Charge storage and its dynamics in porous polytetrafluoroethylene (PTFE) film electrets

The outstanding space charge storage stability of porous polytetrafluoroethylene (PTFE) film electrets is studied by isothermal surface potential decay measurements and open-circuit thermally stimulated discharge (TSD) experiments after corona charging at room and elevated temperatures, or corona charging at RT and then aging at different temperatures. Charge storage properties of porous PTFE, nonporous PTFE (Teflon/spl reg/ PTFE) and nonporous FEP (Teflon/spl reg/ FEP) electrets are compared. The results show that porous PTFE has the best charge storage stability of organic materials for both negative and positive charges, especially at high temperatures. The structure of porous PTFE, investigated by a scanning electron microscope (SEM), is important for understanding the electret properties of this material. Charge dynamics, including the influence of environmental humidity and temperature on charge stability and shift of mean charge depth, and the kinetics of detrapped charges for the porous PTFE film electrets were also investigated by means of isothermal surface potential decay measurements and analysis of the TSD current spectra in combination with the heat pulse technique. It is found that from about RT to 200/spl deg/C slow retrapping plays a dominant role; from about 200/spl deg/C to 300/spl deg/C fast retrapping controls the transport.     

Space charge studies in LDPE using combined isothermal and nonisothermal current measurements

Using a recently developed procedure combining isothermal and nonisothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the nonisothermal measurements appeared a broad peak (40/spl deg/C to 50/spl deg/C) that was possible to decompose into two or three peaks (35, 45 and 65/spl deg/C). At even higher temperature another peak was sometimes present (85/spl deg/C) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of /spl ap/15 /spl mu/m). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the nonisothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.     

Charge storage in corona-charged polypropylene films analyzed by LIPP and TSC methods

Negatively corona-charged 50-/spl mu/m-thick polypropylene (PP) film is measured using laser-induced pressure pulse (LIPP) and thermally stimulated current (TSC) in order to study the charge storage mechanism in the PP film. The LIPP can reveal the space-charge distribution in the depth direction of the PP films and the TSC can be used to measure the energetic depth of the charge trap. The LIPP shows that negative charge is deposited on the charged surface of the sample. Almost all surface charges are removed by soaking the sample in ethanol. However, about 5% of the surface charge is injected into the sample up to a depth of about 7 /spl mu/m from the surface. The injected charge is not removed by the dip-in-ethanol method because the ethanol does not penetrate into the sample. The injection of the surface charge increases with corona-charging temperature. Besides the negative charge injection, the injection of positive charge from the opposite surface is also observed when the sample is charged at higher than 60/spl deg/C. In addition, negative bulk charge is formed when the sample is charged at higher than 80/spl deg/C. The LIPP profile is compared with the TSC spectrum. It is shown that the space charge observed using LIPP disappears when the temperature of the sample exceeds 80/spl deg/C. However, TSC is observed at even higher than 80/spl deg/C. This indicates that the TSC is observed even after the disappearance of the space charge measured using LIPP.     

Analysis of relaxation parameters and their distributions using fractional polarization thermally stimulated discharge currents

It is shown that, in the analysis of fractional polarization thermally stimulated depolarisation current, the values obtained for the activation energy W and the preexponential factor /spl tau//sub 0/ depend on which one is assumed constant. The elemental peak position and shape change significantly for simulations where we assume that the activation energy is variable and the preexponential factor is constant, or vice-versa. A mathematical expression for the current is proposed in the case where W /spl tau//sub 0/ and are both variable. The uncertainties affecting the relaxation parameters produce a broadening of the theoretical peak. The minimum uncertainties in the relaxation parameters are estimated. For a current peak at temperature T/sub m/, a calculated distribution of W with width /spl ap/ kTm, or a calculated distribution of /spl tau/ with width /spl ap/ /spl tau//sub 0/ is physically meaningless.     

Investigations on charge storage and transport in plasma-depositedinorganic electrets

The storage and transport of charges in plasma deposited inorganic electrets with low mechanical stress were investigated under different conditions. Silicon dioxide, nitride and oxynitride were chemically deposited on Si substrates by means of a radio frequency (13.56 MHz) plasma system. The samples were negatively charged by the constant voltage corona method. The surface potential was observed during aging at room temperature, at elevated temperatures of 200, 250 and 300°C and at a relative humidity of 95% at 60°C. The thermal activation process was examined by thermally stimulated discharge (TSC) measurements. Silicon oxynitride possessed the highest charge stability during the different experiments, correlating with the current maximum located at ~400°C. The charge transport in silicon dioxide, with a current maximum located at ~330°C, was described by a theoretical model which considered the drift and the partial retrapping of free charges in the bulk. Despite treatment with hexamethyldisilazane, it was not possible to stabilize the surface potential of silicon nitride samples even at room temperature     

Charge trapping and conduction in pure and iodine-dopedbiaxially-oriented polypropylene

Study of charge trapping and conduction in pure and iodine doped biaxially oriented polypropylene (BOPP) is presented. Structural and chemical modifications induced by iodine were investigated using X-ray, optical and infrared methods. Optical spectra of doped BOPP show absorption at 290 nm from charge transfer complexes. X-ray examination revealed a decrease in crystallinity and crystallite size after doping. The effect of iodine on charge trapping was determined by thermally stimulated current technique. Deep traps (120°C peak) at crystalline-amorphous interfaces are destroyed by iodine, which provides new traps (68°C peak) with activation energy 0.9 eV. Pressure dependence of conductivity indicates ionic conduction in pure samples and electronic conduction in doped samples. Steady state currents in 0.5%wt iodine doped BOPP were measured for fields 1 to 5×10⁵ V cm^-1 and at elevated temperatures 22 to 50°C. Iodine enhances conductivity by ~700× in pure BOPP and the steady state conductivity shows a good fit of the 3-D Poole-Frenkel theory to the experiment. It is proposed that trapped electrons (arising due to donor-acceptor action) thermally released through PF lowering, predominantly contribute to the conduction     

Silicondioxide electret films prepared by the sol-gel process

Sol-gel silica electret films were coated on silicon substrates by a spin coating technique. The process of preparing the films is described and the electret behavior of the films is discussed. The films were corona charged at room temperature. Then, isothermal decay of the surface potential at room temperature and open-circuit thermally stimulated discharge (TSD) experiments were carried out. The results showed that the main peak of the TSD current spectra was situated at ~250°C and charge deposited in the film indicated good stability for the negative charged sample. After storage at room temperature for 50 days, the surface potential of the sample still retained at ~90% of the initial value. The authors found activation energies of 1.3 and 0.7 eV for negative and positive corona charged samples. Experimental results suggest that the sol-gel process could be useful in electret and electronic technique applications     

Thermal resistance characterization of implanted subcollector InP-based HBTs

We present the results of measurements of thermal resistivity of the heterojunction bipolar transistor (HBT) devices, utilizing selective ion implantation to define the subcollector. This new device fabrication technique resulted in high-speed HBT devices with substantially reduced thermal resistivity, compared to devices utilizing the conventional fabrication approach which includes mesa isolation for pattern definition. The measurements were taken on full-thickness 3" InP wafers at T/sub amb/ from 30/spl deg/C to 180/spl deg/C and two separate emitter current densities. We present data on three device epitaxial structures with identical device layouts and discuss the relationship of V/sub be/ to temperature at these elevated power and temperature levels.     

Polarization and dielectric behavior of ac-aged polyethylene

High field polarization and dielectric relaxation behavior have been used to study the aging mechanisms of crosslinked polyethylene (XLPE) and low density polyethylene (LDPE) aged at constant alternating electrical field stress in a humid environment at room temperature. For this study, the dielectric spectroscopy data in the frequency range of 10^-5 to 10⁶ Hz and their comparative analysis, have been used to provide electrical analog models of the aging. The dielectric data in the frequency range of 10^-5 to 5×10 ^-2 Hz were calculated from the desorption current, whereas bridge measurements provide the data in the frequency range of 10 Hz to 10⁶ Hz. In addition, thermal transient current and thermally stimulated discharge current measurements have been carried out in order to determine the polarization and space charge accumulation in ac aged polyethylene. A correlation has been observed in the behavior of polarization and dielectric relaxation in the present work. The dielectric behavior of the XLPE cable samples and of the LDPE flat samples show a presence of both the inter-cluster (low frequency) charge exchange and the intra-cluster (high frequency) charge motion, the former mechanism becoming more dominant as the aging progresses. For the XLPE cable samples ac aged in a humid environment at room temperature, relaxation peaks obeying the fractional power law have been observed in addition to a quasi-dc process     

Single-mode 1.27-/spl mu/m InGaAs:Sb-GaAs-GaAsP quantum well vertical cavity surface emitting lasers

The 1.27-/spl mu/m InGaAs:Sb-GaAs-GaAsP vertical cavity surface emitting lasers (VCSELs) were grown by metalorganic chemical vapor deposition and exhibited excellent performance and temperature stability. The threshold current varies from 1.8 to 1.1 mA and the slope efficiency falls less than /spl sim/35% from 0.17 to 0.11 mW/mA as the temperature is raised from room temperature to 75/spl deg/C. The VCSELs continuously operate up to 105/spl deg/C with a slope efficiency of 0.023 mW/mA. With a bias current of only 5 mA, the 3-dB modulation frequency response was measured to be 8.36 GHz, which is appropriate for 10-Gb/s operation. The maximal bandwidth is estimated to be 10.7 GHz with modulation current efficiency factor of /spl sim/5.25GHz/(mA)/sup 1/2/. These VCSELs also demonstrate high-speed modulation up to 10 Gb/s from 25/spl deg/C to 70/spl deg/C. We also accumulated life test data up to 1000 h at 70/spl deg/C/10 mA.     

The relation of trap distribution of alumina with surface flashover performance in vacuum

Alumina ceramic samples prepared under different sintering temperatures and varied additives were measured to indicate the trap density and trap energy located in alumina materials by using thermally stimulated current (TSC). The surface charges on alumina in vacuum after applying a negative pulse voltage (0.7/4 /spl mu/s), and flashover performances of the materials in vacuum also were measured. We found that the trap distribution in alumina has a correlation with surface charges and flashover performances in vacuum. It is shown that the higher is the trap density in the material, the higher is the surface charge density, and the lower is the flashover voltage on alumina surface. It is believed that the trapping and de-trapping mechanisms of carriers could play an important role during the development of the discharge processes, together with the secondary electron emission mechanism.     

Highly accelerated electromigration lifetime test (HALT) of copper

The reliability of copper interconnects is an important aspect in ULSI technology. The test time of the standard electromigration test is rising with improving interconnect systems. At moderate current densities, lifetime test could last more than 500 h. In this paper, lifetime tests on via-line test structures in a copper dual-damascene technology at extremely high temperatures have been investigated. This method is an alternative solution to the well-known SWEAT method where high current densities are used to accelerate the lifetime test. The used test system was a modified Suss probe station with a self-made reactor. The results have been compared with standard tests performed in commercial oven test equipment. Bimodal behavior was observed above 425/spl deg/C. Only one of the two observed failure types shows the expected thermal dependency and can be extrapolated to the standard test temperatures with Black's equation. The estimated activation energy E/sub A/=0.81 eV is comparable to the activation energy determined by standard tests below 350/spl deg/C. The benefit of this method is a reduction in test time of more than a decade at 425/spl deg/C in comparison to the standard test at 300/spl deg/C and a moderate current density.     

Unleash the military super power

Power systems for military aircraft must tolerate extreme conditions: temperatures as low as -40/spl deg/C or as high as 70/spl deg/C, as well as short-term fluctuations from -55/spl deg/C to 180/spl deg/C; relative humidity as high as 100%; impacts reaching 60G. Difficult conditions can be tackled by using suitable materials and enclosure designs, but a system or unit's weight, size and shape are often governed by its location. These challenges are just the beginning. System reliability and availability are paramount as failures must be minimised at all times. To provide a reliable and safe power system for today's military aviators, many design teams are needed to tackle the architectural, electrical, mechanical and chemical parameters of power units. Exploiting emerging technical innovations, such as high-performance FETs, can only make life easier for designers of the future.     

Charge storage in double layers of thermally grown silicon dioxideand APCVD silicon nitride

Experimental results on charge storage and discharge in double layers of silicon dioxide and silicon nitride will be reported and discussed. SiO₂ with a thickness of 300 nm was thermally grown on silicon wafers, while cover layers of Si₃N₄ with thicknesses of 50, 100, and 150 nm were deposited chemically at atmospheric pressure. The samples were charged by the point-to-grid corona method. At room temperature, the measured surface potential V was stable during a period of almost three years. Isothermal measurements under different environmental conditions showed an improved charge retention compared to a single layer grown silicon dioxide. After ~3 h at 300°C, the observed voltage drop was <10% for the double layers and ~60% for bare SiO₂. Similar results were obtained under a humid condition of 95%RH and 60°C. Besides, thermally stimulated current (TSC) was measured in setup with a temperature ramp of 200°C/h. For the double layers, a current peak with a maximum temperature at ~500°C was observed. The measured current in the range of 300 to 400°C, the location of current maxima observed in thermally grown silicon dioxide or APCVD silicon nitride, was negligible. In addition to improved electret properties the internal stress in the investigated double layers can be adjusted by a proper thickness ratio of oxide layer to nitride layer. Therefore double layers of silicon dioxide and nitride seem to be promising materials for integrated sensors and actuators based on the electret effect     

Temperature dependent dielectric spectroscopy in frequency domain of high-voltage transformer oils compared to physicochemical results

The temperature dependent dielectric spectroscopy in frequency domain is compared to physicochemical and electrical tests that are periodically applied on highly insulating transformer oils. Complex permittivity and tan/spl delta/ data are obtained as a function of frequency and operation temperatures, to demonstrate the polarization phenomena and the induced loss intensification by the thermally stimulated currents. The acquired dielectric data of oil samples collected from 201 operating high voltage (HV) power transformers, were correlated to their corresponding physicochemical ones in order to demonstrate the future potential employment of the proposed measuring technique in the field of high voltage engineering, towards reliability monitoring of oil filled electrical equipment (OFEE).     

Electrical conductivity and space charge in LDPE containing TiO/sub 2/ nanoparticles

Electrical conductivity (DC) and space charge accumulation were studied in samples of low density polyethylene to which nano-sized and micro-sized TiO/sub 2/ (anatase) particles and a dispersant had been added. Sample thicknesses were in the range 150-200 /spl mu/m. At applied field strengths of 10 and 20 kV/mm, the conductivity at 30 /spl deg/C, measured in vacuum in samples containing 10 % w/w nano-sized TiO/sub 2/, decreased by 1-2 orders of magnitude relative to samples with dispersant but without TiO/sub 2/, and by three orders of magnitude at 70 /spl deg/C. In air at 30 /spl deg/C the corresponding decrease was an order of magnitude at 10 kV/mm, and a factor of four at 20 kV/mm. In samples containing 10 % w/w micro-sized TiO/sub 2/ the conductivity increased in air and in vacuum, but only by factors in the range 2-10 depending on temperature and field. Space charge profiles were obtained using the laser-intensity-modulation-method (LIIMM), irradiating both surfaces of the sample. The micro-sized TiO/sub 2/ particles are associated with increased charge injection from the electrodes and increased charge trapping in the sample bulk, increasing the conductivity overall. The nano-sized particles generate very little charge in the sample bulk, but render the electrodes partially-blocking and so lower the conductivity.     

传输线残余电荷放电响应模型的研究

残余电荷放电机制研究对开关操作中快速暂态过电压和电磁干扰研究有着重要意义。基于传输线电荷密度与电压之间的关系,建立了以传输线电荷密度和沿线电流为变量的新传输线模型;结合传输线终端条件,给出了端接负载时域有限差分公式,对该差分公式进行修改获得了终端开路和短路时的差分公式。运用时域有限差分法和无损传输线理论,验证了新传输线模型的正确性,比较了新模型与传统模型在分析残余电荷放电时电压响应中的各自特点。从分析结果发现,无论空间和时间离散步长是否满足Courant条件的等式关系,残余电荷释放产生的响应均发生了振荡现象。对比电荷均匀分布和不均分布传输线电压响应可以看出:电荷不均匀分布对电压幅值和传输时延有很大的影响。     

1.3-/spl mu/m AlGaInAs strain compensated MQW-buried-heterostructure lasers for uncooled 10-gb/s operation

We have successfully fabricated 1.3-/spl mu/m AlGaInAs strain-compensated multiple-quantum-well (MQW) buried-heterostructure (BH) lasers by narrow-stripe selective metalorganic vapor-phase epitaxy. Based on the optimization of AlGaInAs strain compensated MQW and the Al-oxidation-free BH process, we obtained a low-threshold current of 12.5 mA and a relaxation frequency of more than 10 GHz at 85/spl deg/C for Fabry-Perot lasers. For distributed feedback lasers, we demonstrated a 10-Gb/s operation and transmission of over 16 Km for a single mode fiber at 100/spl deg/C. Furthermore, a record-low 25.8-mA/sub p-p/ modulation current for a 10-Gb/s modulation at 100/spl deg/C was demonstrated with shorter cavity and high grating-coupling coefficient. A median life of more than 1/spl times/10/sup 5/ h at 85/spl deg/C was estimated after an aging test of over 5000 h for these lasers. These superior characteristics at high temperatures were achieved by the combination of the high differential gain of AlGaInAs strain compensated MQW and the BH structure.     

The influence of elevated humidity on the stability of the electret state in polyimide films

The influence of environmental humidity on the stability of spatial charge storage in polyimide films has been studied. The decay of the electret potential difference under isothermal conditions and the spectra of thermally stimulated depolarization discharge (TSD) currents have been measured. The experimental results obtained show that as the environmental humidity grows, the stored spatial charge stability drops significantly. It has been shown that the spectra TSD currents in films with increased moisture absorption are shifted towards the lower temperature region. These changes are related to growth in the film’s electric conductivity     

Temperature and current dependences of reliability degradation of buried heterostructure semiconductor lasers

Failure times of semiconductor lasers are usually lengthy under normal aging conditions. Determination of failure times typically involves extrapolation using a sublinear or linear model. It becomes increasingly difficult to experimentally determine activation energy and current exponent since data based on lower temperatures and lower stress currents are required. In this paper, the temperature and current dependences of 1310-nm buried heterostructure (BH) InP lasers were studied. We show that the activation energy of 1310-nm BH lasers based on life test data at 70/spl deg/C-100/spl deg/C is higher than the value of 0.4 eV suggested by Telcordia. The activation energies estimated by sublinear and linear models were 0.87 and 0.55 eV, respectively. We also show that the current exponents are 1.4 and 1.0, respectively, for sublinear and linear models. We discuss the implications of the reliability results in field reliability predictions.