Pinch Off Research Articles

Mobility overestimation due to minority carrier injection and trapping in organic field-effect transistors

Abstract Donor–acceptor (D–A) polymers are promising candidates as semiconductors in flexible transistors because of their high mobility. However, in recent studies on D–A polymer transistors, device characteristics deviate from the idealized transistor model, which is commonly used for extracting mobility, and show an abrupt turn-on in the drain current when measured as a function of gate voltage. To examine the validity of the reported mobility values of D–A polymers, we investigate the detailed mechanism of the nonideal electrical characteristics by device simulation based on a split-channel model, where the gate voltage swing on the drain side is higher than that on the source side. We demonstrate that minority carriers injected into low-bandgap D–A polymers (electrons in the case of usual D–A polymers) at high drain voltages and low gate voltages cause insufficient pinch off, resulting in an overestimation of mobility if the conventional mobility extraction method is used. A more reliable method for estimating mobility from the nonideal transfer characteristics is developed and applied to recent reports. It was determined that the mobility values of D–A polymers in the recent reports were overestimated by a factor of 1.7–94 or more.

Water entry of a constraint posture body under different entry angles and ventilation rates

Water entry experiments of a constraint posture projectile were performed under different entry angles and ventilation rates. The near water surface cavity flow characteristics, including splash sheet, surface closure, pull away, deep closure and cavity ripples, were investigated under different entry angles and ventilation rates without the posture perturbation influence of the projectile. The experimental results demonstrate that, the dimensionless cavity surface closure and pull away time increase gradually with the increasing of the water entry angle, and the dimensionless cavity deep closure time and the pinch off depth experience a slight increase and decrease, respectively. The deep closure patterns shift from the point closure to the line closure as the water entry angle increases for the oblique water entry cases. And, the cavity ripples were observed before the pinch off and after the pull away for ours experiment, which are more likely acoustic in origin. In addition, the surface closure contributed by the splash sheet is clearly delayed at similar entry conditions except for the ventilation rate. The dimensionless cavity deep closure times of the ventilation cases are significantly higher than the cases without ventilation. However, with further increase the ventilation, the deep closure time changes slightly.

Pinch Off Plasma CVD Deposition Process and Material Technology for Nano-Device Air Gap/Spacer Formation

As integrated circuits for high performance CMOS devices scale down to < = 10 nm dimension, further reductions in capacitance are vitally important for device performance. It is important to reduce capacitance in the FEOL and BEOL device structures while maintaining fabrication integration robustness. This paper presents an overview of material and process technology requirements for FEOL air spacer and BEOL air gap formation using a pinch off deposition approach. These approaches utilize established dielectric materials and processes such as Plasma CVD of SiN, SiCN, SiCOH, pSiCOH, in the formation of the air spacer/air gap. The selection of these dielectric materials and processes has a large impact in the void (gap) dimension and volume. The void dimension and volume in airgap/air spacer structures can be controlled with various dielectric deposition processes and materials to facilitate subsequent process fabrication steps, and ultimately to build a robust device with substantial capacitance reduction.

Simultaneous measurement of temperature, stress, and electric field in GaN HEMTs with micro-Raman spectroscopy.

As semiconductor devices based on silicon reach their intrinsic material limits, compound semiconductors, such as gallium nitride (GaN), are gaining increasing interest for high performance, solid-state transistor applications. Unfortunately, higher voltage, current, and/or power levels in GaN high electron mobility transistors (HEMTs) often result in elevated device temperatures, degraded performance, and shorter lifetimes. Although micro-Raman spectroscopy has become one of the most popular techniques for measuring localized temperature rise in GaN HEMTs for reliability assessment, decoupling the effects of temperature, mechanical stress, and electric field on the optical phonon frequencies measured by micro-Raman spectroscopy is challenging. In this work, we demonstrate the simultaneous measurement of temperature rise, inverse piezoelectric stress, thermoelastic stress, and vertical electric field via micro-Raman spectroscopy from the shifts of the E2 (high), A1 longitudinal optical (LO), and E2 (low) optical phonon frequencies in wurtzite GaN. We also validate experimentally that the pinched OFF state as the unpowered reference accurately measures the temperature rise by removing the effect of the vertical electric field on the Raman spectrum and that the vertical electric field is approximately the same whether the channel is open or closed. Our experimental results are in good quantitative agreement with a 3D electro-thermo-mechanical model of the HEMT we tested and indicate that the GaN buffer acts as a semi-insulating, p-type material due to the presence of deep acceptors in the lower half of the bandgap. This implementation of micro-Raman spectroscopy offers an exciting opportunity to simultaneously probe thermal, mechanical, and electrical phenomena in semiconductor devices under bias, providing unique insight into the complex physics that describes device behavior and reliability. Although GaN HEMTs have been specifically used in this study to demonstrate its viability, this technique is applicable to any solid-state material with a suitable Raman response and will likely enable new measurement capabilities in a wide variety of scientific and engineering applications.

Effect of Channel Width Variation on Electrical Characteristics of Double Lateral Gate Junctionless Transistors; A Numerical Study

In this paper, the effect of channel width variation on performance of double lateral gate junctionless transistors in the depletion and accumulation regimes is investigated. The characteristics of the device with various channel widths is comprehensively examined through analysis of on and off state current, threshold voltage (V th), transconductance (g m) and drain conductance (g D) variation in each operating regime. The carriers’ density distribution, electric field components and mobility are investigated through 3-D numerical simulations of the device to illustrate the variation of output characteristics. The results show that as the width decreases, the off-current (I OFF) decreases significantly as a result of better electrostatic control of the lateral gates over the channel. The on-current (I ON) is also decreased mainly due to the doping-dependent mobility degradation.It is also indicated that between the flat-band and fully depleted (pinch off) variation of the majority carriers is the main parameter that modifies the characteristics of the device, while the mobility variation is recognized as the basic factor in the accumulation regime.

Axisymmetric viscous interfacial oscillations – theory and simulations

We study axisymmetric, free oscillations driven by gravity and surface tension at the interface of two viscous, immiscible, radially unbounded fluids, analytically and numerically. The interface is perturbed as a zeroth-order Bessel function (in space) and its evolution is obtained as a function of time. In the linearised approximation, we solve the initial value problem (IVP) to obtain an analytic expression for the time evolution of wave amplitude. It is shown that a linearised Bessel mode temporally evolves in exactly the same manner as a Fourier mode in planar geometry. We obtain novel analytical expressions for the time varying vorticity and pressure fields in both fluids. For small initial amplitudes, our analytical results show excellent agreement with those obtained from solving the axisymmetric Navier–Stokes equations numerically. We also compare our results with the normal mode approximation and find the latter to be an accurate representation at very early and late times. The deviation between the normal mode approximation and the IVP solution is found to increase as a function of viscosity ratio. The vorticity field has a jump discontinuity at the interface and we find that this jump depends on the viscosity and the density ratio of the two fluids. Upon increasing the initial perturbation amplitude in the simulations, nonlinearity produces qualitatively new features not present in the analytical IVP solution. Notably, a jet is found to emerge at the axis of symmetry rising to a height greater than the initial perturbation amplitude. Increasing the perturbation amplitude further causes the jet to undergo end pinch off, giving birth to a daughter droplet. This can happen either for an advancing or a receding jet, depending on the viscosity ratio. A relation is found between the maximum jet height and the perturbation amplitude. Hankel transform of the interface demonstrates that at large perturbation amplitudes higher wavenumbers emerge, sharing some of the energy of the lowest mode. When these additional higher modes are present, the interface has pointed crests and rounded troughs.

Water entry of deformable spheres

When a rigid body collides with a liquid surface with sufficient velocity, it creates a splash curtain above the surface and entrains air behind the sphere, creating a cavity below the surface. While cavity dynamics has been studied for over a century, this work focuses on the water entry characteristics of deformable elastomeric spheres, which has not been studied. Upon free surface impact, an elastomeric sphere deforms significantly, giving rise to large-scale material oscillations within the sphere resulting in unique nested cavities. We study these phenomena experimentally with high-speed imaging and image processing techniques. The water entry behaviour of deformable spheres differs from rigid spheres because of the pronounced deformation caused at impact as well as the subsequent material vibration. Our results show that this deformation and vibration can be predicted from material properties and impact conditions. Additionally, by accounting for the sphere deformation in an effective diameter term, we recover previously reported characteristics for time to cavity pinch off and hydrodynamic force coefficients for rigid spheres. Our results also show that velocity change over the first oscillation period scales with the dimensionless ratio of material shear modulus to impact hydrodynamic pressure. Therefore, we are able to describe the water entry characteristics of deformable spheres in terms of material properties and impact conditions.

Small Signal Parameter Extraction and DCSimulation of Asymmetric Dual Channel AlGaN/GaN Heterojunction Field Effect Transistor

Objectives: This article reports extraction of small signal parameters and analysis of dc behavior of enhanced dual channel (DC) AlGaN/GaN HFET first time. Enhanced device for which small signal model proposed incorporates double channels, field plate and nucleation layer as additional feature for better reliability. Methods/Analysis: Direct method for small signal parameter extraction is employed using cold FET pinch off biasing at lower and upper frequency band. Lower frequency band for analysis is chosen to ensure extraction accuracy for parasitic capacitive elements of our proposed small signal model parameters. Also TCAD simulation environment is utilized for extensive analysis and characterization of device for dc and small signal performance. Findings: The small signal extrinsic and intrinsic parameters are extracted for the proposed device structure that can predict physical as well as RF performance correctly. Simulation results support clear insight about dc performance and functional reliability of device. Novelty/Improvements: Proposed model takes in to consideration most of the important parasitic as well as intrinsic components of device for extraction purpose. The method employed for model is capable for providing higher accuracy in comparison of other approaches. The extracted model and simulation results give clear insight about enhanced device performance in higher frequency range. The reported results are compared with latest published data and found to be in good agreement.

The influence of Gulf Stream eddies and meanders on near-surface chlorophyll

The Gulf Stream region contains strong mesoscale variability that significantly influences planktonic ecosystems residing therein. Meanders of the Gulf Stream can be identified as eastward propagating features in maps of sea level anomaly. These meanders can become unstable and pinch off to form nonlinear mesoscale eddies (rings) that trap large parcels of water. Following formation, ecosystems trapped within these eddies are subjected to temporally varying vertical velocities throughout their lifetime. As a result of both horizontal advection and vertical fluxes, multiple physical-biological mechanisms can simultaneously influence phytoplankton communities trapped in eddies. In this study we examine how the near-surface chlorophyll field (CHL) evolves in meanders and eddies by comparing satellite observations with an eddy-resolving ocean model.Prior in situ and satellite observations have revealed that during the formation of cyclonic Gulf Stream meanders, water with elevated CHL is transported southward. In anticyclonic meanders, water with reduced CHL is transported northward. Alternating submesoscale patches of upwelling and downwelling occur along the meandering front; however, evidence of a biological response to meander-induced vertical motion was not observed in meander-centric composite averages. During the formation of nonlinear Gulf Stream eddies, elevated and suppressed CHL is trapped and subsequently transported westward in cyclones and anticyclones, respectively. Following formation, CHL is observed to increase in the cores of anticyclones. The observed positive trend in CHL in anticyclones is consistent with the influence of eddy-induced Ekman pumping (eddy/wind interaction) that generates upwelling in anticyclones and downwelling in cyclones.To substantiate the influence of eddy-induced Ekman pumping on CHL in Gulf Stream eddies, two separate eddy-resolving physical-biological simulations are compared. The first simulation is forced with a realistic surface stress that includes the influence of ocean surface currents. The second simulation neglects this process. The time evolution of CHL within eddies is very different in these two simulations. The model that includes eddy-induced Ekman pumping generates temporal trends in CHL that are similar to the observations.

Catheter Pinch off with Foreign Body Retrieval.

Catheter Pinch off with Foreign Body Retrieval.

Bending and growth of entrained air filament under converging and asymmetric rotational fields

Here we have proposed the increase of the entrainment rate by extruding an air filament under the action of convergent but asymmetric rotational field. By varying the source speed and the diameter of rotational fields, we showed the bending of an air filament towards the higher strength direction of the asymmetric inertia. Interfacial profiles like bubble ejection from the air filament and non-collapsible entrainment with air accumulation in a stagnant zone are obtained in finite volume based numerical simulations, on gradual increase of average rotational fields. Physical understanding of bent interface profile reveals the presence of multiple stages in filament growth depending upon the inertia of surrounding medium. Accumulation of air in the stagnant zone is found to be more prominent in case of rotational speed based asymmetry in contrast to its counterpart having diametric asymmetry of imposing sources. Relative comparison between these two methods of producing asymmetric field showed faster growth of filament upon varying the source diameter, while keeping the speed same. In case of extreme retardation and enhancement of rotational asymmetry, film pinch off and formation of bubble train have been reported. The shape of ejected bubbles is governed by the inertia of the surrounding medium, and bubbles have taken elliptical shapes with their major axis aligned parallel to the adjacent velocity field.

Free surface over a horizontal shear layer: vorticity generation and air entrainment mechanisms

Two air entrainment mechanisms driven by vortex instability are reported in the unstable relaxation of a horizontal shear layer below a free surface. This flow is experimentally investigated by means of planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) coupled with surface profilometry. PLIF identifies counter-rotating vortex pairs (CRVP) emanating from the surface following the growth of high steepness two-dimensional millimetre-size waves for Reynolds and Weber numbers based on the momentum thickness of 177 to 222 and 7.59 to 13.9, respectively. High spatio-temporal resolution PIV reveals the role of surface-generated vorticity and flow separation in the highly curved trough of the waves on the injection of a CRVP. Air bubbles are entrapped in the wake of these CRVPs at Reynolds number above 190. PIV data and spanwise PLIF images show two initiation mechanisms: primary vortex instability modulating the spanwise location where the flow separates, resulting in the pinch off of an air ligament, and secondary vortex instability turning a CRVP into$\unicode[STIX]{x1D6FA}$-shaped loops pulling the surface down. Instability wavelengths agree with linear stability analysis, and models for these new air entrainment mechanisms are proposed.

Catheter Fracture of a Totally Implantable Venous Device Due to Pinch Off Syndrome in Breast Cancer: A Case Report

Totally implantable venous devices are used in medical care for parenteral nutrition, vascular access, administrating chemotherapeutic agents and so on. Although the large variety of catheter complications, catheter fracture is a rare but serious complication. The pinch off syndrome is caused by the compression of the catheter between the clavicle and first rib, and may lead to fracture and possible dislocation of the catheter. We report here the case history of a patient with metastatic breast cancer who developed a rare complication of subclavian catheter fracture as a consequence of pinch off syndrome.

Towards the understanding of bubble-bubble interaction upon formation at submerged orifices: A numerical approach

Present study reports numerical investigations on the bubble evolution at submerged orifices and their mutual interactions in quiescent liquid under constant inflow condition of air. In finite volume based 3D domain, breaking and making of interfaces are tracked using volume of fluid (VOF) solver. Binary interaction dynamics is studied for a wide range of fluid properties (3.85×10−11<Mo<4.645×10−6), orifice diameters (1, 1.5 and 2mm) and spacings (2–10mm). Apart from observed coalescence and non-coalescence, fusion of bubbles before and after pinch off from orifice mouth at different Mo are presented. Using velocity patterns inside the bubble, explanation of different merging pattern is proposed. A 3D map for distinct behaviour while merging, in terms of Mo, orifice diameter and spacing is also tried. Finally, effect of velocity of gaseous phase on merging between sister bubbles are assessed numerically.

Giant acoustoelectric current in suspended quantum point contacts

We present results on the acoustoelectric current driven through a quantum point contact (QPC) placed on a suspended nanobridge, which is subject to surface acoustic waves (SAWs). The magnitude of this current is much larger than that of a two-dimensional gas, and the system has enhanced sensitivity to perturbations. In particular, the current oscillations as a function of magnetic field exhibit additional features associated with the spin splitting. Furthermore, the negative voltage applied to the QPC gates induces the oscillations revealing the subband structure not seen in transport measurements at the elevated temperatures of our experiment. Near the pinch off conditions, the acoustoelectric current becomes negative which we attribute to the enhanced backscattering caused by the SAW-phonons' absorption and emission.

Electric field dependence of optical phonon frequencies in wurtzite GaN observed in GaN high electron mobility transistors

Due to the high dissipated power densities in gallium nitride (GaN) high electron mobility transistors (HEMTs), temperature measurement techniques with high spatial resolution, such as micro-Raman thermography, are critical for ensuring device reliability. However, accurately determining the temperature rise in the ON state of a transistor from shifts in the Raman peak positions requires careful decoupling of the simultaneous effects of temperature, stress, strain, and electric field on the optical phonon frequencies. Although it is well-known that the vertical electric field in the GaN epilayers can shift the Raman peak positions through the strain and/or stress induced by the inverse piezoelectric (IPE) effect, previous studies have not shown quantitative agreement between the strain and/or stress components derived from micro-Raman measurements and those predicted by electro-mechanical models. We attribute this discrepancy to the fact that previous studies have not considered the impact of the electric field on the optical phonon frequencies of wurtzite GaN apart from the IPE effect, which results from changes in the atomic coordinates within the crystal basis and in the electronic configuration. Using density functional theory, we calculated the zone center E2 (high), A1 (LO), and E2 (low) modes to shift by −1.39 cm−1/(MV/cm), 2.16 cm−1/(MV/cm), and −0.36 cm−1/(MV/cm), respectively, due to an electric field component along the c-axis, which are an order of magnitude larger than the shifts associated with the IPE effect. Then, we measured changes in the E2 (high) and A1 (LO) Raman peak positions with ≈1 μm spatial resolution in GaN HEMTs biased in the pinched OFF state and showed good agreement between the strain, stress, and electric field components derived from the measurements and our 3D electro-mechanical model. This study helps to explain the reason the pinched OFF state is a suitable reference for removing the contributions of the electric field and the IPE-induced stress from the temperature rise in the ON state and suggests that the IPE-induced stress in the GaN buffer is an order of magnitude smaller than previously believed. Our analysis and experimental results support previous theoretical studies discussing the electric field dependence of optical phonon frequencies apart from the IPE effect and suggest that this is a general phenomenon occurring in all wurtzite and zincblende crystals. The total electric field dependence of the optical phonon frequencies in piezoelectric crystals is a critical consideration in accurately characterizing the stress, strain, electric field, and temperature distributions in microelectronic devices via micro-Raman spectroscopy.

Thermodynamic coarsening arrested by viscous fingering in partially miscible binary mixtures.

We study the evolution of binary mixtures far from equilibrium, and show that the interplay between phase separation and hydrodynamic instability can arrest the Ostwald ripening process characteristic of nonflowing mixtures. We describe a model binary system in a Hele-Shaw cell using a phase-field approach with explicit dependence of both phase fraction and mass concentration. When the viscosity contrast between phases is large (as is the case for gas and liquid phases), an imposed background flow leads to viscous fingering, phase branching, and pinch off. This dynamic flow disorder limits phase growth and arrests thermodynamic coarsening. As a result, the system reaches a regime of statistical steady state in which the binary mixture is permanently driven away from equilibrium.

Bubble formation process from a novel nozzle design in liquid cross-flow

An experimental study is reported that investigated the bubble formation from a novel nozzle design in a liquid cross-flow using high speed imaging. Different configurations and orientations of the novel nozzle design were considered over a range of gas-to-liquid flow rate ratios (GLR) from 0.00031 to 0.00204. The results show that for all cases, the novel nozzle generated smaller bubbles at higher detachment frequency compared to the standard nozzle. At low liquid velocities, the novel nozzle generated bubbles that were 30% smaller in size at a detachment frequency 2–3 times higher than that for the standard nozzle. It was also found that the bubble diameter and the detachment frequency are almost independent of the liquid velocity. The underlying physical process of bubble formation and detachment in the novel nozzle under liquid cross-flow was also investigated. It was observed that the process comprised of three phases: expansion, collapse and pinch off. It was also found that the rebound force of the bubble from a side-hole under the influence of liquid drag force and hydrostatic pressure plays a key role in the early bubble detachment. The results demonstrated that the novel nozzle design performs better than the standard nozzle in the liquid cross-flow, especially at high GLRs.

Hydrogen-terminated diamond vertical-type metal oxide semiconductor field-effect transistors with a trench gate

The hydrogen-terminated diamond surface (C-H diamond) has a two-dimensional hole gas (2DHG) layer independent of the crystal orientation. A 2DHG layer is ubiquitously formed on the C-H diamond surface covered by atomic-layer-deposited-Al2O3. Using Al2O3 as a gate oxide, C-H diamond metal oxide semiconductor field-effect transistors (MOSFETs) operate in a trench gate structure where the diamond side-wall acts as a channel. MOSFETs with a side-wall channel exhibit equivalent performance to the lateral C-H diamond MOSFET without a side-wall channel. Here, a vertical-type MOSFET with a drain on the bottom is demonstrated in diamond with channel current modulation by the gate and pinch off.

Contributed Review: Experimental characterization of inverse piezoelectric strain in GaN HEMTs via micro-Raman spectroscopy

Micro-Raman thermography is one of the most popular techniques for measuring local temperature rise in gallium nitride (GaN) high electron mobility transistors with high spatial and temporal resolution. However, accurate temperature measurements based on changes in the Stokes peak positions of the GaN epitaxial layers require properly accounting for the stress and/or strain induced by the inverse piezoelectric effect. It is common practice to use the pinched OFF state as the unpowered reference for temperature measurements because the vertical electric field in the GaN buffer that induces inverse piezoelectric stress/strain is relatively independent of the gate bias. Although this approach has yielded temperature measurements that agree with those derived from the Stokes/anti-Stokes ratio and thermal models, there has been significant difficulty in quantifying the mechanical state of the GaN buffer in the pinched OFF state from changes in the Raman spectra. In this paper, we review the experimental technique of micro-Raman thermography and derive expressions for the detailed dependence of the Raman peak positions on strain, stress, and electric field components in wurtzite GaN. We also use a combination of semiconductor device modeling and electro-mechanical modeling to predict the stress and strain induced by the inverse piezoelectric effect. Based on the insights gained from our electro-mechanical model and the best values of material properties in the literature, we analyze changes in the E2 high and A1 (LO) Raman peaks and demonstrate that there are major quantitative discrepancies between measured and modeled values of inverse piezoelectric stress and strain. We examine many of the hypotheses offered in the literature for these discrepancies but conclude that none of them satisfactorily resolves these discrepancies. Further research is needed to determine whether the electric field components could be affecting the phonon frequencies apart from the inverse piezoelectric effect in wurtzite GaN, which has been predicted theoretically in zinc blende gallium arsenide (GaAs).