Virtual Source Model Research Articles

A Quasi-Ballistic Model for Short Channel Monolayer Graphene Field Effect Transistor Including Scattering Effects

In this work, the Landauer approach-based short channel model of a graphene field effect transistor has been presented. The quasi-ballistic and ballistic transport mechanism is explained by this approach for nanoscale transistors. The concept of the virtual source is considered to obtain the carrier density in active regions. Here, we apply the Landauer approach for short-channel graphene field effect transistors relating to the parameter in the virtual source model. The mobility of the model has been improved including various scattering effects like impurity scattering, acoustic phonon scattering, and optical phonon scattering. The dynamic resistance model is considered due to asymmetry transport in the electron branch and hole branch. The proposed model has been verified with several experimental works for model validation. The comparisons of the model simulation with several experimental results that have been reported in the literature match well for different short channel lengths of graphene.

A photon source model for alpha-emitter radionuclides

Objective. A Monte Carlo virtual source model named PHID (photon from Ion decay) that generates photons emitted in the complex decay chain process of alpha-emitter radionuclides is proposed, typically for use during the simulation of SPECT image acquisition. Approach. Given an alpha-emitter radionuclide, the PHID model extracts from Geant4 databases the photon emission lines from all decaying daughters for both isometric transition and atomic relaxation processes. According to a given time range, abundances and activities in the decay chain are considered thanks to the Bateman equations, taking into account the decay rates and the initial abundances. Main results. PHID is evaluated by comparison with analog Monte Carlo simulation. It generates photons with the correct energy and temporal distribution, avoiding the costly simulation of the complete decay chain thus decreasing the computation time. The exact time gain depends on the simulation setup. As an example, it is 30× faster for simulating 1 MBq of 225Ac in water for 1 section Moreover, for 225Ac, PHID was also compared to a simplified source model with the two main photon emission lines (218 and 440 keV). PHID shows that 2 times more particles are simulated and 60% more counts are detected in the images. Significance. PHID can simulate any alpha-emitter radionuclide available in the Geant4 database. As a limitation, photons emitted from Bremsstrahlung are ignored, but they represent only 0.7% of the photons above 30 keV and are not significant for SPECT imaging. PHID is open-source, available in GATE 10, and eases the investigation of imaging photon emission from alpha emitters.

The Study of Hydrogen Volume Fraction Effects on the Flame Temperature of Turbulence Diffusion Propane Jet Flames

This paper studies the influence of hydrogen volume fraction effects on the temperature distribution of diffusion turbulent propane jet flames. Numbers of experimental scenarios have been carried out to investigate the evolution of temperature distribution under different hydrogen volume fractions. In the continuous region, these experimental results show that the temperature distribution and the maximum temperature of diffusion of turbulent jet flames are proportional to the hydrogen volume fraction under the same heat release rate of propane. Considering the model of virtual point source and the three-stage model, the theoretical model between the hydrogen volume fraction and flame temperature has been analyzed. The relationship among the temperature distribution, hydrogen volume fraction, and heat release rate has been modified. It can provide some important references for the fire risk assessment of turbulent diffusion jet flames.

Design of a high performance CNFET 10T SRAM cell at 5nm technology node

This article proposes a CNFET 10T SRAM cell based on Stanford Virtual Source model at 5nm technology node, through optimization design and simulation analysis to select optimum gate widths of transistors to ensure best performance in terms of stability, speed and power consumption. We compare the proposed 10T CNFET SRAM with the optimized 6T CNFET SRAM in [9]. It was found that the timing and power characteristics of the proposed 10T SRAM cell is better than that of the 6T structure, the static power consumption is greatly reduced while the RSNM is improved by 93.5%, read and write EDP are improved by 68.5% and 96%, respectively.

Experimental investigation of rarefied flows through supersonic nozzles

In this paper, we report experimental investigation to improve the shape of a supersonic nozzle for rarefied flows for high axial density at extended distances from the nozzle. The reported work is significant for molecular jet/beam applications that require high center-line density and narrow jet profile. We investigate a parabolic nozzle whose profile is generated using the virtual source model of free expansion and compare its performance with a set of conical nozzles having different cone angles using simulations as well as experiments. Axial density and lateral spread of the jets are measured using a pitot tube assembly. The accuracy and operational limit of the pitot tube for rarefied flow is quantified by using established mathematical and empirical models for a sonic nozzle. The study demonstrates that the performance of the parabolic nozzle is comparable or slightly better as compared to conical counterparts. Moreover, the parabolic profile can be used for optimizing the opening angle for conical nozzles of various lengths.

Developing a new Monte Carlo algorithm as an alternative tool to simulate virtual source model on an Elekta Versa HD Linac

The aim of this study is to develop in-house Monte Carlo (MC) algorithm as an alternative tool to simulate all mechanisms of interactions for dose calculation in radiation therapy. Additionally, evaluating the MC-SCAN algorithm with beam modeling of Elekta VersaHD Linac was aimed. Mohan 6 MV photon spectrum was used and the source was modeled as a point source. The absorbed dose is calculated via modeling all interaction mechanisms to gain energy deposition. TPR20/10, PDD, photon profile of different field sizes as well as point dose was calculated after that, a comparison of all the achieved results with experimental measurements went through. The difference between calculated mean energy and theoretical one was obtained <1%. The calculated TPR20/10 value was (0.685 ± 0.03). The difference between experimental PDD and photon profile, and the calculated ones by new model was <3% for all field sizes. A new MC algorithm can be used as an alternative tool to simulate virtual source model on an Elekta VersaHD Linac. All calculated values turned out to present the difference between experimental measurements to be <3% for homogeneous mediums. Nonetheless, further research was needed to be done for the better improvement.

Elaboration and experimental validation of a Monte Carlo source model for linac 6 MV photon beams with and without Flattening Filter

The concept of virtual source model has been developed to provide an efficient and precise particle generator mainly involved in standard photon beam treatment planing and Monte Carlo dosimetry simulations. In this work, we propose a source model for 6 MV linac photon beam for both irradiation modes, with and without flattening filter. Two sources are considered to represent primary and a scattered radiations. Energy spectra taking into account the correlation between energy and diffusion angle have been used. Model parameters were tuned through a dose distribution iterative optimisation approach. The convergence condition relies on γindex evaluation. The simulations were performed in two steps, phase space files were generated using Geant4 toolkit, then energy deposit in water were computed using DPM code. Monte Carlo dose distributions of the model were compared to experimental data of two linac systems, an Elekta Synergy® with flattening filter and a Varian TrueBeam® with and without flattening filter. Good results were obtained in terms of γindex 2%/2 mm criterion, dose difference and distance to agreement (DTA) analysis for field size range from 3 × 3 cm2 to 30 × 30 cm2.

Cryogenic Quasi-Ballistic Transport Enhanced by Strained Silicon Technologies in 14-nm Complementary Fin Field Effect Transistors Through Virtual Source Model

Cryogenic Quasi-Ballistic Transport Enhanced by Strained Silicon Technologies in 14-nm Complementary Fin Field Effect Transistors Through Virtual Source Model

Performance, Analysis, and Modeling of III-V Vertical Nanowire MOSFETs on Si at Higher Voltages

Heterostructure engineering in III-V vertical nanowire (VNW) MOSFETs enables tuning of transconductance and breakdown voltage. In this work, an In_<i>x</i>Ga<inline-formula> <tex-math notation="LaTeX">$_{{1}-{x}}$ </tex-math></inline-formula>As channel with a Ga-composition grading (<inline-formula> <tex-math notation="LaTeX">${x} \,\,=$ </tex-math></inline-formula> 1&#x2013;0.4) in the channel and drain region, combined with field plate engineering, enables breakdown voltage above 2.5 V, while maintaining transconductance of about 1 mS/<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>, in VNW MOSFETs. The field plate consists of a vertically integrated SiO₂ layer and a gate contact, which screens the electric field in the drain region, extending the device operating voltage. By scaling the field plate, a transconductance of 2 mS/<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>, alongside the breakdown voltage of 1.5 V, is obtained, demonstrating the benefit of field engineering in the drain. The scalability of the field plate and the gate is measured, showing an ON-resistance increase by 23 <inline-formula> <tex-math notation="LaTeX">$\Omega \cdot \mu \text{m}$ </tex-math></inline-formula>, and transconductance decrease by 5 <inline-formula> <tex-math notation="LaTeX">$\mu \text{S}/\mu \text{m}$ </tex-math></inline-formula>, per nm field plate length. This behavior is captured in a new and modified virtual source model, where device transmission and drain resistance are altered to capture the field plate scaling effect. The modeling is applied to nanowire (NW) devices with field plate lengths ranging from 5 to 115 nm, capturing accurately essential device performance parameters. Finally, a modified band-to-band (BTB) tunneling approach is used to accurately describe the device behavior above 1.5 V.

Development of a new Hybrid Virtual Source Model to simulate Elekta Synergy MLCi2 linac

Monte Carlo (MC) simulations of medical linear accelerators applying the phase space approach provide excellent results and substantially reduce execution time. This approach becomes impractical because a new complete MC simulation is needed for any new generation of linac heads. To overcome this disadvantage, the investigation exhibits an innovative hybrid Virtual Source Model (VSM) based on a theoretical reconstruction model utilizing the unchangeable parameters of linacs, taking as input the distribution of dose profiles and energy spectra weight. This VSM involves a couple of principal components, virtual source of photons and scattered particle sources from the Virtual Flattening Filter (VFF) and the asymmetric jaws. Extensive calculations and comparisons between the experimental dose distributions and the hybrid VSM are performed to evaluate VSM accuracy. The TOpas and Gate MC codes are utilized to achieve dose computational speedup and memory management optimization. For validation, several regular squared fields of varying sizes ranging from 3 × 3 to 20×20cm2 are considered, as well as two complex IMRT plans based on head&neck and pelvic treatments. Examining the VSM accuracy in the penumbra and the high dose gradient regions, applying gamma index (2%/2mm) criterion confirmed satisfactory results. A two and three dimensional γ index examination demonstrated an excellent agreement concerning the (3%/3mm) criteria for IMRT treatments. Furthermore, the speedup factor reached is 290 when applying this hybrid VSM compared to the entire simulation. Consequently, and according to the results achieved in this task, the hybrid VSM proposed is considered suitable for treatment planning systems (TPS) calculation purposes.

Development of a virtual source model for Monte Carlo-based independent dose calculation for varian linac.

Monte Carlo (MC) independent dose calculations are often based on phase‐space files (PSF), as they can accurately represent particle characteristics. PSF generally are large and create a bottleneck in computation time. In addition, the number of independent particles is limited by the PSF, preventing further reduction of statistical uncertainty. The purpose of this study is to develop and validate a virtual source model (VSM) to address these limitations. Particles from existing PSF for the Varian TrueBeam medical linear accelerator 6X, 6XFFF, 10X, and 10XFFF beam configurations were tallied, analyzed, and used to generate a dual‐source photon VSM that includes electron contamination. The particle density distribution, kinetic energy spectrum, particle direction, and the correlations between characteristics were computed. The VSM models for each beam configuration were validated with water phantom measurements as well as clinical test cases against the original PSF. The new VSM requires 67 MB of disk space for each beam configuration, compared to 50 GB for the PSF from which they are based and effectively remove the bottleneck set by the PSF. At 3% MC uncertainty, the VSM approach reduces the calculation time by a factor of 14 on our server. MC doses obtained using the VSM approach were compared against PSF‐generated doses in clinical test cases and measurements in a water phantom using a gamma index analysis. For all tests, the VSMs were in excellent agreement with PSF doses and measurements (>90% passing voxels between doses and measurements). Results of this study indicate the successful derivation and implementation of a VSM model for Varian Linac that significantly saves computation time without sacrificing accuracy for independent dose calculation.

Large Injection Velocities in Highly Scaled, Fully Depleted Silicon on Insulator Transistors

State-of-the-art Fully-Depleted Silicon-on-Insulator Transistors of different gate lengths were measured down to <inline-formula> <tex-math notation="LaTeX">${\text{L}}_{G}=20$ </tex-math></inline-formula> nm. A quasi-ballistic virtual source model was found to be in good agreement with the observed data for both NFET and PFET. The extracted injection velocity increases with decreasing channel length, as expected, reaching <inline-formula> <tex-math notation="LaTeX">${9.51}\times {10}^{{6}}\text {cm/s}$ </tex-math></inline-formula> for electrons and <inline-formula> <tex-math notation="LaTeX">${7.16}\times {10}^{{6}}\textit {cm/s}$ </tex-math></inline-formula> for holes at <inline-formula> <tex-math notation="LaTeX">${\text{L}}_{G}=20$ </tex-math></inline-formula> nm. These values are more than 80&#x0025; of the thermal velocity in lightly-doped bulk silicon. Analysis shows that quantum confinement in the thin SOI channel as well as strain effects are potentially responsible for such ultra-high velocity. These results indicate that further scaling of the channel length could make it possible to approach the non-degenerate thermal velocity.

The Graphene Field Effect Transistor Modeling Based on an Optimized Ambipolar Virtual Source Model for DNA Detection

The graphene-based Field Effect Transistors (GFETs), due to their multi-parameter characteristics, are growing rapidly as an important detection component for the apt detection of disease biomarkers, such as DNA, in clinical diagnostics and biomedical research laboratories. In this paper, the non-equilibrium Green function (NEGF) is used to create a compact model of GFET in the ballistic regime as an important building block for DNA detection sensors. In the proposed method, the self-consistent solutions of two-dimensional Poisson’s equation and NEGF, using the nearest neighbor tight-binding approach on honeycomb lattice structure of graphene, are modeled as an efficient numerical method. Then, the eight parameters of the phenomenological ambipolar virtual source (AVS) circuit model are calibrated by a least-square curve-fitting routine optimization algorithm with NEGF transfer function data. At last, some parameters of AVS that are affected by induced charge and potential of DNA biomolecules are optimized by an experimental dataset. The new compact model response, with an acceptable computational complexity, shows a good agreement with experimental data in reaction with DNA and can effectively be used in the plan and investigation of GFET biosensors.

Implementation of a new virtual source model in Gate 9.0 package to simulate Elekta Synergy MLCi2 6 MV accelerator

Monte Carlo simulation is appreciated as an extraordinary technique to investigate particle physic processes in Radiation Therapy. This task offers a new Virtual Source Model (VSM) based on an innovative reconstruction method to extract energy and angular distribution from the Python phase space output data. Extensive comparisons of dose distributions are performed to evaluate VSM simulation precision. Four squared field configurations extending from 3 × 3 to 20 × 20 cm2 are chosen for dose calculation to test field size and symmetry influences. To evaluate simulation accuracy, the beam quality parameters (such as D 10 (%), d max (cm), d 80 (cm), and ) also validation tests (gamma index formalism for 2%/2 mm criteria, Distance To Agreement DTA, and the estimator standard error (ϵ, ϵ max)) are determined. Good agreement is achieved in terms of beam quality parameters and validation tests for each evaluated beam size, within a computation time of 58 hours and 17 hours on 20 nodes (presents 160 CPUs) of the full simulation and the VSM, respectively. This advanced VSM generated for the Elekta Synergy MLCi2 platform displays an uncomplicated approach. It is a great example of reconstructing different x-ray beams of various linac accelerators to facilitate its integration in cancer treatment.

PHYSICS OF NANOTRANSISTORS: UNIFICATION OF TRANSMISSION MODEL WITH VIRTUAL SOURCE MODEL – MVS-TRANSMISSION MODEL

In the eigth one from the line our new tutorial reviews, directed to serve students, university teachers and researchers, іt is shown that the transmission model can be closely related to the virtual source model. By simply replacing the diffusion mobility μ in the virtual source model by the apparent mobility app μ for a linear current, we obtain the correct results from the ballistic limit to the diffusion limit. By replacing the saturation rate sat v limited by scattering by the injection rate inj v , we obtain the correct value of the current ON I . A comparison of the experimentally measured characteristics shows that nanotransistors on a silicon substrate work very far from the ballistic limit, while nanosized III-V FETs work very close to the ballistic limit. There are two serious complaints about the model of passage. One of them is connected with the difficulty of calculating the dependence IDS ∝VDS due to the problems of calculating the dependence DS T ∝V . The other is due to the difficulties in predicting the current ON I , which in turn is due to the difficulty of calculating the critical length  at a high voltage on the drain, as a result of which it is difficult to predict the magnitude of SAT T . Because of these limitations, the transmission model and the virtual source model are combined in such a way that the parameters of the transmission model are taken from the insertion of the experimental results into the virtual source model, and the physical meaning of the parameters is taken from the transmission model. It is shown how it is possible to analyze the VAC of nanotransistors on the basis of the MVS/ passage model. The variety of types of transistors generates new peculiar problems, however, the methodology for analyzing experimental data does not change fundamentally. It was stressed that the application of the MVS/transmission model is justified if the transistor is assembled qualitatively. For such transistors, the model makes it possible to obtain physically meaningful parameters reliably.

Modeling of Dirac voltage for highly p-doped graphene field-effect transistor measured at atmospheric pressure

In this paper, the modeling approach of Dirac voltage extraction of highly p-doped graphene field-effect transistor (GFET) measured at atmospheric pressure is presented. The difference of measurement results between atmospheric and vacuum pressures was analyzed. This work was started with actual wafer-scale fabrication of GFET with the purposes of getting functional device and good contact of metal/graphene interface. The output and transfer characteristic curves were measured accordingly to support on GFET functionality and suitability of presented wafer fabrication flow. The Dirac voltage was derived based on the measured output characteristic curve using ambipolar virtual source model parameter extraction methodology. The circuit-level simulation using frequency doubler circuit shows the importance of accurate Dirac voltage value to the device practicality towards design integration.

Validation of histogram‐based virtual source models for different IGRT kV‐imaging systems

Image-guided radiotherapy (IGRT) improves tumor control but its intensive use may entrain late side effects caused by the additional imaging doses. There is a need to better quantify the additional imaging doses, so they can be integrated in the therapeutic workflow. Currently, no dedicated software enables to compute patient-specific imaging doses on a wide range of systems and protocols. As a first step toward this objective, we propose a common methodology to model four different kV-imaging systems used in radiotherapy (Varian's OBI, Elekta's XVI, Brainlab's ExacTrac, and Accuray's Cyberknife) using a new type of virtual source model based on Monte Carlo calculations. We first describe our method to build a simplified description of the photon output, or virtual source models (VSMs), of each imaging system. Instead of being constructed using measurement data, as it is most commonly the case, our VSM is used as the summary of the phase-space files (PSFs) resulting from a first Monte Carlo simulation of the considered x-ray tube. Second, the VSM is used as a photon generator for a second MC simulation in which we compute the dose. Then, the proposed VSM is thoroughly validated against standard MC simulation using PSFs on the XVI system. Last, each modeled system is compared to profiles and depth-dose-curve measurements performed in homogeneous phantom. Comparisons between PSF-based and VSM-based calculations highlight that VSMs could provide equivalent dose results (within 1% of difference) than PSFs inside the imaging field-of-view (FOV). In contrast, VSMs tend to underestimate (for up to 20%) calculated doses outside of the imaging FOV due to the assumptions underlying the VSM construction. In addition, we showed that the use of VSMs allows reducing calculation time by at least a factor of 2.8. Indeed, for identical simulation times, statistical uncertainties on dose distributions computed using VSMs were much lower than those obtained from PSF-based calculations. For each of the four imaging systems, VSMs were successfully validated against measurements in homogeneous phantoms, and are therefore ready to be used for future preclinical studies in heterogeneous or anthropomorphic phantoms. The cross system modeling methodology developed here should enable, later on, to estimate precisely and accurately patient-specific 3D dose maps delivered during a large range of kV-imaging procedures.

Virtual Source Extended Range Points Migration Method for Auto-Focusing 3-D Terahertz Imaging

This letter presents an auto-focusing imaging algorithm for terahertz (THz) band imaging by extending the range point migration (RPM) method using a dielectric lens model. The traditional THz time-domain spectroscopy (THz-TDS) imaging system has an essential problem for depth dependence of the azimuth resolution, which becomes more crucial in subsurface imaging, because the depth of target is unknown in most cases. This letter thus introduces an RPM-based auto-focusing imaging method using an equivalent virtual source model to accurately represent both near- and far-range areas from the focal point. Experimental validation using THz-TDS demonstrated that the proposed algorithm accurately compensates image distortions by out-of-focus effect with much lower complexity than traditional radar-imaging approaches.

PHYSICS OF NANOTRANSISTORS: BALLISTIC INJECTION RATE AND UNIFICATION OF BALLISTIC MODEL WITH THE VIRTUAL SOURCE MODEL

In the sixth one from the line our new tutorial reviews, directed to serve students, university teachers and researchers, the dependence of the electron velocity on the drain voltage in ballistic MOSFETs, as well as its dependence on the gate voltage and on the inversion charge, are considered. At first glance, it seems unusual that in the ballistic MOSFET the speed is saturated with the increase in the drain voltage in the absence of electron scattering, however, the physics of this phenomenon is now quite understandable. In ballistic MOSFET, the electron velocity is saturated not at the drain end of the conduction channel, as in massive transistors, where the electric field is the largest and the scattering intense, but where the source ends and the conduction channel begins, i.e, at the top of the barrier where the electric field is zero.The saturation of the velocity, also known as the ballistic injection rate, is also discussed. It is this speed that is the upper limit of the injection rate in real MOSFETs. If 2 /2 S D n N  , then the ballistic injection rate is constant, however, for 2 / /2 S D n N &gt;  this speed increases with increasing surface density of electrons. Simple calculations of the ballistic injection rate have been made, which can serve as a starting point for more thorough calculations. It is shown how the ballistic model and the virtual source model are interrelated. By simply replacing the traditional mobility, which is limited by scattering, in the virtual source model by ballistic mobility, we obtain the correct course of the ballistic linear current. By replacing the saturation velocity sat v in the massive conduction channel by the ballistic injection rate ball inj v , we obtain the correct value of the ballistic current ON I . It is also shown that the ballistic model predicts larger currents than the experimental data. This is due to the effects of electron scattering, understanding of which is extremely important for modeling nanotransistors.

A Compact Model of MoS2 Field-Effect Transistors From Drift-Diffusion to Ballistic Carrier Transport Regimes

In this letter, a compact model for charge and drain current in molybdenum disulfide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) field-effect transistors (FETs) is developed, which is valid from ballistic to quasi-ballistic to drift-diffusion electronic transport regimes. Considering the influence of trap charges in MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> transistors, a physical-based and analytical charge model is derived. Based on the virtual source model which applies to both ballistic and quasi-ballistic transports, the carrier number density and current expressions are combined to yield the current-voltage (I-V) characteristics. Furthermore, the presented model is validated by experimental data as well as recently reported simulations for MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FETs with different gate lengths. It shows that our model is accurate, straight-forward, scalable and compatible for short- and long-channel devices.