an annular formation around the beam entrance hole. Different parameters like FWHM, FWTM, peak-to-background ratio and peak-to-valley ratio for each individual detector element as well as their sum spectra have been investigated. The data handling, i.e. how eight parallel signals are individually stored and summed together, has been discussed. The possibility to use the detector for simultaneous measurement of high and low energy photons, i.e. PIXE combined with PIGE has been demonstrated.

Measurement of Mobility in HEMT Devices Using High-Order Derivatives

In this paper, a novel approach to the measurement of mobility of GaAs HEMT devices is presented. The new approach employs high-order derivatives as a means of determining the parameters of the proposed new mobility equation. The new approach is compared to established mobility measurement methods, and shown to offer better accuracy. The results presented also consider the behavior of mobility in the linear and saturation bias regions. The mobility value extracted by this new method has permitted improvements to the MESFET/HEMT model when simulating the behavior of the device in the linear region. This is critical in many applications, such as in low current linear-mixing applications.

Model and design of PCB parallel winding for planar transformer

In today's high-frequency low-profile switching-mode power converters, paralleling winding in planar transformers is needed, especially for low voltage and high current applications. Although this practice is believed to be effective to increase the current handling capacity of the windings, the effect might be offset due to the unequal current sharing within parallel layers if the winding layers are not properly arranged and connected. In this work, the particular problems raised from parallel winding are addressed and modeled based on a one-dimensional field model. The factors affecting winding losses with parallel connection are analyzed. Software to predict the current density distribution and AC loss for planar windings has been developed.

A submicron PHEMT nonlinear model suitable for RFID low current amplifier design

A new nonlinear submicron double heterostructure PHEMT model is presented that is suitable for low current application. It is found that this model accurately predicts the bias-dependent S-parameters for PHEMT up to 40 GHz, and good agreement is obtained between simulated and measured results for 2.45 GHz and 5.8 GHz amplifiers.

The National University of Singapore high energy ion nano-probe facility: Performance tests

The Research Centre for Nuclear Microscopy, National University of Singapore incorporates three state-of-the-art beam lines, connected to a high brightness High Voltage Engineering Europa 3.5 MV Singletron accelerator. One of these lines is a NEC (National Electrostatics Corporation, USA) ion channeling facility, utilising broad beam ion beam analysis techniques for advanced materials research. The other two lines are microbeam facilities; one is designed for nuclear microscopy of biomedical samples and advanced materials, where relatively high currents (>50 pA) are required, and the other for proton beam micromachining (PBM) and materials modification, where lower currents can be utilised. The resolution performances of the two microbeam lines have been measured, and the results are as follows: (1) The nuclear microscope line incorporates the Oxford Microbeams OM2000 endstation with the OM50 quadrupole lenses configured in the high excitation triplet mode. This line has achieved the world’s best performances for analytical applications of 290 × 400 nm for a 50 pA current of 2 MeV protons. (2) The PBM line, which is the first of its kind worldwide, utilizes the new generation of compact (OM52) quadrupole lenses (Oxford Microbeams Ltd.) also configured in a high excitation, triplet configuration. This facility, which has superior demagnification properties, has achieved the world’s best performances for low current applications. Spot sizes of 35 × 75 nm have been measured using direct scanning transmission ion microscopy for beam currents of 10,000 protons per second.

Steady State Characteristics of Two-Phase Isolated Converter for Step-down Applications.

This paper presents a method of minimizing the smoothing capacitor of the buck converter with isolated transformers and synchronous rectifiers, which is used for the low voltage-high current applications. The principle is to remove the ripple voltage by applying two-phase configuration of asymmetry switching. A detailed analysis is given using averaging method and cleared the operations of the circuit. Especially the smoothing capacitor can be shifted to the high voltage side, as a results the capacity is extremely lessened to be few μF. In this circuit, small capacitor of several hundred μF is necessary to use in the output (low voltage side). The capacitors in the primary and secondary sides being both small, the ceramic capacitors of low ESR are possible to use. From the experimental result, the efficiency of about 92% was obtained under the conditions of the input 140V and the output 1.5V/20A with the primary capacity of 0.33μF, the secondary capacity of 200μF and the switching frequency of 100kHz.

New developments in design and applications for pelletron accelerators

Most of the developments over the last several years related to Pelletron accelerator are in the field of accelerator mass spectrometry (AMS) and other low beam current applications with the exception of a very high DC electron recirculation Pelletron. High precision AMS systems based on tandem Pelletrons from 500 kV to 5 MV terminal potential are now in use for routine high precision AMS measurements. Their performance will be reported. In addition, there has been significant advancement in the design of the multi-cathode SNICS source for the use of both gas and solid samples within a single source. The latest performance of these sources will be discussed. New diagnostics is being developed for very low beam currents. The latest design of the low current beam profile monitor (LCBPM) will also be presented.

Novel approaches for retinal drug delivery.

Novel methods of ophthalmic drug delivery are being developed to facilitate treatment of a variety of eye diseases. Pharmaceuticals administered intravitreally are able to bypass the blood-ocular barrier to achieve constant therapeutic levels in the eye, while minimizing systemic side effects. Sustained-release intravitreal implants are being developed to enhance further the intravitreal route of administration. Liposomes, microscopic vesicles with a membrane-like lipid bilayer surrounding an aqueous compartment, are being developed to incorporate a wide variety of drug molecules, proteins, nucleotides, and even plasmids giving them great potential for use in ophthalmology. Biodegradable scleral plugs containing pharmaceuticals have the advantage of biodegradability and the ability to fit in a relatively small diameter of 1 mm. Conjugate drugs, which are covalently linked, decrease drug solubility, which increases their half-life and also limits the amount of active drug present at a given time. Viral vectors have been investigated in the delivery of genetic material to the posterior segment of the eye. Retrovirus, adenovirus, adenoma-associated virus, herpes virus, and lentivirus have been investigated for gene transfer to the retina. Finally, iontophoresis, a method of drug delivery typically involving the application of low electric currents to drive molecules across barriers, such as skin, is being explored for ophthalmic applications.

Proton beam micromachined resolution standards for nuclear microprobes

The quest for smaller spot sizes has long been the goal of many nuclear microprobe groups worldwide, and consequently there is a need for good quality resolution standards. Such standards have to be consistent with the accurate measurement of state-of-the-art nuclear microbeam spot sizes, i.e. 400 nm for high current applications such as Rutherford backscattering spectrometry and proton-induced X-ray emission, and 100 nm for low current applications such as scanning transmission ion microscopy or ion beam-induced charge. The criteria for constructing a good quality nuclear microprobe resolution standard is therefore demanding: the standard has to be three dimensional with a smooth surface, have an edge definition better than the state-of-the-art beam spot resolutions, and exhibit vertical side walls. Proton beam micromachining (PBM) is a new technique of high potential for the manufacture of precise 3D microstructures. Recent developments have shown that metallic microstructures (nickel and copper) can be formed from these microshapes. Prototype nickel PBM resolution standards have been manufactured at the Research Centre for Nuclear Microscopy, NUS and these new standards are far superior to the 2000 mesh gold grids currently in use by many groups in terms of surface smoothness, vertical walls and edge definition. Results of beam resolution tests using the new PBM standards with the OM2000 microprobe end station/HVEE Singletron system have yielded spot sizes of 290 nm×450 nm for a 50 pA beam of 2 MeV protons.

Suppression of Bulk Defects in Antimonide Superlattice Infrared Photodiodes

AbstractWhile physical properties of ideal antimonide superlattices (ASL) indicate that they should significantly outperform mercury cadmium telluride (MCT) based infrared photodiodes for low dark current applications in the long and very long wave-infrared (LWIR and VLWIR), this potential has not yet been fully realized. Even though measured Auger and tunneling rates in ASL's are reduced as predicted, overall carrier lifetimes remain much shorter, and dark currents much higher than expected. The large carrier losses are the result of defects in the ASL structure, with contributions measured from large bulk defects and surface channels along mesa sidewalls, and the remaining component believed to be due to midgap states.In this presentation we report on several studies of epitaxial growth parameters and their influence on defect formation. X-ray photoelectron spectroscopy analysis of oxide desorption from GaSb substrates shows the presence of both antimony and gallium oxides, along with their decomposition and desorption behavior with anneal temperature. A study of buffer growth shows that defect density and size are critically dependent on growth temperature, with an optimal growth window between 480 and 500 °C.. Side-by-side GaSb buffer growths on vicinal ((100) + 1 ° (111)) and flat (100) substrates show that while growing on vicinal material can suppress mound formation, it does not yield epilayers as flat as can be obtained on (100) substrates grown under optimal conditons. Finally, the ratio of As to In flux during superlattice growth can be used to tune the lattice parameter both above and below that of GaSb, with strain-related defects appearing when the mismatch reaches roughly 0.1%.

Analytical correction for temperature and diode characteristics for application in a wide dynamic range logarithmic electrometer

A study has been carried out on the I–V characteristics of junction diodes for low current applications. An analytical correction for temperature on the forward voltage is estimated using experimentally derived parameters. The parameters can all be easily measured. The analytical correction is found to be accurate within 4% for a current range of 10−11–10−4 A and for a temperature range of −30–70 °C. This study will be useful for logarithmic electrometers for low current measurement where space and power are at a premium.

EBSD in FESEM and LVSEM

Abstract Background Electron backscattered diffraction (EBSD) is a very powerful analysis technique to understand the crystallography of a sample in the SEM. Typically, a high beam current is needed to obtain good results because of inefficient collection of the patterns. In W-filament SEM's, the high beam current necessitates a large spot size and requisite low image resolution. The large beam current can also be a source of problems for insulating samples or sample mounts by causing charging and the resulting image drift. To reduce the spot size with adequate beam currents for imaging, field-emission (FE) guns are used. For most FESEM's, the beam current is markedly reduced from that produced by W-filament instruments. Even though the image resolution is better, the EBSD performance tends to suffer at low beam currents for standard hardware configurations. To regain the performance of a comparable W-filament system, the EBSD hardware should be changed for the low beam current operation. This paper will address the hardware considerations for low beam current applications.

Nuclear microprobe analysis and imaging: Current state of the art performances

The current state-of-the-art performances claimed for nuclear microbeam spatial resolutions are: (a) 400 nm spot sizes for high current (100 pA) proton beams, (b) 400 nm for high current alpha particle beams, (c) 100 nm for low proton current (<0.1 pA) applications such as Scanning Transmission Ion Microscopy (STIM) and Ion Beam Induced Charge (IBIC) and (d) 50 nm for low current alpha particles. These claims, however, have been undermined not only by the lack of a common and generally accepted resolution standard, but also by the lack of a consistent approach between groups regarding a recognised method of beam spot measurement. The lack of a definitive method for assessing spatial resolution is hindering the future development of nuclear microprobes to higher spatial resolutions. We require the manufacture of resolution standards specifically tailored for nuclear microprobe spot size measurements, for both high and low current operations. Until such resolution standards are available, one commercially available standard which has high potential for high current proton beam resolution tests is the Ebeam test chip manufactured for the characterisation of electron beam testers. This chip has patterns as small as 0.5 μm in size produced in 0.5 μm thick aluminium by direct electron beam writing. Using both analytical Particle Induced X-ray Emission (PIXE) imaging and PIXE line scans, the nuclear microscope facility at the National University of Singapore has demonstrated spatial resolutions of less than 400 nm for a 100 pA beam of 2 MeV protons. Low current resolution standards have more stringent requirements. One potential standard tested in Singapore is a prototype self-supporting X-ray mask used in X-ray lithography. This test standard, however, is not commercially available at the moment, is fragile and therefore difficult to handle and easily fractured. Using off-axis STIM imaging and line scanning over this mask, the NUS nuclear microscope facility has demonstrated beam spot sizes less than 130 nm for a 1 pA 2 MeV proton beam.

Iontophoresis of amphetamine in the neostriatum and nucleus accumbens of awake, unrestrained rats

When administered systemically to ambulant animals, amphetamine (AMPH) has both excitatory and inhibitory effects on single-unit activity in the neostriatum and nucleus accumbens. To determine the extent to which these results reflect a direct action of the drug, AMPH was applied iontophoretically to neostriatal and accumbal neurons under naturally occurring behavioral conditions. AMPH dose-dependently (5–40 nA) inhibited the vast majority of spontaneously active units. The inhibition, which was evident at low ejection currents (5–10 nA), had relatively short onset (4–12 s) and offset (6–24 s) latencies, and was positively correlated with basal firing rate. Even stronger dose-dependent inhibitory responses were recorded when neurons having no or a very low rate of spontaneous activity were tonically activated by continuous, low-current applications of glutamate (Glu). Systemic injection of either SCH-23390 (0.1 mg/kg) or haloperidol (0.2 mg/kg), relatively selective D 1 and D 2 receptor antagonists, respectively, blocked the AMPH-induced inhibition. Prolonged AMPH iontophoresis (2–3 min; 5–30 nA) inhibited both spontaneous impulse activity and Glu-induced excitations, resulting in a complete blockade of the Glu response at relatively high AMPH ejection currents (≥20 nA). Taken together, these results suggest that although dopamine is largely responsible for the inhibitory effects of iontophoretic AMPH, dopamine alone cannot account for the complex response of neostriatal and accumbal neurons to systemic AMPH administration.

An analytical MOS transistor model valid in all regions of operation and dedicated to low-voltage and low-current applications

Afully analytical MOS transistor model dedicated to the design and analysis of low-voltage, low-current analog circuits is presented. All the large-and small-signal variables, namely the currents, the transconductances, the intrinsic capacitances, the non-quasi-static transadmittances and the thermal noise are continuous in all regions of operation, including weak inversion, moderate inversion, strong inversion, conduction and saturation. The same approach is used to derive all the equations of the model: the weak and strong inversion asymptotes are first derived, then the variables of interest are normalized and linked using an appropriate interpolation function. The model exploits the inherent symmetry of the device by referring all the voltages to the local substrate. It is shown that the inversion chargeQ inv is controlled by the voltage differenceV P — Vch whereV ch is the channel voltage, defined as the difference between the quasi-Fermi potentials of the carriers. The pinch-off voltageV P is defined as the particular value of Vch, such that the inversion charge is zero for a given gate voltage. It depends only on the gate voltage and can be interpreted as the equivalent effect of the gate voltage referred to the channel. The various modes of operation of the transistor are then presented in terms of voltagesV P —V S andV P —V D Using the charge sheet model with the assumption of constant doping in the channel, the drain currentIDis derived and expressed as the difference between a forward componentI F and a reverse componentI R. Each of these is proportional to a function ofV P —V S respectivelyV P —V D through a specific currentI S This function is exponential in weak inversion and quadratic in strong inversion. The current in the moderate inversion region is then modelled by using an appropriate interpolation function resulting in a continuous expression valid from weak to strong inversion. A quasi-static small-signal model including the transconductances and the intrinsic capacitances is obtained from an accurate evaluation of the total charges stored on the gate and in the channel. The transconductances and the intrinsic capacitances are modelled in moderate inversion using the same interpolation function and without any additional parameters. This small-signal model is then extended to higher frequencies by replacing the transconductances by first order transadmittances obtained from a non-quasi-static calculation. All these transadmittances have the same characteristic time constant which depends on the bias condition in a continuous manner. To complete the model, a general expression for the thermal noise valid in all regions of operation is derived. This model has been successfully implemented in several computer simulation programs and has only 9 physical parameters, 3 fine tuning fitting coefficients and 2 additional temperature parameters.

Simulation of arc-circuit instability in a low current DC automotive switching application

The dynamic arc produced in a switch breaking an inductive load sometimes interacts with the rest of the circuit to produce a highly nonlinear response characterized by growing, high frequency oscillations, current chopping and transient overvoltages. System simulation, therefore, requires an approach that includes switch arc behavior. A technique for employing a well-known equation from power circuit breaker design to analyze such arc-circuit interaction for switch arcs of less than 2 mm in length is described. The so-called Mayr arc equation provides a basis for accurate modeling of arc-circuit instability after one recognizes that the drawn switch arc exists in two distinct phases, for which the mechanisms of arc erosion and instability are effectively decoupled. This approach provides a technique for predicting instabilities in switched automotive electrical systems that could result in either radiofrequency interference to vehicle electronics, or potentially damaging high voltage transients. >

Behavior of a respiratory rate-responsive pacemaker during and after cardiac surgery

DVANCES IN pacemaker technology now allow devices to increase their paced rates in response to information from biologic sensors.’ Previous case reports have described unexpected adverse device behavior related to sensor activation during surgery in the presence of these “rate-responsive” devices. 2-5 In this report, a patient with a minute ventilation-sensing device who underwent coronary artery bypass surgery had sensor and device function assessed during the operation. The device estimates minute ventilation by calculating thoracic impedance from measurements of voltage obtained during a low energy current application (1 mA, 7.6 usec, 18 Hz) delivered between the extrathoracic pulse generator and the proximal pole of the intrathoracic bipolar pacemaker electrode. It was hypothesized that the marked changes in thoracic impedance consequent to chest opening would affect sensor behavior. In addition, the effects of cautery and mechanical ventilation settings on device sensor function were observed in a prospective manner. A 65year-old woman was admitted for urgent coronary revascularization because of unstable angina. Three months previously, she received a Telectronics Meta II VU-R 1204H minute volume monitoring rate-responsive pacemaker (Denver, CO) for episodes of symptomatic bradycardia with fatigue. Her predominant rhythm was sinus with normal atrioventricular conduction. Her baseline 12-lead electrocardiogram showed evidence of old inferior infarction. Device telemetry preoperatively revealed that all beats were sensed with no evidence of paced beats. In order to maintain a stable baseline for the sensor, the device was in a “WI + adapt” mode for the 2 hour period before the operation. In this mode, the device obtains sensor data but does not adjust the paced rate. The sensitivity of the sensor or “rate-response factor” was 18 with an upper rate of 120 beats/min and a minimum rate of 60 beats/min. The patient was anesthetized using a moderate-dose fentanyl and benzodiazepine technique. With the sternum open, the pacemaker was interrogated. This showed sensing only with no paced beats. During electrocautery, high frequency (> 10 Hz) continuous interference was detected and appropriately led to no more than two to three beats of temporary asynchronous (noise reversion mode) pacing. WI-R pacing was then instituted at the preoperative rateresponse settings. The device immediately showed sensing only. There was no evidence of a change in paced rate consequent to sensor activation with the chest open (Fig 1 A).

Improved performance of carbon-doped GaAs base heterojunction bipolar transistors through the use of InGaP

Carbon-doped GaAs/AlGaAs heterojunction bipolar transistors (HBTs) typically exhibit severe leakage at the base-emitter interface which limits their utility for low-current applications. Furthermore, the device breakdown voltage, and hence power handling capability, is limited due to the band gap of the GaAs collector material. In this letter we will demonstrate for the first time that both of these limitations can be overcome through the use of InGaP. Since InGaP is not readily doped with carbon, it does not suffer from compensation due to carryover of carbon from the GaAs base. Hence, the ideality factor of the base-emitter junction improves from 1.3 to 1.09 when the conventional n-AlGaAs emitter layer is replaced with n-InGaP. Moreover, InGaP eliminates the crossover of the base and collector currents typically observed in heavily carbon doped GaAs HBTs. This results in the maintenance of gain even at very low collector currents. As a collector material, we have found that InGaP produces significantly higher breakdown voltage than GaAs (19 V vs 12 V) of the same thickness and doping, due to its larger band gap. As in the emitter, InGaP collectors exhibit excellent ideality factors of ∼1.05.