Two-dimensional Monte Carlo Simulation Research Articles

Monte Carlo study of kink effect in short-channel InAlAs/InGaAs high electron mobility transistors

A semiclassical two-dimensional ensemble Monte Carlo simulator is used to perform a physical microscopic analysis of the kink effect in short-channel InAlAs/InGaAs lattice-matched high electron mobility transistors (HEMTs). Due to the small band gap of InGaAs, these devices are very susceptible to suffer impact ionization processes, with the subsequent hole transport in the channel, both supposedly implicated in the kink effect and easy to be implemented in a Monte Carlo simulation. The results indicate that for high enough VDS, holes, generated by impact ionization, tend to pile up in the channel under the source side of the gate due to the attracting potential caused by the surface charge at the recess and, mostly, by the gate potential. Due to this pile up of positive charge, the potential barrier controlling the current through the channel is lowered, so that the channel is further opened and ID increases, leading to the well known kink effect in the current–voltage characteristics. The microscopic understanding of this phenomenon provides valuable information to conceive the optimum fabrication process for kink-effect-free HEMTs.

Hydrocarbon transport in the MkIIa divertor of JET

This contribution presents two-dimensional Monte-Carlo simulations of the local transport of hydrocarbons that are chemically eroded in the JET MkIIa divertor. The effect of a given background carbon flux flowing from the main plasma down to the divertor is also taken into account. The influence of local plasma temperatures and densities and the influence of different assumptions for the sticking of re-deposited particles hitting the tiles is analysed. Even under the assumption that the sticking of re-deposited hydrocarbon fragments is zero, a large amount (about 75%) of the eroded particles is re-deposited in the form of ionized carbon on the tiles. A reasonable agreement between the simulation and the experimental observation of the carbon deposition at the inner louvers in the MkIIa JET divertor is achieved, if high chemical erosion yields of about 20% together with a negligible effective sticking of hydrocarbons are assumed. Although there are experimental observations indicating such high erosion yields, such yields cannot be applied as a stationary value for the effective erosion since it would turn the inner divertor into a net erosion area in contrast to the experimental findings. The possibility of applying different erosion rates for (re)-deposited layers and the substrate material is discussed. To further improve the modelling a better knowledge about the formation and erosion of re-deposited layers is required. A first conclusion concerning the erosion and re-deposition behaviour in ITER will be drawn.

Ballistic nanodevices for terahertz data processing: Monte Carlo simulations

By using a semi-classical two-dimensional Monte Carlo simulation, simple devices(T-branch junctions (TBJs) and rectifying diodes) based on AlInAs/InGaAsballistic channels are analysed. Initially, the model is validated by means ofHall-effect measurements of mobility and electron concentration in long (diffusive)channels. Then, quasi-ballistic transport at room temperature is confirmed in a100 nm channel. Our simulations qualitatively reproduce the experimental resultsof electric potential measured in a TBJ appearing as a result of electron ballistictransport, and in close relation with the presence of space charge inside thestructure. As examples of devices exploiting the ballistic transport of electrons,preliminary simulations of a multiplexor/demultiplexor and a rectifying diodeare presented, demonstrating their capability for terahertz operation.

Two-dimensional Monte Carlo Simulations of a Colloidal Dispersion Composed of Polydisperse Ferromagnetic Particles: For the Case of No External Magnetic Field

Two-dimensional Monte Carlo Simulations of a Colloidal Dispersion Composed of Polydisperse Ferromagnetic Particles: For the Case of No External Magnetic Field

Numerical and experimental study of a 0.25 µm fully-depleted silicon-on-insulator MOSFET: static and dynamic radio-frequency behaviour

We investigate the static and dynamic characteristics of a 0.25 µm gate-length fully-depleted silicon-on-insulator metal–oxide–semiconductor field effect transistor. Considering properly the physical topology of the transistor with its accesses, numerical simulations are performed using a two-dimensional ensemble Monte Carlo simulator and these are compared with experimental results. Moreover, in the simulation we include important effects that appear in real transistors, such as surface charges, contact resistances, impact ionization phenomena and extrinsic parasitic capacitances. The appearance of a velocity overshoot region near the drain, together with the presence of hot carriers in the channel, is observed in the saturation regime. The dynamic behaviour of the device is described through significant small signal equivalent circuit parameters, which are examined by means of internal quantities (such as profiles of the internal electron concentration), provided by numerical simulations. In general, the device shows excellent performance in terms of important design parameters, such as transconductance-to-current ratio, transconductance and cut-off frequency together with a reduced capacitive coupling to the substrate. The results of the Monte Carlo simulations show an exceptional agreement with the experimental data, thus confirming the validity of the simulator as a reliable tool for the analysis of the influence of geometrical and electrical parameters on the static and dynamic performance of the device.

The effect of high-K gate dielectrics on deep submicrometer CMOS device and circuit performance

The potential impact of high permittivity gate dielectrics on device short channel and circuit performance is studied over a wide range of dielectric permittivities (K/sub gate/) using two-dimensional (2-D) device and Monte Carlo simulations. The gate-to-channel capacitance and parasitic fringe capacitances are extracted using a highly accurate three-dimensional (3-D) capacitance extractor. It is observed that there is a decrease in parasitic outer fringe capacitance and gate-to-channel capacitance in addition to an increase in internal fringe capacitance, when the conventional silicon dioxide is replaced by a high-K gate dielectric. The lower parasitic outer fringe capacitance is beneficial for the circuit performance, while the increase in internal fringe capacitance and the decrease in the gate-to-channel capacitance will degrade the short channel performance contributing to higher DIBL, drain leakage, and lower noise margin. It is shown that using low-K gate sidewalls with high-K gate insulators can decrease the fringing-induced barrier lowering. Also, from the circuit point of view, for the 70-nm technology generation, the presence of an optimum K/sub gate/ for different target subthreshold leakage currents has been identified.

Relating Interactions between Neurofilaments to the Structure of Axonal Neurofilament Distributions through Polymer Brush Models

Neurofilaments (NFs) have been proposed to interact with one another through mutual steric exclusion of their unstructured C-terminal “sidearm” domains, producing order in axonal NF distributions and conferring mechanical strength to the axon. Here we apply theory developed for polymer brushes to examine the relationship between the brush properties of the sidearms and NF organization in axons. We first measure NF–NF radial distribution functions and occupancy probability distributions for adult mice. Interpreting the probability distributions using information theory, we show that the NF distributions may be represented by a single pair potential of mean force. Then, to explore the relationship between model parameters and NF architecture, we conduct two-dimensional Monte Carlo simulations of NF cross-sectional distributions. We impose purely repulsive interaction potentials in which the sidearms are represented as neutral and polyelectrolyte chains. By treating the NFs as telechelic polymer brushes, we also incorporate cross-bridging interactions. Both repulsive potentials are capable of reproducing NF cross-sectional densities and their pair correlations. We find that NF structure is sensitive to changes in brush thickness mediated by chain charge, consistent with the experimental observation that sidearm phosphorylation regulates interfilament spacing. The presence of attractive cross-bridging interactions contributes only modestly to structure for moderate degrees of cross-bridging and leads to NF aggregation for extensive cross-bridging.

Role of the antiphase-boundary energy in periodic-to-quasiperiodic superlattice transitions: a Monte Carlo simulation of ordering in Al-rich γ-TiAl

Abstract Results of the strip projection method have established that formation of antiphase boundaries (APBs) is the initial stage of a periodic-to-quasiperiodic transformation in superlattices. The Al5Ti3−to-quasiperiodic superlattice transformation, observed in Al-rich γ-TiAl intermetallics, is a classic example of such a transition. Energy of the relevant APBs would, therefore, be expected to play an important role in the occurrence of this transition. In the two-dimensional Monte Carlo simulations carried out in this work, change in the APB energy, accomplished by suitably altering the pair interaction energies, was found to explain the diffraction effects observed in the alloys with various compositions depicting this transition. The results of the simulation bring out salient features of the atomic arrangements in imperfectly ordered quasiperiodic and near-quasiperiodic structures encountered in this transition.

Role of the antiphase-boundary energy in periodic-to-quasiperiodic superlattice transitions: A Monte Carlo simulation of ordering in Al-rich γ-TiAl

Abstract Results of the strip projection method have established that formation of antiphase boundaries (APBs) is the initial stage of a periodic-to-quasiperiodic transformation in superlattices. The Al5Ti3−to-quasiperiodic superlattice transformation, observed in Al-rich γ-TiAl intermetallics, is a classic example of such a transition. Energy of the relevant APBs would, therefore, be expected to play an important role in the occurrence of this transition. In the two-dimensional Monte Carlo simulations carried out in this work, change in the APB energy, accomplished by suitably altering the pair interaction energies, was found to explain the diffraction effects observed in the alloys with various compositions depicting this transition. The results of the simulation bring out salient features of the atomic arrangements in imperfectly ordered quasiperiodic and near-quasiperiodic structures encountered in this transition.

Validation of mesoscopic theory and its application to computing concentration dependent diffusivities

Despite numerous molecular simulation studies on the diffusion of small molecules through nanoporous films and on surfaces, most simulations have been limited to equilibrium conditions and relatively short time and length scales. Currently, there is a lack of a suitable computational framework which links molecular scale information with diffusion over larger scales. Here we employ a recently derived mesoscopic theory, which is exact in the limit of an infinite range potential, to model diffusion by a hopping mechanism through nanoporous films or on surfaces. One- and two-dimensional lattice gradient continuous time Monte Carlo simulations are compared to solutions of mesoscopic models for various finite range potentials and microscopic diffusion dynamics. The latter are solved using finite difference and spectral methods. We have found that the mesoscopic models quantitatively describe diffusion even for relatively short-range potentials, enabling comparison of model predictions to experimental data. It is shown that the accuracy of mesoscopic theories for a finite range potential improves with increasing system dimensionality. We have found that parameterization of a concentration dependent diffusion coefficient depends on various parameters in a complex way and can be multiple-valued. The implications of these results for modeling diffusion through membranes are also discussed.

A model for molecular adsorption on the surface of a columnar structure including steric effects and adsorbate–adsorbate repulsive interactions

Abstract Two-dimensional Monte Carlo simulations for adsorption on a columnar structured substrate were made to explore the steric effects of pore width and depth on molecular adsorption under adsorbate-adsorbate repulsive interactions. Simulation adsorption data fit a Frumkin-type isotherm for each set of pore width, depth and top column dimensions. As pore depth increases and pore width approaches the adsorbate molecular size, the surface coverage by adsorbed particles decreases sharply. Therefore, the adsorbate–adsorbate interaction parameters derived from the simulated isotherms exhibit a marked dependence on the substrate roughness factor (R). Phenomenologically, the influence of R on the isotherms is smoothed down by introducing a correction factor (f), which allowed us to collapse all simulated isotherms into a single isotherm. Results from the model are qualitatively compared to earlier observations about the influence of columnar structured gold surface roughness on the adsorption of 1,10-phenantroline in acid solution (oph) at constant potential (E>Epzc).

Aspect ratio dependent plasma-induced charging damage in rf precleaning of a metal contact

As the packing density increases in the fabrication of semiconductor chips, the aspect ratio and the critical dimension (CD) of a metal contact are exponentially aggravated in dry etch processing. The aspect ratio dependency of plasma-induced charging damage during the rf precleaning of a metal contact has been evaluated with a two-dimensional Monte Carlo simulation and with related experiments. From the simulation of a metal contact opened on a gate metal, it is found that the potential on a metal contact bottom, which is directly related to plasma-induced charging damage, is saturated near an aspect ratio of 4 after initially linearly increasing with the aspect ratio. However, the linear decrease of CD of a metal contact exponentially increases the potential stress on the gate oxide. These simulation results are confirmed with the two different experiments, an in situ charge-up monitoring and the electrical test of fully fabricated complementary metal–oxide–semiconductor wafers. A phase controlled inductively coupled plasma is newly proposed to suppress the plasma-induced charging damage. With this system, the plasma-induced damage is strongly suppressed when the phase delay of the bias power to the source power is near 180°.

Reconstruction of doses from radionuclide inhalation for nuclear power plant workers using air concentration measurements and associated uncertainties.

This paper presents a case study to illustrate the influence of parameter uncertainties on calculating an internal radiation dose of one actual nuclear plant worker, alias Mr. X, as well as the utility of air sampling for internal dose reconstruction. Input probability distributions for air concentrations of radionuclides were derived from empirical air measurements taken by fixed area air samplers. The total internal dose was calculated by multiplying radionuclide intake by dose conversion factors in Monte-Carlo simulations. There is significant variability in dose conversion factors and uncertainty in the estimated concentrations of radionuclides in the air to which Mr. X was exposed. The high variability and uncertainty of the model parameters contributed to large ranges of predicted internal doses for Mr. X. Two-dimensional Monte-Carlo simulations were conducted to separate contributions of the uncertainty and variability. Sensitivity analysis was conducted to determine which of the input parameters contributed most to uncertainty in internal dose estimates. Our analysis suggests that the uncertainty resulting from use of general air surveys contributes more to the internal dose ranges than the variability from DCFs and other population-derived parameters. Reduction of the uncertainty in reconstructed internal dose can be achieved by using personal air sampling and/or individual bioassays and regular whole-body counting.

Monte Carlo simulation of abnormal grain growth in two dimensions

Abnormal grain growth in the presence of second-phase particles is investigated with the help of a two-dimensional Monte Carlo simulation. An aggregate of equiaxed grains is considered with constant grain boundary energy and mobility. The only driving force accounted for stems from the grain boundary curvature. The process of abnormal grain growth is investigated as a function of two governing parameters, the initial degree of pinning of the matrix grains by the particles and the initial size advantage of the anomalous grain. In such conditions, moderate growth is obtained whose specific features are discussed with respect to the available models. It is shown that it is possible to obtain drastic grain growth by introducing the thermally activated unpinning of grain boundaries from particles. For this purpose, a simplified but effective procedure is proposed and discussed that includes the influence of the capillary force on the height of the local energy barrier for grain unpinning.

A quantitative risk assessment for fumonisins B1 and B2 in US corn

Quantitative risk analysis permits modifying risk estimates with changes in variables such as exposure. This analysis for exposure to the mycotoxin fumonisin describes the magnitude of adverse effects, variability in the population and uncertainty of models as a range of possible outcomes. The most sensitive adverse response in rats, nephrotoxic lesions, was used for the dose-response analysis. Dietary intake of corn products was estimated from a 3-day consumption survey. Levels of corn in each product were estimated by standard methods. Fumonisin levels in corn products were estimated from Food and Drug Administration (FDA) surveillance data and distributions of fumonisin consumption were modelled for each eater in the survey population. Uncertainty for predictions made from each model and uncertainty resulting from model selection were described. Results of the dose-response and exposure analyses were assimilated in a two-dimensional Monte-Carlo simulation. Distributions representing variability and uncertainty were iteratively selected to form an array of estimates of the risk. On the basis of this analysis, current dietary levels of fumonisin would not result in renal lesions even at upper levels of exposure. To avoid toxicity at much higher doses, limiting corn intake would be more effective than would limiting the level of fumonisin in corn.

Breakdown quenching in high electron mobility transistor by using body contact

In this paper, the effect of a body contact (BC) to quench breakdown effects and increase the breakdown voltage in high-electron mobility transistors (HEMTs) is theoretically investigated. The body contact is formed by a highly p-type doped substrate connected to an ohmic back contact. By means of a two-dimensional (2-D) self consistent Monte Carlo simulator we show that the BC prevents holes generated by impact ionization (II) from accumulating in the channel and in the buffer, thus inhibiting the parasitic bipolar effect (PBE). This improves the breakdown behavior and extends the range of the usable drain voltages.

Simulation of enzymatic cellular reactions complicated by phase separation.

We present two-dimensional Monte Carlo simulations of enzymatic cellular reaction occurring via the Michaelis-Menten scheme in the case of attractive interactions between the reaction products. The model employed predicts phase separation in the cell provided that the reaction is relatively fast. The shape of the corresponding patterns varies from a few separate islands to a large patch located in the center of the cell. The fluctuations of the reaction rate during such regimes are found to be much higher than those predicted by the Poissonian distribution.

COMPUTATIONAL ASPECTS OF THE LOCAL ITERATIVE MONTE CARLO TECHNIQUE

Lately, the Local Iterative Monte Carlo technique was introduced for an efficient simulation of effects connected to sparsely populated regions in semiconductor devices like hot electron effects in silicon MOSFETs. This paper focuses on computational aspects of this new Monte Carlo technique, namely the reduction of the computation time by parallel computation and the reuse of drift information. The Local Iterative Monte Carlo technique combines short Monte Carlo particle flight simulations with an iteration process to a complete device simulation. The separation between short Monte Carlo simulations and the iteration process makes a simple parallelization strategy possible. The necessary data transfer is small and can be performed via the Network File System. An almost linear speed up could be achieved. Besides by parallelization, the computational expenses can be significantly reduced, when the results of the short Monte Carlo simulations are memorized in a drift table and used several times. A comparison between a bulk, a one-dimensional and the two-dimensional Local Iterative Monte Carlo simulation reveals that by using the drift information more than once, becomes increasingly efficient with increasing dimension of the simulation.

Monte Carlo study of the dynamic breakdown effects in HEMT's

We present a theoretical investigation of the near-breakdown scenario in pseudomorphic high electron mobility transistors (HEMT's). We show that the main mechanism for the enhanced drain current is a parasitic bipolar effect due to holes, generated by impact ionization, which accumulate in the channel and in the substrate close to the source contact. The dynamic of this charge accumulation and of the consequent drain current increase is studied by means of a two-dimensional (2-D) Poisson Monte Carlo simulator.

Analysis of electric field distribution in GaAs metal–semiconductor field effect transistor with a field-modulating plate

Electric field distribution in the channel of a field effect transistor (FET) with a field-modulating plate (FP) has been theoretically investigated using a two-dimensional ensemble Monte Carlo simulation. This analysis revealed that the introduction of FP is effective in canceling the influence of surface traps under forward bias conditions and in reducing the electric field intensity at the drain side of the gate edge under pinch-off bias conditions. This study also found that a partial overlap of the high-field region under the gate and that at the FP electrode is important for reducing the electric field intensity. The optimized metal–semiconductor FET with FP (FPFET) (LGF∼0.2 μm) exhibited a much lower peak electric field intensity than a conventional metal–semiconductor FET. Based on these numerically calculated results, we have proposed a design procedure to optimize the power FPFET structure with extremely high breakdown voltages while maintaining reasonable gain performance.