Pinch-off Region Research Articles

Physics of enhanced impact ionization in strained-Si p-channel metal-oxide-semiconductor field-effect transistors

This paper investigates the impact ionization phenomenon in strained-Si p-channel metal-oxide-semiconductor field-effect transistors with different strained-Si cap layer thicknesses at a Ge content of 20%. From the relationship between the impact ionization efficiency and the electric field in the pinch-off region, the strain-induced enhancement of the impact ionization efficiency appears to be quite consistent despite the high hole mobility and considerable scattering in the strained-Si cap layer; this consistent enhancement can be attributed to a decrease in the bandgap energy, regardless of the modification of the mean free path of the holes.

Investigation of Impact Ionization in Strained-Si n-Channel Metal–Oxide–Semiconductor Field-Effect Transistors

In this study, we have systematically re-investigated impact ionization (II) characteristics in strained-Si n-channel metal–oxide–semiconductor field-effect transistors (nMOSFETs) with different strained-Si cap layers at two Ge contents. The strained-Si nMOSFETs can supply further II experimental conditions with band-gap energy narrowing, higher electron mobility, and greater scattering caused by the Ge out-diffusion effect. Despite such II conditions, no marked difference in the II multiplication coefficient as a function of drain voltage, M-1(VD), between unstrained- and strained-Si nMOSFETs is found for widely accepted strain-enhanced II efficiency, implying that II efficiency depends on the maximum channel electric field Em in the pinch-off region. Through the translation of M-1(VD) into M-1(Em), it is found that strain-enhanced II efficiency is attributed to the narrowing of band-gap energy, taking into account the difference in source/drain junction depth between unstrained- and strained-Si nMOSFETs.

A Low LO Power Mixer Utilizing the Body Effect

We propose a single-stacked CMOS mixer that can operate at low local oscillator (LO) power condition with a new switching mechanism. Gating the body terminal makes it possible for the mixer to operate in a more ideal switching mode by utilizing the body effect. Biasing at near pinch-off region gives rise to beneficial aspect, low power dissipation. This circuit is composed of all PMOS transistors which draw only 0.275 mA from a supply voltage of 1.8 V. This circuit features gain and noise enhancement characteristic, low power consumption, and simple topology. The proposed mixer achieves conversion gain of 18 dB, noise figure of 9.1 dB with 0 dBm LO power, and power consumption as low as 0.5 mW.

Quasi 3-D Velocity Saturation Model for Multiple-Gate MOSFETs

This paper presents a quasi-3D velocity saturation model for a multiple-gate MOSFET based on a calculation of Gauss's equation. A new and compact velocity saturation region length is derived from a simple approximation of the electric field distribution in the pinchoff region. It is found that the length of the velocity saturation region increases with the increment of gate length and fin width. This new model is used to derive an analytical expression of a substrate current for a trigate MOSFET. In order to improve the accuracy of the substrate current model, the newly derived substrate current model introduces a parasitic potential drop across a thin-extension region and a dimensionless fitting parameter. A body-tied trigate MOSFET is fabricated on a bulk wafer so that the substrate current could be measured by its body contact. The new substrate current model is then compared with measurement data for a trigate MOSFET, and good agreement is observed

Experiments on bubble pinch-off

A bubble is slowly grown from a vertical nozzle until it becomes unstable and pinches off. We use ultra-high-speed video imaging, at frame-rates up to 1millionfps, to study the dynamics and shape of the pinch-off neck region. For bubbles in water (Bo≃1.0) the radius of the neck reduces with a power law behavior R∼tα, over more than 2 decades, with an exponent in the range α=0.57±0.03, in good agreement with other available studies, but which is slightly larger than 1∕2 predicted by Rayleigh-Plesset theory. The vertical curvature in the neck increases more slowly than the azimuthal curvature, making the neck profiles more slender as pinch-off is approached. Self-similar shapes are recovered by normalizing the axial coordinate by a separate length scale which follows a different power law, Lz∼tγ, where γ=0.49±0.03. Results for air, He, and SF6 gas are identical, suggesting that the gas density plays a minimal role in the dynamics. The pinch-off in water leaves behind a tiny satellite bubble, around 5μm in diameter and the flow-field inside the liquid is shown to be consistent with simple sink flow. The effects of liquid viscosity on the pinch-off speed and neck shapes, are also characterized. The speed starts to slow down at a viscosity of about 10 times that of water, which corresponds to Reμ≃2000. This also changes the power law, increasing the exponent to α≃1 for viscosities above 70cP (Reμ≃40). For surrounding liquid of viscosity above 10cP, we observe just before pinch-off, that the neck is stretched into a thin filament of air, which then breaks into a stream of microbubbles. In some cases we observe a cascade of bubble sizes. While some of the details differ, our results are in overall agreement with those of Burton, Waldrep, and Taborek [Phys. Rev. Lett. 94, 184502 (2005)], except we do not observe the rupture of the air cylinder as it reduces to 50μm size. For water we observe a continuous necking down to the pixel-resolution of our optical system, which at the largest frame-rates is ∼10μm.

Deformation and breakup of a stretching liquid bridge covered with an insoluble surfactant monolayer

The breakup of surfactant-laden drops and jets is of technological interest and fundamental scientific importance. Surfactants are routinely used to control the breakup of drops and jets in applications ranging from inkjet printing to crop spraying. Accurate computation of breakup of surfactant-laden drops and jets is often the key to the development of new applications and to providing a rational fundamental understanding of both existing and emerging applications. While highly accurate algorithms for studying the breakup of surfactant-free drops and jets are well documented and much is now known about the dynamics in such situations, little is known by contrast about the closely related problem of interface rupture when surfactant effects cannot be neglected. The deformation and breakup of a stretching liquid bridge of an incompressible Newtonian fluid whose surface is covered with an insoluble surfactant monolayer are analyzed here experimentally and computationally. In the experiments, high-speed visualization is used to capture the transient deformation of a bridge. The dynamic shapes of bridges (captive between two rods of 3.15 mm diameter) are captured and analyzed with a time resolution of 1 ms. The bridge lengths are 3.15 mm initially and about 4–7 mm at breakup, which occurs after stretching for about 0.1–0.2 s, depending on the volume and viscosity of the liquid and the surface density of spread monolayers. The dynamics of a surfactant-covered bridge is governed by the Navier-Stokes and convection-diffusion equations. First, these equations are solved with a three-dimensional, but axisymmetric, or two-dimensional (2D), finite element algorithm using elliptic mesh generation. Second, the governing set of 2D equations is reduced to a set of one-dimensional (1D) equations by means of the slender-jet approximation and the resulting set of 1D equations is solved with a 1D finite element algorithm. The presence of surfactant results not only in the lowering of surface tension and the capillary pressure, but also in surface tension gradients and Marangoni stresses, both of which affect the transient dynamics leading to breakup. In particular, the role of Marangoni stresses in delaying bridge breakup and on formation of satellite droplets is investigated as a function of the initial surface density and surface activity of the surfactant, and surface Peclet number that measures the importance of convection relative to diffusion. The predictions of the 2D algorithm are confirmed to be faithful to the physics by demonstrating that the computed results accord well with the experiments and existing scaling theories. In the pinch-off region, the surfactant is swept out of a thinning neck by strong convection. The calculations thus reveal that the scaling behavior in the presence of surfactant parallels that observed in the absence of surfactant, in accordance with recent reports by others. The 2D computations and the experiments are used in tandem to identify regions in the space of governing parameters where the 1D equations can be used with confidence.

On the Origin of Increase in Substrate Current and Impact Ionization Efficiency in Strained-Si n- and p-MOSFETs

Strain dependence of substrate current and impact ionization efficiency in the strained-Si n- and p-MOSFETs has been systematically investigated with the wide range of strain, by utilizing the universal relationship between the impact ionization efficiency and the electrical field in the pinch-off region. It was found that the impact ionization efficiency increased with increasing strain, leading to the drastic increase of substrate current, and that the strain dependence of impact ionization efficiency was well represented by simply using the strain dependence of the bandgap energy regardless of carrier type. This result strongly suggests that the strain-induced enhancement of impact ionization efficiency is not due to the modification of the carrier transport properties but to the narrowing of the bandgap, and presumably, the resulting decrease in threshold energy for impact ionization. It was also found that the impact ionization efficiency increased with the increase of temperature while its dependence did not exhibit distinct strain dependence. These results are attributable mainly to the temperature dependence of energy distribution function.

A Compact Model of the Pinch-off Region of 100 nm MOSFETs Based on the Surface-Potential

We have developed a model for circuit-simulation which describes the MOSFET region from pinch-off to drain contact based on the surface potential. The model relates the surface-potential increase beyond the pinch-off point to the channel/drain junction profile by applying the Gauss law with the assumption that the lateral field is greater than the vertical one. Explicit equations for the lateral field and the pinch-off length are obtained, which take the potential increase in the drain overlap region into account. The model, as implemented into a circuit simulator, correctly reproduces measured channel conductance and overlap capacitance for 100 nm pocket-implant technologies as a function of bias condition and gate length.

Drop formation - an overview

We give a brief description of the technological challenges underlying the dynamics of drop formation, noting that a number of significant early advances are associated with the TH Dresden. In particular, to understand the size distribution of drops, one has to consider the non-linear dynamics leading up to the point where the radius of the fluid neck goes to zero. In this pinch-off region, both the neck shape and the velocity inside it are described by scaling laws, which we investigate from the point of view of dimensional analysis. We consider the crossover between different scaling regions, and briefly discuss more recent developments.

Carrier relaxation inGaAsv-groove quantum wires and the effects of localization

Carrier relaxation processes have been investigated in $\mathrm{GaAs}∕{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ v-groove quantum wires (QWRs) with a large subband separation $(\ensuremath{\Delta}E\ensuremath{\simeq}46\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$. Signatures of inhibited carrier relaxation mechanisms are seen in temperature-dependent photoluminescence (PL) and photoluminescence-excitation measurements; we observe strong emission from the first excited state of the QWR below $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This is attributed to reduced intersubband relaxation via phonon scattering between localized states. Theoretical calculations and experimental results indicate that the pinch-off regions, which provide additional two-dimensional confinement for the QWR structure, have a blocking effect on relaxation mechanisms for certain structures within the v groove. Time-resolved PL measurements show that efficient carrier relaxation from excited QWR states into the ground state occurs only at temperatures $\ensuremath{\gtrsim}30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Values for the low-temperature radiative lifetimes of the ground- and first excited-state excitons have been obtained ($340\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ and $160\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, respectively), and their corresponding localization lengths along the wire estimated.

2D ensemble Monte Carlo modelling of bulk and FD SOI MOSFETs: active layer thickness and noise performance

In this work, the RF and microwave noise performance of bulk and fully-depleted silicon-on-insulator (FD SOI) transistors with directly comparable topologies is investigated by means of an ensemble Monte Carlo simulator, and in the case of FD SOI devices, the influence of varying the active layer thickness is also studied. Intrinsic noise sources are calculated and analysed, and important figures of merit such as NFmin or Rn are also determined and evaluated. Moreover, a microscopic understanding of the noise phenomena is achieved, which is essential for the optimization of the transistors. FD SOI devices show a reduced influence of induced gate noise and drain noise as compared to the bulk transistor due to the minor influence of phonon scattering and energetic carriers at the pinch-off region. Whereas reducing the active layer thickness minimizes the effect of hot carriers in the noise parameters, it can lead to a degradation of other important high-frequency figures of merit such as gm or fT.

Simulation of current fluctuations in submicron n-MOSFETs operating in the hot-carrier regime at RF frequencies using two-dimensional impedance field method

We simulate transport and fluctuations in 0.3 µm gate length n-MOSFETs operating in the hot-carrier regime using ISE-TCAD software. Our results are compared with other results given by microscopic simulations. As a first step, drift-diffusion and hydrodynamic models are employed to study the static and dynamic characteristics of these devices in common-source configuration. The velocity overshoot and other hot electron phenomena in the pinch-off region of the channel as well as the admittance parameters and other elements of small-signal equivalent scheme are in good agreement with the results of Monte Carlo simulations. The calculation of the spectral density of current fluctuations is implemented within the two-dimensional direct impedance field method of ISE-TCAD software at RF frequencies, when current fluctuations are dominated by diffusion noise. Among the two macroscopic models, only the hydrodynamic scheme simulates correctly the excess noise due to the presence of hot carriers.

Microwave performance evaluation of diamond surface channel FETs

Diamond is expected to become an important semiconductor for high power and high frequency applications. Recent progress in fabrication technology has enabled DC, small signal RF, large signal RF and switching experiments on devices with short gatelength leading to the first comprehensive assessment of the performance capability of these devices. Using high-quality diamond buffer layers the highest cut-off frequencies for L G=0.2 μm namely f T=24.6 GHz, f max(MAG)=63 GHz and f max(U)=80 GHz could be extracted from small signal measurements near the pinch-off region. First noise measurements have resulted in a minimum noise figure F min of approximately 1 dB at 3 GHz. Power measurement at 1 GHz indicate a saturated output power density of 0.35 W/mm, which is, however, limited by the on-wafer tuning range.

Effects of Geometry on the Tensile Strength of Extrusion Blow Molding Pinch-Off Welds

The pinch-off weld is formed when the mold closes on the parison. A strong weld is dependant on the correctly designed pinch-off geometry of the mold. A pinch-off design relies mostly on past experience and trial and error. There are few equations available to properly design the pinch-off. Material suppliers have provided some guidelines on pinch-off geometry for their materials. The experiments performed in this study show the effect of pinch-off geometry on the tensile load of the weld in extrusion blow molding parts. When the mold closes on the parison and it is squeezed, the geometry affects how the material flows in the pinch-off region. Previous research on pinch-off load was done without an accurate measurement of the parison thickness. The geometry tested was the land length, first angle, second angle, distance to second angle, and flash pocket. The tensile load of the weld was examined in these experiments by the use of design of experiments.

Investigation of the electrical behavior of an asymmetric MOSFET

In this study a possible approach for improving breakdown voltage while maintaining f T for a MOSFET, is presented. In a conventional MOSFET process with LDD the S/D is implanted with a large tilt angle, which gives an asymmetry due to the shadowing effect by the gate. This asymmetry results in a longer drain-LDD region, which in combination with a lower LDD dose, could reduce the electrical field near the drain pinch-off region. A simulation study for different LDD doses and angles has been performed. It is shown that there exist an optimum range of LDD doses where the asymmetric device has higher figure-of-merit, concerning breakdown voltage and cut-off frequency, than the symmetric MOSFET structure.

Modeling of kink effect in polysilicon thin film transistor using charge sheet approach

A current–voltage model based on the charge sheet approach in post saturation that includes the “kink effect” for polycrystalline silicon thin film transistor is presented. To explain the kink effect, the increase in the channel charge density due to the impact ionization caused by the high electric field in the pinch-off region near the drain is evaluated. The charge density thus obtained is used to obtain the drain current in the post saturation and the results are matched with the experimental data, showing good agreement.

Pinch-off modes and satellite formation in liquid/liquid jet systems

Forced jets of glycerin/water solution flowing into silicone oil were investigated. Forcing amplitudes were sufficient to induce stable or semi-stable pinch-off modes in the jet. The indices of refraction in the two liquids were matched, and planar slices of flow were visualized using laser induced fluorescence. Two-dimensional velocity fields were obtained by particle image velocimetry (PIV). For a viscosity ratio, η i/ η o=1.6, satellites form at lower Reynolds numbers as well as lower Strouhal numbers. Satellite occurrence depends on the interface shape approaching pinch-off as well as the detailed velocity gradients surrounding the pinch-off region. Satellite formation required a zone of concave curvature downstream of but close to the jet neck as pinch-off approached. This zone was associated with intense vorticity that first deformed the interface into an upstream-facing cone attached to the main drop and then deformed the cone into a satellite shape. Increasing the ambient viscosity suppressed satellite formation for the Reynolds and Strouhal numbers examined.

A novel drain current I-V model for MESFET

The conventional approach for modeling the dc I-V characteristics of a MESFET transistor usually adopts the hyperbolic tangent dependence on V/sub ds/. On the contrary, our new empirical model describes the device drain current as a polynomial of effective gate-source voltage V/sub eff/. The derived model is capable of accurately modelling the subthreshold effect and the device current-voltage behavior at different operating regions, in particular, the device operation around the pinchoff region. Measured and modeled results of a 0.5-/spl mu/m gatelength MESFET device are compared and good agreement has been obtained. Comparisons between the proposed model, Curtice model, Chalmers model, and Parker model are also made in this paper. In addition, a single-stage class-AB amplifier was built with a commercial high-power MESFET transistor to verify the new model.

Monte Carloanalysis of dynamic and noise performance of submicron MOSFETs at RF and microwave frequencies

In this paper, an ensemble 2D bipolar Monte Carlosimulator is employed for the study of static characteristics,high-frequency response and noise behaviour in a 0.3 µmgate-length n-MOSFET in common source configuration.Short-channel effects, such as velocity overshoot in the pinch-off region, together with the appearance of hot electrons at thedrain end of the channel are observed in the staticcharacteristics. Admittance parameters and the small-signalequivalent circuit have been calculated in order tocharacterize the dynamic response of the device. The use of abipolar simulator allows one to study the dynamics of both types ofcarriers simultaneously. While the static results are dominatedby the electron transport, the contribution of holes mainly affects the drain-substrate capacitive coupling. The noisebehaviour of the simulated MOSFET is also studied (up to40 GHz) by means of different parameters, such as the spectral densities of the current fluctuations at the drain and gateterminals (and their cross-correlation), normalized α,β and C parameters and NFmin . In the saturationregime, due to the presence of hot carriers, an increase indrain and gate noise with respect to the long-channelprediction has been found. Moreover, a stronger correlationbetween drain and gate noise is observed, especially at lowdrain current. Induced gate noise is found to play a crucialrole in the determination of NFmin at high draincurrents.

Dependence of polycrystalline silicon thin-film transistor characteristics on the grain-boundary location

Dependence of transistor characteristics on the grain-boundary location in polycrystalline silicon (poly-Si) thin-film transistors (TFTs) has been analyzed using device simulation. In the linear region, degradation is similar wherever the grain boundary is located. On the other hand, in the saturation region, degradation is less when the grain boundary is in the pinch-off region near the drain edge and degradation is similar when the grain boundary is elsewhere. Although this dependence is similar to the dependence on the trap location in single-crystal silicon transistors, the mechanism is different. This dependence in poly-Si TFTs is because the coulombic potential barrier caused by the grain boundary is lowered in the high electric field in the pinch-off region. This is a kind of Poole–Frenkel effect.