Pinching Off Research Articles

Improvement in U-Shaped Channel SOI Lateral IGBT for Wider Reverse Bias Safe Operating Area

In this article, the inductive turn-off failure mechanism of U-shaped channel silicon-on-insulator lateral insulated-gate bipolar transistor (SOI-LIGBT) under high-voltage and high-current is investigated by experiments and numerical simulations. First, the measured turn-off waveforms before and after failure are compared to infer the cause of the failure. Then, the failure is reproduced by calibrated three-dimensional simulations using Sentaurus TCAD. According to the simulation and measured results, the turn-off failure is resulted from the high impact ionization in the bird’s beak region. To reveal the failure mechanism, the turn-off failure process is divided into two phases. In phase I, the electrons generated by drastic impact ionization in the bird’s beak region act as the base current to keep the operation of parasitic PNP transistor after channel pinching off. In phase II, the parasitic NPN transistor is activated due to the hole current amplified by PNP, and thus, turn-off failure occurs. Based on the revealed turn-off failure mechanism, an improved U-shaped channel SOI-LIGBT with a P-top is proposed. The P-top is arranged in the bird’s beak region to reduce the impact ionization rate. The measured results demonstrate that no failure occurs when the improved device turns off under high-voltage and high-current conditions. Compared with the conventional U-shaped channel SOI-LIGBT, the improved device obtains 275% and 23% improvements in turn-off current at a bus voltage of 450 V and reverse bias safe operation area, respectively.

Microfluidic Jetting Deformation and Pinching-off Mechanism in Capillary Tubes by Using Traveling Surface Acoustic Waves

To date, there has been little research attention paid to jetting deformation and pinching-off of microfluidic flows induced by the surface acoustic wave (SAW) mechanism. Further, such studies were almost limited to one sessile drop actuation without any confinement mechanisms. Such a scenario is likely attributable to the mechanism’s relatively poor controllability, the difficulty of maintaining the fluid loading position and issues related to stability and repeatability. In this paper, a novel SAW-microfluidic jetting system with a vertical capillary tube was designed, accompanied by a large number of experiments investigating the single droplet jetting mechanism with different device dimensions, resonance frequencies and radio frequency (RF) power capabilities. The study began with the whole jetting deformation and droplet pinching off through the use of a microscope with a high-speed camera, after which the results were discussed to explain the droplet jetting mechanism in a vertical capillary tube. After that, the study continued with experimental and theoretical examinations for high-quality single droplet jetting conditions. Jetting characterization parameters, including threshold RF power, resonance frequency, liquid volume, pinching off droplet dimensions, were thoroughly analyzed. Lastly, the Weber number range, a significant parameter in SAW-microfluidic jetting, was verified, and the pinching off microdroplet dimension was analyzed and compared via experiments. The significance of this study lies in the realization of microfluidic drop-on-demand based on SAW technology.

Experimental I–V and C–V Analysis of Schottky-Barrier Metal-Oxide-Semiconductor Field Effect Transistors with Epitaxial NiSi2 Contacts and Dopant Segregation**Supported by the National Natural Science Foundation of China under Grant No 61674161, and the Open Project of State Key Laboratory of Functional Materials for Informatics.

We present an experimental analysis of Schottky-barrier metal-oxide-semiconductor field effect transistors (SB-MOSFETs) fabricated on ultrathin body silicon-on-insulator substrates with a steep junction by the dopant implantation into the silicide process. The subthreshold swing of such SB-MOSFETs reaches 69 mV/dec. Emphasis is placed on the capacitance-voltage analysis of p-type SB-MOSFETs. According to the measurements of gate-to-source capacitance with respect to at various , we find that a maximum occurs at the accumulation regime due to the most imbalanced charge distribution along the channel. At each peak, the difference between and is equal to the Schottky barrier height (SBH) for NiSi2 on highly doped silicon, which indicates that the critical condition of channel pinching off is related with SBH for source/drain on channel. The SBH for NiSi2 on highly doped silicon can affect the pinch-off voltage and the saturation current of SB-MOSFETs.

Direct observational evidence for the merging of equatorial plasma bubbles

AbstractIn this work we present direct ground‐based observational evidence for the merging of individual equatorial plasma bubbles (EPBs) obtained through the imaging of OI 630.0 nm airglow. Three potential mechanisms have been identified: (1) One of the EPBs tilts and reaches location of the adjacent growing EPB finally merging with it. (2) Some of the branches of an EPB arising from secondary instabilities reach out to adjacent EPB and merge with it. (3) The eastward zonal drift of the EPB on the eastern side slows down while the adjacent EPB on the western side drifts relatively faster and catches up. In one of the cases, a branch of an EPB was observed to get interchanged with another EPB as a result of merging and consequent pinching off from the parent EPB.

Direct and remote constriction of membrane necks.

The physical properties of membrane necks are relevant in vesiculation, a process that plays an essential role in cellular physiology. Because the neck's radius is, in general, finite, membrane scission and the consequent pinching off of the vesicle can only occur if it is narrowed to permit the necessary membrane topological reformation. Here we examine, in a simple single phase lipid vesicle, how external forces can promote neck constriction not only by direct compression at the neck but also, counterintuitively, by dilation at remote locations. These results provide a new perspective on the role played by actin polymerization in the process of endocytosis.

Wrapping Our Heads around Endocytosis

​EukaryoticEukaryotic cells use a process called endocytosis to take up materials from their environment and package them for use within the cell. The best-studied form of endocytosis—receptor-mediated endocytosis—has several stages. In the first step, the cell's outer membrane dimples to form a pocket. A protein collar then assembles around this pocket, forming a thin neck leading to a bulb of membrane. Next, this bulb pinches off to form a membrane-bound sphere, or vesicle. The progression of vesicle formation in yeast. Yeast and mammalian cells follow the same general steps of endocytosis (forming the pocket, collaring it, and pinching it off) and use similar ensembles of proteins to achieve them. Both cells use a protein called clathrin to coat the sites where pockets form, and pocket formation is associated with a burst of actin assembly. Mammalian cells also rely on the enzyme dynamin to collar and help pinch off vesicles. Yeast, meanwhile, manage to pinch off vesicles efficiently without dynamin, raising questions about the fundamental pinching mechanism. Yeast and mammals also use Bin-Amphiphysin-Rvs (BAR)–domain proteins to generate and sense membrane curvature. How do mammalian and yeast cells coordinate the activities of these various participants to form endocytic vesicles? In this issue of PLoS Biology, Jian Liu and colleagues theorize that endocytosis is coordinated in both types of cells by processes that affect and respond to the curvature of cell membranes. In both mammalian and yeast cells, the membrane lipid phosphatidylinositol (4,5) bisphosphate (PIP2) plays a crucial role in endocytosis. Membrane levels of PIP2 are regulated by enzymes that phosphorylate (that is, add a phosphate group to) lipid precursors to create more PIP2, and enzymes called phosphatases that dephosphorylate (remove a phosphate group from) PIP2 to reduce its levels. Early on in endocytosis, there are high levels of PIP2 in the membrane that serve to recruit clathrin or other coat proteins to the site where membrane dimples will form. Later, though, PIP2 levels must be reduced to allow these coating proteins to drop off and be replaced by a different set of proteins that will direct trafficking of the vesicle within the cell. Liu and colleagues wondered whether this cycle regulating PIP2 levels might also be responsible for other critical steps in endocytosis, such as the pinching off of vesicles. They created a conceptual and computational model to help explore this idea. In their model, the coat proteins that are recruited by PIP2 to the endocytic site cause the cell membrane to deform into a dimple. Collar proteins bind better to curved membranes, so the formation of this initial curve promotes the recruitment of collaring proteins. Collar proteins also increase membrane curvature where they attach, so their recruitment is a self-reinforcing process. As the growing protein collar constricts the neck of the pocket, the membrane at the end of the collar bulges out into a bulb. The lipid membrane in this bulb is stretched into a sharp curve. Recent work by other groups has demonstrated that some lipid phosphatases are more efficient at cleaving PIP2 as membrane curvature increases. Therefore, Liu and coauthors suggest that PIP2 in the bulb, and especially at the interface between the bulb and the neck of the pocket, gets quickly dephosphorylated as the neck gets more tightly curved. Nothing exciting would happen if PIP2 levels decreased universally across the surface of the membrane pocket. But, what if PIP2 in the neck region is protected from dephosphorylation by the collar proteins, so that only PIP2 on the bulb is vulnerable? The dephosphorylated lipid takes up less space in a membrane, so differences in PIP2 levels at the neck/bulb juncture would create a circumferential force that acts like a noose. This noose would collapse the membrane at the neck/bulb interface, pinching off the vesicle. The authors' model encompasses these ideas, and computer simulations using the model recapitulate what is observed in nature. The model successfully describes the general process of endocytosis and, what's more, allows the authors to make specific predictions about what may change if certain protein players are tinkered with experimentally (e.g., through mutation). Although these predictions have yet to be explicitly tested, the model offers a roadmap to finally understanding the cascade of events involved in endocytosis. Liu J, Sun Y, Drubin DG, Oster GF (2009) The Mechanochemistry of Endocytosis. doi:10.1371/journal.pbio.1000204

Zymogen granule exocytosis is characterized by long fusion pore openings and preservation of vesicle lipid identity.

The dynamics of the fusion pore that forms between a secretory vesicle and the plasma membrane are important in the regulation of both exocytosis and endocytosis. Here, we describe characteristics of fusion during zymogen granule exocytosis in exocrine pancreatic acinar cells. By using fluorescence recovery after photobleaching techniques, we show that the fusion pore remains open to allow free aqueous exchange with the vesicle lumen. There is no lipid interchange between the plasma and granule membranes during this time, and at the end of its life, the intact granule shrinks in situ, probably by a gradual pinching off of membrane patches. We propose that the protracted fusion pore lifetime is adapted to permit compound exocytosis, whereby the lingering primary granule acts as a conduit through which the contents of a secondary granule can be released. The lack of lipid intermixing may then facilitate selective recycling of granule membrane and preservation of apical membrane integrity.

Dynamics of pinch-off in liquid/liquid jets with surface tension

Full two-dimensional velocity fields were measured by PIV in forced jets of glycerin/water solution flowing into and pinching off in ambients of Dow Corning 200 Series silicone fluid. The Reynolds, Froude, and Bond numbers computed at the nozzle exit were 34, 0.20, and 6.1, respectively. Viscosity ratios between the inner and outer fluids of 0.17 and 1.7 were examined in jets forced at a Strouhal number of 4.0. The experiments revealed that the viscosity ratio strongly affected the evolving jet flow, the pinch-off process, and the resulting droplet shapes. The higher viscosity ambient yielded less gravitational acceleration within the jet fluid, a broader cone shape upstream and a more rounded drop interface downstream of the pinch-off zone, and eventually spheroidal droplets with weak oscillations in shape. The lower viscosity ambient yielded greater gravitational acceleration within the jet, a narrower cone shape upstream and flatter drop interface downstream of the pinch-off zone, and eventually strong oscillations including inverted curvature within the droplets that formed. The difference in downstream interface angles at pinch off could be explained by the velocity and vorticity distributions within the two flows.

Structure and function of the cytoskeleton of a Dictyostelium myosin-defective mutant.

To study the role of conventional myosin in nonmuscle cells, we determined the cytoskeletal organization and physiological responses of a Dictyostelium myosin-defective mutant. Dictyostelium hmm cells were created by insertional mutagenesis of the myosin heavy chain gene (De Lozanne, A., and J. A. Spudich. 1987. Science (Wash. DC). 236: 1086-1091). Western blot analysis using different mAbs confirms that hmm cells express a truncated myosin fragment of 140 kD (HMM-140 protein) instead of the normal 243-kD myosin heavy chain (MHC). Spontaneous revertants appear at a frequency less than 4 x 10(-5), which synthesize normal myosin and are capable of forming thick filaments. In hmm cells, the HMM-140 protein is diffusely distributed in the cytoplasm, indicating that it cannot assemble into thick filaments. The actin distribution in these mutant cells appears similar to that of wild-type cells. However, there is a significant abnormality in the organization of cytoplasmic microtubules, which penetrate into lamellipodial regions. The microtubule networks consist of approximately 13 microtubules on average and their pattern is abnormal. Although hmm cells can form mitotic spindles, mitosis is not coordinated with normal furrow formation. The hmm cells are clearly defective in the contractile events that lead to normal cytokinesis. The retraction of different regions of the cell can result in the occasional pinching off of part of the cell. This process is not coupled with formation of mitotic spindles. There is no specific accumulation of HMM-140 in such constrictions, whereas 73% of such cells show actin concentrated in these regions. The mutant hmm cells are also deficient in capping of Con-A-bound surface receptors, but instead internalize this complex into the cytoplasm. The hmm cells display active phagocytosis of bacteria. Whereas actin is concentrated in the phagocytic cups, HMM-140 protein is not localized in these regions. cAMP, a chemoattractant that induces drastic rounding up and formation of surface blebs in wild type cells, does not induce rounding up in the hmm cells. A Triton-permeabilized cell model of the wild-type amebae contracts on reactivation with Mg-ATP, whereas a model of the hmm cell shows no detectable contraction. Our data demonstrate that the conventional myosin participates in the significant cortical motile activities of Dictyostelium cells, which include rounding up, constriction of cleavage furrows, capping surface receptors, and establishing cell polarity.

Energetics of the break-up of dislocation dipoles into prismatic loops

The energetics of two different models leading to the formation of a row of prismatic loops from the fluctuation and subsequent break-up of dislocation dipoles or elongated dislocation loops have been evaluated as a function of fluctuation amplitude and wavelength. Break-up most likely occurs by spontaneous pinching off of small circular loops at the ends of closed-ended dipoles or elongated loops. The theoretical results agree well with experimental observations of the dislocation substructure in Ti 4+-doped sapphire (α-Al 2O 3) deformed by basal slip.

The development of cells in the coenocytic endosperm of the African blood lily Haemanthus Katherinae

The endosperm of the African blood lily Haemanthus Katherinae Bak. follows the helobial pattern of development in which two chambers of endosperm are formed. In the earliest observed stage of development a large micropylar chamber and a smaller dome-shaped chalazal chamber of endosperm are present. Both are coenocytic and contain wall ingrowths of the transfer cell type along the embryo sac wall. Freely growing walls grow centripetally from the embryo sac wall, branch, and eventually meet, forming a layer of cells along the embryo sac wall. This process occurs first in the micropylar chamber. After four or more layers of endosperm cells are present, phragmoplasts form in association with karyokinesis and give rise to cross walls situated between the freely growing walls. When 10 or more layers of endosperm cells are present, free wall-less cells are present in the central vacuole near the edge of the cellular endosperm of the micropylar chamber. The free cells originate from mitosis of nuclei at the inner wall-less edge of the endosperm and the subsequent pinching off and release of the free cells into the large central vacuole. The free cells may undergo karyokinesis and become binucleate. The chalazal chamber of endosperm also becomes cellular by means of freely growing walls.

Recent studies on the comparative biochemistry of the cholinergic neuron.

It is well known that the exposure of mammalian brain tissue to moderately intense liquid shear forces (“homogenization”) results in the breakup of neuronal and glial cell bodies and processes, the release of nuclei, mitochondria and other cell organelles, and the “pinching off” of presynaptic nerve terminals to form synaptosomes (Gray and Whittaker 1962; for recent review, see Whittaker 1973). As can be shown by the subsequent isolation of the synaptosomes by density gradient centrifugation, their formation accounts for the survival, in a particle-bound form, of a considerable proportion of the total tissue content of acetylcholine and other transmitters. The survival of acetylcholine is particularly striking since the tissue homogenate is rich in choline esterases which destroy any free acetylcholine liberated by homogenization. Synaptosome formation is an efficient process (Clementi et al. 1966) and occurs when any mammalian brain is homogenized: man, monkey, ox, sheep,...

Measurement of domain wall mobility in GdIG

The promise of memory and logic applications for magnetic bubble domains has heightened the general interest in the occurrence and behavior of cylindrical domains in various materials. A train of short magnetic field pulses transforms the usual demagnetized snakelike domain pattern into a minimum-energy circular domain pattern which is stable with no dc bias field. This work maps the limits on field pulse width and height for producing the array. Domain wall mobility studies have also been made making use of the simple domain structure. The mobility studies suggest that there are regions of the sample where the local mobility is as much as ten times the average value. This leads to a simple model for the formation of the bubble array from the labyrinth structure, involving the “pinching off” of the snakes. Similar experiments on orthoferrite having very uniform properties fail to produce bubble domains, tending to support the model. Detailed results are presented.