High field transport in PbTe

The drift velocity and Hall coefficient of tn and p-type PbTe at 77K were measured for various crystallographic directions of current up to field strengths of 1.5 kV/cm. Both the conductivity and the Hall effect are anisotropic with the latter depending also strongly on the magnetic field. These effects are attributed to nonuniform heating of the equivalent conduction band L-valleys of PbTe and equivalent intervalley transfer. For field strengths beyond 1 kV/cm oscillations of the current and the potential distribution occur. The Hall coefficient decreases sharply above this threshold indicating avalanche breakdown. At still higher field strengths a sign reversal of the Hall coefficient is observed. Probing the potential distribution, it could be shown that this instability is caused by high field domains, which travel along the sample with the drift velocity of the carriers. Possible mechanisms for the formation of high field domains are discussed.     

Analysis of the relationship between defect site generation anddielectric breakdown utilizing A-mode stress induced leakagecurrent

The dielectric breakdown mechanism is studied from the viewpoint of the relationship with the generation of defect sites in the oxide film, utilizing the “A-mode” stress induced leakage current (A-mode SILC) under the constant-voltage stressing. It is demonstrated that the breakdown occurs when the A-mode SILC becomes a threshold level, I_th. In spite of that, the constant I_th for various stress fields is expected by the conventional model which assumes that each defect site is generated randomly in the oxide film, I _th, increases with the stress field. To explain this variety of I_th by the stress field, the concept of “breakdown-path creation efficiency” (γ_BPC), is proposed, which represents the amount of defect sites in the whole gate area required to create a breakdown path from one side of oxide film to the other side at a local spot. According to this concept, it is demonstrated that the efficiency becomes smaller with the increase in the stress field. These results require us to take account the nonuniform distribution of defect sites in the oxide film into the model for the breakdown mechanism. The introduction of the stress-field dependent depth profile of defect sites allows to explain the variety of I_th     

长脉冲GW级HPM大气击穿时间分析

结合电子流体方程与Maxwell方程组,对单脉冲高功率微波(HPM)大气击穿过程进行仿真,采用时域有限差分方法(FDTD)并结合HPM自生等离子体的特征参数,仿真了不同压强和场强下单脉冲HPM自生等离子体的参量变化,分析了HPM频率为6.4 GHz时,不同场强、压强下的大气击穿时间,并开展了大气击穿实验加以验证。理论分析与实验结果表明,实验与理论分析结果一致,压强与场强的变化对大气击穿时间均有显著影响,原因在于场强和压强对大气击穿种子电子浓度的变化起决定性作用,进而影响大气击穿时间。场强为kV/cm量级时,大气击穿时间在10 ns量级,在相同的场强下,随着压强的增大,击穿时间会先减小再增加。相同的大气压强条件下,场强越高,大气击穿时间越短。     

非均匀厚度漂移区SOI高压器件及其优化设计

提出非均匀厚度漂移区SOl高压器件新结构及其优化设计方法.非均匀厚度漂移区调制SOI层的电场并增强埋层电场,从而 提高器件击穿电压.考虑到这种调制效应.提出解析模型用以优化设计该新器件的结构参数.借助解析模型,研究了电场分布和器件击穿电压与结构参数的关系.数值仿真'证实了解析模型的正确性.具有3阶梯的非均匀厚度漂移区SOl器件耐压为常规结构SOl器件的2倍,且保持较低的导通电阻.     

非均匀厚度漂移区SOI高压器件及其优化设计

提出非均匀厚度漂移区SOI高压器件新结构及其优化设计方法. 非均匀厚度漂移区调制SOI层的电场并增强埋层电场,从而提高器件击穿电压. 考虑到这种调制效应,提出解析模型用以优化设计该新器件的结构参数. 借助解析模型,研究了电场分布和器件击穿电压与结构参数的关系. 数值仿真证实了解析模型的正确性. 具有3阶梯的非均匀厚度漂移区SOI器件耐压为常规结构SOI器件的2倍,且保持较低的导通电阻.     

A numerical approach to modeling the ultrashort-gate MESFET

A simple numerical model compatible to small computers is developed for an ultrashort-gate GaAs MESFET that takes into account the transient electron dynamics leading to velocity overshoot of electrons. It is assumed that because of the velocity overshoot phenomenon, the carriers move with constant high mobility at fields greater than the threshold field necessary for intervalley scattering for a certain time before relaxing to the equilibrated velocity of the low-mobility satellite valley. These time constants are taken from results of Monte Carlo calculations. The model also takes into account nonuniform channel doping as well as the nonabrupt depletion boundary. It is shown that changing the gate length from 1.0 to 0.5 µm does result in improved gain bandwidth although not in perfect proportion to the gate length reduction. This is due to excess charge dipole buildup in saturation as well as capacitance from fringing effects. The effects of profiles on the peak electric fields are also pointed out.     

Ultrafast pulse generation in photoconductive switches

Carrier and field dynamics in photoconductive switches are investigated by electrooptic sampling and voltage-dependent reflectivity measurements. We show that the nonuniform field distribution due to the two-dimensional nature of coplanar photoconductive switches, in combination with the large difference in the mobilities of holes and electrons, determine the pronounced polarity dependence. Our measurements indicate that the pulse generation mechanism is a rapid voltage breakdown across the photoconductive switch and not a local field breakdown     

Gate field emission induced breakdown in power SiC MESFETs

The breakdown mechanism of SiC MESFETs has been analyzed by careful investigation of gate leakage current characteristics. It is proposed that gate current-induced avalanche breakdown, rather than drain avalanche breakdown, is the dominant failure mechanism for SiC MESFETs: thermionic-field emission and field emission are dominant for the ON state (above pinch-off voltage) and the OFF state (below pinch-off voltage), respectively. The effect of Si/sub 3/N/sub 4/ passivation on breakdown voltage has been also investigated. Si/sub 3/N/sub 4/ passivation decreases the breakdown voltage due to higher electric field at the gate edge compared to edge fields before passivation. A reduction in surface trapping effects after passivation results in the higher electric field because the depletion region formed by trapped electrons is reduced significantly.     

Dependence of induced transmembrane potential on cell density,arrangement, and cell position inside a cell system

A nonuniform transmembrane potential (TMP) is induced on a cell membrane exposed to external electric field. If the induced TMP is above the threshold value, cell membrane becomes permeabilized in a reversible process called electropermeabilization. Studying electric potential distribution on the cell membrane gives us an insight into the effects of the electric field on cells and tissues. Since cells are always surrounded by other cells, we studied how their interactions influence the induced TMP. In the first part of our study, we studied dependence of potential distribution on cell arrangement and density in infinite cell suspensions where cells were organized into simple-cubic, body-centered cubic, and face-centered cubic lattice. In the second part of the study, we examined how induced TMP on a cell membrane is dependent on its position inside a three-dimensional cell cluster. Finally, the results for cells inside the cluster were compared to those in infinite lattice. We used numerical analysis for the study, specifically the finite-element method (FEM). The results for infinite cell suspensions show that the induced TMP depends on both: cell volume fraction and cell arrangement. We established from the results for finite volume cell clusters and layers, that there is no radial dependence of induced TMP for cells inside the cluster.     

Photoelectric fields in semiconductors under strong excitation

Photoelectric fields induced in semiconductors with nonuniform impurity profiles cancel the built-in electric field. In the case of strong photoexcitation, the maximum open-circuit voltage approaches the internal built-in potential difference due to the impurity gradient between the ohmic contact probes on the semiconductor slice.     

一种SOI nLDMOS的开态特性设计

介绍了一种用于高压电平位移电路的SOInLDMOS器件结构,对其漂移区、sink区、bufferN场板等结构参数进行器件工艺联合仿真,并分析相关结构参数对开态特性的影响,用于指导高压MOS器件尤其是基于SOI的横向MOS器件的开态特性设计。采用优化的参数,器件关态击穿电压为296V,开态击穿电压为265V,比导通电阻为60．9mΩ·cm^2。     

High Breakdown Field Dielectric Elastomer Actuators Using Encapsulated Polyaniline as High Dielectric Constant Filler

A novel method allowing rapid production of reliable composites with increased dielectric constant and high dielectric strength for dielectric elastomer actuators (DEA) is reported. The promising approach using composites of conductive particles and insulating polymers generally suffers from low breakdown fields when applied to DEA devices. The present publication shows how to overcome this deficiency by using conductive polyaniline (PANI) particles encapsulated into an insulating polymer shell prior to dispersion. PANI particles are encapsulated using miniemulsion polymerization (MP) of divinylbenzene (DVB). The encapsulation process is scaled up to approximately 20 g particles per batch. The resulting particles are used as high dielectric constant (?′) fillers. Composites in a polydimethylsiloxane (PDMS) matrix are prepared and the resulting films characterized by dielectric spectroscopy and tensile tests, and evaluated in electromechanical actuators. The composite films show a more than threefold increase in ?′, breakdown field strengths above 50 V μm^?1, and increased strain at break. These novel materials allow tuning the actuation strain or stress output and have potential as materials for energy harvesting.     

Potential distribution for a spheroidal cell having a conductivemembrane in an electric field

When a cell is situated in a uniform electric field, the field is modified due to the relatively low conductance of the cell membrane compared to that of the surrounding fluids. In certain cases, such as in the estimation of internal and external electrokinetic forces, one requires a means of estimating the magnitude of the electric field inside and outside the cell. Most treatments consider the case when the membrane has zero conductivity, or the case of only a spherical cell. The authors solve Laplace's equation for the electric potential distribution inside and outside a cell having a prolate spheroidal shape and having a membrane with a finite, nonzero conductivity     

Influence of Nonuniform Initial Porosity Distribution on Adhesive Failure in Electronic Packages

Adhesives in electronic packages contain numerous pores and cavities of various size-scales. Moisture diffuses into these voids. During reflow soldering, the simultaneous action of thermal stresses and moisture-induced internal pressure drives both pre-existing and newly nucleated voids to grow and coalesce, causing adhesive failure. In this work, a nonuniform initial porosity distribution in the adhesive is assumed. The entire adhesive is modeled by void-containing cells that incorporate vapor pressure effects on void growth and coalescence through an extended Gurson porous material model. Our computations show that increasing nonuniformity in the adhesive's initial porosity f ₀ drives the formation of multiple damage zones. Under the influence of vapor pressure or residual stresses, interface delamination becomes the likely failure mode in low mean porosity adhesives with nonuniform f ₀. For high mean porosity adhesives, the combination of vapor pressure and nonuniform f ₀ distribution induces large-scale voiding throughout the adhesive. Residual stresses further accelerate voiding activity and growth of the damage zones, resulting in brittle-like adhesive rupture.     

A dynamic action potential model analysis of shock-inducedaftereffects in ventricular muscle by reversible breakdown of cellmembrane

To elucidate the subcellular mechanism underlying the aftereffects of high-intensity dc shocks, a small pore, which mimics reversible breakdown of the cell membrane (electroporation), was incorporated into the phase-2 Luo-Rudy (L-R) model of ventricular action potentials. The pore size was set to occupy 0.15%-4.25% of the total cell membrane during the 10-ms shock. The pore was assumed to decrease after the shock exponentially with a time constant of 100-1,400 ms to simulate resealing process. In normal myocytes, the pore formation results in a delay of repolarization of the shocked action potential, which is followed by prolonged depolarization and oscillation of membrane potential like early afterdepolarization (EAD). Time- and voltage-dependent changes in the delayed rectifier K+ currents (IKr, IKs) in combination with those of L-type Ca2+ current (ICa,(L)) and ion flux through the pore (I(pore)) are responsible for the potential changes. Spontaneous excitation from the oscillation depends on activation of ICa,(L). In myocytes overloaded with Na+ and Ca2+ secondary to 90% inhibition of Na+-K+ pump, the pore formation results in a delay of repolarization of the shocked action potential, which is followed by slower cyclic depolarization in response to spontaneous release of Ca2+ from the sarcoplasmic reticulum (SR). This delayed afterdepolarization-type oscillation is abolished by complete block of Ca2+ release from the SR. These findings suggest that high-intensity electric field application will cause arrhythmogenic responses through a transient rupture of sarcolemma with different subcellular events in ventricular cells under normal and pathological conditions.     

Electroporation-Induced Cell Modifications Detected with THz Time-Domain Spectroscopy

Electroporation (electropermeabilization) increases the electrical conductivity of biological cell membranes and lowers transport barriers for normally impermeant materials. Molecular simulations suggest that electroporation begins with the reorganization of water and lipid head group dipoles in the phospholipid bilayer interface, driven by an externally applied electric field, and the evolution of the resulting defects into water-filled, lipid pores. The interior of the electroporated membrane thus contains water, which should provide a signature for detection of the electropermeabilized state. In this feasibility study, we use THz time-domain spectroscopy, a powerful tool for investigating biomolecular systems and their interactions with water, to detect electroporation in human cells subjected to permeabilizing pulsed electric fields (PEFs). The time-domain response of electroporated human monocytes was acquired with a commercial THz, time-domain spectrometer. For each sample, frequency spectra were calculated, and the absorption coefficient and refractive index were extracted in the frequency range between 0.2 and 1.5 THz. This analysis reveals a higher absorption of THz radiation by PEF-exposed cells, with respect to sham-exposed ones, consistent with the intrusion of water into the cell through the permeabilized membrane that is presumed to be associated with electroporation.     

Study of MOS gate dielectric breakdown due to drain avalanche breakdown

This work reports the effects of drain impact ionization injection on the gate dielectric breakdown. Results show that due to the high energy hot carrier injection, the gate oxide can break down twice at a low oxide electric field (<1.2 MV/cm). The first breakdown occurs simultaneously with the drain avalanche breakdown whereas the second breakdown occurs beyond the drain breakdown. It is further identified that the first gate oxide breakdown is governed by the thermionic emission of hot electrons at low oxide fields (<1.0 MV/cm) and by the scattering processes at higher oxide fields. The second breakdown is attributed to the Fowler–Nordheim (F–N) tunneling.     

Correlation between oxide breakdown and defects in SiC wafers

A correlation between gate oxide breakdown in metal oxide semiconductor (MOS) capacitor structures and structural defects in SiC wafers is reported. The oxide breakdown under high applied fields, in the accumulation regime of the MOS capacitor structure, is observed to occur at locations corresponding to the edge of bulk structural defects in the SiC wafer such as polytype inclusions, regions of crystallographic misorientation, or different doping concentration. Breakdown measurements on more than 50 different MOS structures did not indicate any failure of the oxide exactly above a micropipe. The scatter in the oxide breakdown field across a 10 mm × 10 mm square area was about 50%, and the highest breakdown field obtained was close to 8 MV/cm.     

A theoretical and experimental analysis of the buried-source VMOS dynamic RAM cell

The buried-source dynamic RAM cell combines a VMOS transistor (VMOST) and a buried junction capacitor to make a one-transistor cell (1TC) providing large storage capacitance, long charge retention, and high density. The threshold voltage, breakdown voltage, and weak inversion current for the forward and reverse modes of operation of the VMOST and the junction capacitance are experimentally related to the nonuniform doping profile of the channel. Equations are developed for the VMOST short-channel threshold voltage and storage capacity of the cell. The charge capacity (per unit of cell area) of the buried-source cell is calculated to be 2.5 times that of the conventional 1TC cell. The cell charge retention time was measured at more than 1 s at 100°C, proving operation of the device as a dynamic memory element. The technology is capable of producing an 80-µm²cell using 4-µm minimum features, no cell contacts, and a single level of interconnect.     

Wannier-Stark localization in the natural superlattice of silicon carbide polytypes

Results of a study of silicon carbide polytypes under high electric fields are presented. The presence of a natural superlattice in silicon carbide polytypes is shown to introduce a miniband structure into the conduction band, which leads to a number of effects: negative differential conductivity in the Bloch oscillation mode, electron-phonon resonances, localization in the lower miniband, resonance tunneling between the first and second minibands, prevalence of holes in the impact ionization in a wide range of fields, anomalously high avalanche breakdown fields (with a negative temperature dependence of the breakdown field) and other unusual effects. Notice that all the phenomena mentioned above are parts of a single Wannier-Stark localization process, which sets in as the mean electric field increases from 100 to 2900 kV/cm (the maximal field magnitude achieved in an abrupt p-n junction is twice larger).