A full 3D non-equilibrium Green functions study of a stray charge in a nanowire MOS transistor

The effect of the location of a negative stray charge associated with an acceptor type defect state in the channel of a nanowire transistor has been investigated using a Non Equilibrium Green’s Function Formalism in the effective mass approximation. Due to the fact that the nanowire cross-section is 2.2×2.2 nm², we have calculated the effective masses using Tight Binding (TB) calculations. A third neighbor sp ³ TB model has been used. We have found that the on current is two time smaller when the charge is located in the source end as compared to its location in the drain end. We have also studied the effect on the current of the spatial distribution of the acceptor charge. The calculations show that when the charge is more distributed (de-localized) the effect of the blocking of the current is less efficient, so the current is higher.     

Modeling drive currents and leakage currents: a dynamic approach

The dynamics of electrons and holes propagating through the nano-scaled channels of modern semiconductor devices can be seen as a widespread manifestation of non-equilibrium statistical physics and its ruling principles. In this respect both the devices that are pushing conventional CMOS technology towards the final frontiers of Moore’s law and the upcoming set of alternative, novel nanostructures grounded on entirely new concepts and working principles, provide an almost unlimited playground for assessing physical models and numerical techniques emerging from classical and quantum mechanical non-equilibrium theory. In this paper we revisit the Boltzmann as well as the Wigner–Boltzmann equation which offers a valuable platform to study transport of charge carriers taking part in drive currents. We focus on a numerical procedure that regained attention recently as an alternative tool to solve the time-dependent Boltzmann equation for inhomogeneous systems, such as the channel regions of field-effect transistors, and we discuss its extension to the Wigner–Boltzmann equation. Furthermore, we pay attention to the calculation of tunneling leakage currents. The latter typically occurs in nano-scaled transistors when part of the carrier distribution sustaining the drive current is found to tunnel into the gate due the presence of an ultra-thin insulating barrier separating the gate from the channel region. In particular, we discuss the paradox related to the very existence of leakage currents established by electrons occupying quasi-bound states, while the (real) wave functions of the latter cannot carry net currents. Finally, we describe a simple model to resolve the paradox as well as to estimate gate currents provided the local carrier generation rates largely exceed the tunneling rates.     

Confined phonon scattering in multivalley Monte Carlo simulation of quantum cascade lasers

We present a detailed investigation of the effects that optical phonon confinement has on the electronic transport properties of GaAs-based multiple-quantum-well (MQW) quantum cascade laser (QCL) structures. Both confined and interface phonon modes are included based on the macroscopic dielectric continuum model. Interface phonon dispersions are obtained using the transfer matrix method with periodic boundary conditions. Scattering rates of both Γ- and X-valley electrons by all the interface and confined phonon modes are calculated and fully incorporated in the multivalley Monte Carlo simulation of a deep-active-well 6.7 μm GaAs-based MQW QCL. We find that the inclusion of phonon confinement enhances the electron-phonon scattering rates and output current to a relatively small extent with respect to the bulk phonon approximation.     

Full band Monte Carlo simulation of dislocation effects on 250 nm AlGaN-GaN HEMT

In this work, we present a full band Monte Carlo simulation of the effects of dislocation scattering on the performance of a 0.25 μm AlGaN/GaN HEMT (high electron mobility transistor). We performed a full characterization of the device and validated the simulation results with experimental data (Lee et al. in IEEE Electron. Dev. Lett. 24:613–615, [2003]). Here we show a study of the DC device performance as a function of the density of thread dislocations.     

Design of random doping fluctuation resistant structures of semiconductor devices

An automated technique is presented for the computation of the doping profiles that minimize the intrinsic fluctuations of various parameters induced by random doping fluctuations in semiconductor devices. The technique is based on the computation of the doping sensitivity functions of the parameters under consideration and the constrained minimization of the standard deviation of fluctuations by using the Lagrange multipliers technique. The technique is then applied to the minimization of the random doping induced fluctuations of the threshold voltage in 40-nm channel length MOSFET device.     

Magnetization transfer imaging of cortical bone in vivo using a zero echo time sequence in mice at 4.7 T: a feasibility study

Objective

To investigate the feasibility of magnetization transfer (MT) imaging in mice in vivo for the assessment of cortical bone.

Materials and methods

MT-zero echo time data were acquired at 4.7 T in six mice using MT preparation pulses with two different flip angles (FAs) and a series of ten different off-resonance frequencies (500–15000 Hz). Regions of interest were drawn at multiple levels of the femoral cortical bone. The MT ratio (MTR) was computed for each combination of FAs and off-resonance frequencies. T1 measurements were used to estimate the direct saturation (DS) using a Bloch equation simulation. Estimation of the absorption line width of cortical bone from T2* measurements was also performed.

Results

MTR values were higher using 3000° FA than 1000° FA. MTR values decreased toward higher off-resonance frequencies. Maximum mean MTR ± standard deviation (SD) of 58.57 ± 5.22 (range 50.44–70.61) was measured with a preparation pulse of 3000° and off-resonance frequency of 500 Hz. Maximum “true” MT effect was estimated at around 2–3 and 5 kHz, respectively, for 1000° and 3000° FA. Mean full width at half maximum ± SD of 577 ± 91 Hz was calculated for the absorption spectral line of the cortical bone.

Conclusion

MT imaging can be used for the assessment of cortical bone in mice in vivo. DS effects are negligible using preparation pulses with off-resonance frequencies greater than 3 kHz.

     

An investigation of performance limits of conventional and tunneling graphene-based transistors

In this paper we perform a simulation study on the limits of graphene-nanoribbon field-effect transistors (GNR-FETs) for post-CMOS digital applications. Both conventional and tunneling FET architectures are considered. Simulations of conventional narrow GNR-FETs confirm the high potential of these devices, but highlight at the same time OFF-state leakage problems due to various tunneling mechanisms, which become more severe as the width is made larger and require a careful device optimization. Such OFF-state problems are partially solved by the tunneling FETs, which allow subthreshold slopes better than 60 mV/dec, at the price of a reduced ON-current. The importance of a very good control on edge roughness is highlighted by means of a direct simulation of devices with non-ideal edges.     

Advance Compensated Mho Relay Algorithm for a Transmission System with Shunt Flexible AC Transmission System Device

Abstract—The presence of shunt flexible AC transmission system devices adversely affect the performance of distance relay and create security and reliability issues. This article introduces a noble compensated Mho relay algorithm for the protection of transmission line employing shunt flexible AC transmission system devices, such as a static VAR compensator and static synchronous compensator. A detailed model of transmission system employing a shunt flexible AC transmission system device is explained. Then compensated impedance inserted by a shunt device in the transmission line is calculated, and finally, a compensated Mho relay algorithm is proposed to protect zone one of the transmission line. Simulation work is carried out in PSCAD/EMTP software. Results show that the proposed relay is secure, accurate, and reliable under the wide variation in power system parameters, such as load angle, fault resistance, fault location, and compensation level.     

Analytical and self-consistent quantum mechanical model for a surrounding gate MOS nanowire operated in JFET mode

We derive an analytical model for the electrostatics and the drive current in a silicon nanowire operating in JFET mode. We show that there exists a range of nanowire radii and doping densities for which the nanowire JFET satisfies reasonable device characteristics. For thin nanowires we have developed a self-consistent quantum mechanical model to obtain the electronic structure.     

Subjective perception of safety in healthy individuals working with 7 T MRI scanners: a retrospective multicenter survey

Objective

To retrospectively assess perception of safety of healthy individuals working with human 7 Tesla (T) magnetic resonance imaging (MRI) scanners.

Materials and methods

A total of 66 healthy individuals with a mean age of 31 ± 7 years participated in this retrospective multicentre survey study. Nonparametric correlation analysis was conducted to evaluate the relation between self-reported perception of safety and prevalence of sensory effects while working with 7 T MRI scanners for an average 47 months.

Results

The results indicated that 98.5 % of the study participants had a neutral or positive feeling about safety aspects at 7 T MRI scanners. 45.5 % reported that they feel very safe and none of the participants stated that they feel moderately or very unsafe while working with 7 T MRI scanners. Perception of safety was not affected by the number of hours per week spent in the vicinity of the 7 T MRI scanner or the duration of experience with 7 T MRI. More than 50 % of individuals experienced vertigo and metallic taste while working with 7 T MRI scanners. However, participants’ perceptions of safety were not affected by the prevalence of MR-related symptoms.

Conclusions

The overall data indicated an average perception of a moderately safe work environment. To our knowledge, this study delineates the first attempt to assess the subjective safety perception among 7 T MRI workers and suggests further investigations are indicated.

     

An advanced description of oxide traps in MOS transistors and its relation to DFT

Recently, an advanced model for defects in the insulating regions of semiconductor devices has been suggested, which can explain the removable component of the negative bias temperature instability (NBTI) and recoverable random telegraph/flicker noise. We give a brief introduction to the atomic scale physics behind the model and show how model parameters can be extracted from density functional theory (DFT) calculations. The central link between DFT calculations and device simulation is the carrier energy dependent part of the capture cross section, the line shape function. Calculations of the line shape functions of model defect structures using a simple harmonic approximation are presented. The calculations show a considerable shift in the oscillator frequency upon charge state transitions for the defects investigated.     

Ex-vivo cellular MRI with b-SSFP: quantitative benefits of 3 T over 1.5 T

INTRODUCTION: The use of MRI with iron-based magnetic nanoparticles for imaging cells is a rapidly growing field of research. We have recently reported that single iron-labeled cells could be detected, as signal voids, in vivo in mouse brains using a balanced steady-state free precession imaging sequence (b-SSFP) and a customized microimaging system at 1.5 T. METHODS: In the current study we assess the benefits, and challenges, of using a higher magnetic field strength for imaging iron-labeled cells with b-SSFP, using ex vivo mouse brain specimens imaged with near identical systems at 1.5 and 3.0 T. RESULTS: The substantial banding artifact that appears in 3 T b-SSFP images was readily minimized with RF phase cycling, allowing for banding-free b-SSFP images to be compared between the two field strengths. This study revealed that with an optimal 3 T b-SSFP imaging protocol, more than twice as many signal voids were detected as with 1.5 T. CONCLUSION: There are several factors that contributed to this important result. First, a greater-than-linear SNR gain was achieved in mouse brain images at 3 T. Second, a reduction in the bandwidth, and the associated increase in repetition time and SNR, produced a dramatic increase in the contrast generated by iron-labeled cells.     

Numerical and experimental evaluation of RF shimming in the human brain at 9.4 T using a dual-row transmit array

Objective

To provide a numerical and experimental investigation of the static RF shimming capabilities in the human brain at 9.4 T using a dual-row transmit array.

Materials and methods

A detailed numerical model of an existing 16-channel, inductively decoupled dual-row array was constructed using time-domain software together with circuit co-simulation. Experiments were conducted on a 9.4 T scanner. Investigation of RF shimming focused on B₁ ⁺ homogeneity, efficiency and local specific absorption rate (SAR) when applied to large brain volumes and on a slice-by-slice basis.

Results

Numerical results were consistent with experiments regarding component values, S-parameters and B₁ ⁺ pattern, though the B₁ ⁺ field was about 25 % weaker in measurements than simulations. Global shim settings were able to prevent B₁ ⁺ field voids across the entire brain but the capability to simultaneously reduce inhomogeneities was limited. On a slice-by-slice basis, B₁ ⁺ standard deviations of below 10 % without field dropouts could be achieved in axial, sagittal and coronal orientations across the brain, even with phase-only shimming, but decreased B₁ ⁺ efficiency and SAR limitations must be considered.

Conclusion

Dual-row transmit arrays facilitate flexible 3D RF management across the entire brain at 9.4 T in order to trade off B₁ ⁺ homogeneity against power-efficiency and local SAR.     

Unified simulation of transport and luminescence in optoelectronic nanostructures

Computer simulation of microscopic transport and light emission in semiconductor nanostructures is often restricted to an isolated system of a single quantum well, wire or dot. In this work we report on the development of a simulator for devices with various kinds of nanostructures which exhibit quantization in different dimensionalities. Our approach is based upon the partition of the carrier densities within each quantization region into bound and unbound populations. A bound carrier is treated fully coherent in the directions of confinement, whereas it is assumed to be totally incoherent with a motion driven by classical drift and diffusion in the remaining directions. Coupling of the populations takes place through electrostatics and carrier capture. We illustrate the applicability of our approach with a well-wire structure.     

30 years of sodium/X-nuclei magnetic resonance imaging

In principle, all nuclei with nonzero spin can be employed for magnetic resonance imaging (MRI). Special scanner hardware and MR sequences are required to select the nucleus-specific frequency and to enable imaging with “sufficient” signal-to-noise ratio. This Special Issue starts with an overview of different nuclei that can be used for MRI today, followed by a review article about techniques required for imaging of quadrupolar nuclei with short relaxation times. Sequence developments to improve image quality and applications on different organs and diseases are presented for different nuclei (²³Na, ³⁵Cl, ¹⁷O, and ¹⁹F), with a focus on imaging at natural abundance.     

Radar observations of electrical atmospheric discharges occurring above the altitude of 20 km

Analysis of actual radar data of upper atmospheric phenomena indicates that neglecting refraction of electromagnetic waves causes the computed altitudes of the reflecting structures to be overestimated. Curves of target altitude against elevation angle are presented. They reveal that it is unlikely that the atmospheric phenomena occur at altitudes below 20 km. The Earth equivalent radius factor is proposed as an indicator of the plausibility of the computed altitudes.     

Modeling heating effects in nanoscale devices: the present and the future

In this review paper we give an overview on the present state of the art in modeling heat transport in nanoscale devices and what issues we need to address for better and more successful modeling of future devices. We begin with a brief overview of the heat transport in materials and explain why the simple Fourier law fails in nanoscale devices. Then we elaborate on attempts to model heat transport in nanostructures from both perspectives: nanomaterials (the work of Narumanchi and co-workers) and nanodevices (the work of Majumdar, Pop, Goodson and recently Vasileska, Raleva and Goodnick). We use our own simulation results which we have used to examine heat transport in nanoscaling devices to point out some important issues such as the fact that thermal degradation does not increase as we decrease feature size due to the more pronounced non-stationary transport and ballistic transport effects in nanoscale devices. We also point out that instead of using SOI, if one uses Silicon on Diamond technology there is much less heat degradation and better spread of the heat in the Diamond material. We also point out that tools for thermal modeling of nanoscale devices need to be improved from the present state of the art as 3D tools are needed, for example, to simulate heat transport and electrical transport in a FinFET device. Better models than the energy balance equations for the acoustic and optical phonons what we presently use in our simulators are also welcomed. The ultimate goal is to design the tool that can be efficient enough but at the same time can simulate most accurately both electrons and phonons within the particle pictures by solving their corresponding Boltzmann transport equations self-consistently. Investigations in integration of Peltier coolers with CMOS technology are also welcomed and much needed to reduce the problem of heat dissipation in nanoscale devices and interconnects.     

Use of density gradient quantum corrections in the simulation of statistical variability in MOSFETs

With the scaling of field-effect transistors to the nanometre scale, it is well recognised that TCAD simulations of such devices need to account for quantum mechanical confinement effects. The most widely used method to incorporate quantum effects within classical and semi-classical simulators is via density gradient quantum corrections. Here we present our methodologies for including the density gradient method within our Drift-Diffusion and Monte Carlo simulators and highlight some of the additional benefits that this provides when dealing with the charge associated with random discrete dopants.