Numerical simulation of clustering phenomena for point-defects in HgCdTe

We investigated the formation of cluster defects in HgCdTe materials by numerical simulation. The equations used for the simulation include change in the activation energy for Hg vacancy diffusion dependent on the local cadmium composition. This study demonstrated how defects grow during thermal treatment. We simulated the annealing process for initial composition fluctuations in uniform n-type Hg_0.78Cd_0.22Te. When the local composition fluctuations increase, we found that the concentration of vacancies around the initial core increases because of the composition difference between the initial core and bulk region. Concurrently, if the fluctuation range is narrow, the resulting large constant for interstitial diffusion accelerates this process. We determined that the critical fluctuation range is in the order of 50 nm for annealing at 150°C. The concentration of clustering vacancies around the initial core reached 10¹⁶ cm⁻³, meaning that the conductivity of the cluster region changed to the p-type. We conclude that such cluster defects have a structure consisting of a core surrounded by a p-type shell.     

Eccentric spheres models of the head

Equations are derived for electric potentials (electroencephalograms) and magnetic fields (magnetoencephalograms) produced by dipolar sources in three eccentric spheres models of the head. In these models, I) the thickness of the layer representing the skull varies around the model, II) the thickness of the scalp layer varies, and III) the electrical conductivity of an eccentric spherical "bubble" in the brain region varies. Using these equations, it was found that variations in these features of the models have at most only small effects on the general spatial patterns of the electric potentials and the radial component of the magnetic fields. However, some significant effects on the amplitudes were found. The effects of the variations in the skull and scalp layer thicknesses on the field amplitudes were found to be significantly smaller than on the potential amplitudes. The effects on the field amplitudes of the variations in the bubble conductivity were found to be only somewhat smaller than on the potential amplitudes. It was also found that the effects of variations in these features of the models on source localization accuracy were significantly smaller for inverse solutions using fields than for solutions using potentials.     

Standardizing compact models for IC simulation

The idea of standard compact (SPICE-like) model equations has gained support recently throughout the semiconductor industry. In the past, compact models have been developed independently either by a single company or by a university or research group. These models have lacked diverse technology coverage and normally were not fully tested or productized. The concept of standardization is embraced by the semiconductor industry in several other areas, yet simulation has lagged behind due to the difficult notion of standardizing software. In this article, the idea of a standard compact model will be described as well as the industry consortium supporting the standardization effort     

Direct tunnelling models for circuit simulation

This paper presents a review of models for direct tunnelling with a view to identifying suitable models for inclusion in a circuit simulator. For thin oxides, the critical quantities required for the derivation of tunnel current are the transparency of the barrier, the oxide field and the supply of carriers for tunnelling. This paper reviews different approaches to the incorporation of these quantities in analytical models. A new model for direct tunnelling, which includes quantum effects in a format suitable for circuit simulation, is outlined. Recent developments in MOSFET models, which include gate current, are briefly discussed.     

Critical connectivity phenomena in multihop radio models

The percolation of a broadcast in a multihop radio network modeled by a spatial Poisson process is studied. The effect of station density and transmission radius on the extent of broadcast percolation is examined. For broadcast percolation in one spatial dimension, analytical expressions for the average extent of percolation are derived. A model for two-dimensional spatial percolation is presented along with related simulation results. The results suggest that in optimizing transmission radius as a function of communication performance measures, the choice of radius may be bounded from below by the need to maintain a desired network connectivity. The connectivity constraint can be relaxed to some extent in certain non-Poisson spatial models     

Circuit simulation models for the high electron mobility transistor

A three-terminal model is formulated for the high electron mobility transistor (HEMT) using charge-voltage relationships derived from the Boltzmann transport. Emphasis is placed on modeling the current transport of the two-dimensional electron gas (TEG) and the capacitance of the embedded parasitic MESFET structure. Furthermore, a distributed circuit topology is used to better model high-frequency effects, such as the transit time delay, in both small-signal and large-signal-transient analysis. The HEMT model is implemented in the circuit simulation program HP-SPICE. Both dc and ac simulation results are discussed.     

Spatially independent VCSEL models for the simulation of diffusiveturn-off transients

Carrier diffusion and spatial hole burning can have a severe impact on vertical cavity surface emitting laser (VCSEL) performance. In particular, these phenomena can produce secondary pulses, bumps, and optical tails in the VCSEL turn-off transient which limit both the system bit rate and the bit error rate (BER). To study these effects, laser rate equation models that include both spatial and temporal dependence are often employed; however, simulations which require discretization of both space and time, while accurate, typically consume vast amounts of computational power. In this paper, we demonstrate that models based on well-accepted spatially independent rate equations can be used to simulate these effects. These models exhibit the advantages of the full spatio-temporal approach but execute much more quickly. We also integrate these models into electronic computer-aided design (CAD) tools which will enable circuit and system designers to simultaneously simulate electrical and optical performance     

An improved buffer for bioelectric signals

We propose an ac-coupled amplifier that offers a high input impedance, thus making it suitable for bioelectric signal amplification. We also present the necessary formulas for calculating its input impedance and transfer function in order to facilitate its adaptation to different applications.     

DC machine models for SPICE2 simulation

A four-level computer model is proposed for DC machine simulation using SPICE2 to meet different simulation requirements. The most complex model takes account of magnetic saturation, armature reaction, current dependence of winding circuit parameters, and eddy current effects. The models have been developed to enable designers to simulate the static and dynamic characteristics of a complete converter drive system including the DC machine more simply, practically, and reliably in one simulation run. Some simulations have been investigated to demonstrate the benefits of the SPICE2 machine models     

Techniques for fast simulation of models of highly dependablesystems

With the ever-increasing complexity and requirements of highly dependable systems, their evaluation during design and operation is becoming more crucial. Realistic models of such systems are often not amenable to analysis using conventional analytic or numerical methods. Therefore, analysts and designers turn to simulation to evaluate these models. However, accurate estimation of dependability measures of these models requires that the simulation frequently observes system failures, which are rare events in highly dependable systems. This renders ordinary Simulation impractical for evaluating such systems. To overcome this problem, simulation techniques based on importance sampling have been developed, and are very effective in certain settings. When importance sampling works well, simulation run lengths can be reduced by several orders of magnitude when estimating transient as well as steady-state dependability measures. This paper reviews some of the importance-sampling techniques that have been developed in recent years to estimate dependability measures efficiently in Markov and nonMarkov models of highly dependable systems     

Low-noise two-wired buffer electrodes for bioelectric amplifiers

Active buffer electrodes are known to improve the immunity of bioelectric recordings against power line interferences. A survey of published work reveals that buffer electrodes are almost exclusively designed using operational amplifiers (opamps). In this paper, we discuss the advantage of utilizing a single transistor instead. This allows for a simple electrode, which is small and requires only two wires. In addition, a single transistor adds considerably less noise when compared to an opamp with the same power consumption. We then discuss output resistance and gain as well as their respective effect on the common mode rejection ratio (CMRR). Finally, we demonstrate the use of two-wired buffer electrodes for a bioelectric amplifier.     

Self consistent bipolar transistor models for computer simulation

Extended Ebers-Moll models of bipolar junction transistors have proven useful in computer-aided design packages for many years. The most recent refinement has been the incorporation of basewidth modulation or Early effect through a single model parameter called the Early voltage. This paper delineates the conditions under which a unique Early voltage can be defined and shows that a necessary condition is the base current must be independent of collector-base voltage. Further, a linearization of the large-signal model around a d.c. operating point leads to a small-signal hybrid-pi model in which the collector-base resistor r_u is necessarily absent. The absence of r_u has many implications in the design of high performance amplifier circuits.     

Two-dimensional computer simulation models for MOSLSI fabrication processes

A two-dimensional process simulation program has been developed. The process models used for this program are oxidation, diffusion, ion implantation, and deposition/etching of CVD films. The numerical models are based on a finite-difference approximation to diffusion equation. A large number of equations derived from the diffusion equation are solved by Stone's method because of its excellent rate of convergence. Attention is paid primarily to lateral impurity diffusion and lateral oxidation near the edge of the oxidation mask. Oxidation enhanced diffusion of boron is also included. We have obtained good quantitative agreement between calculated and experimentally observed diffused line capacitance variation with reverse bias voltage which is strongly affected by the lateral channel stop diffusion in a locally oxidized process.     

A method for localizing EEG sources in realistic head models

A computationally practical method for performing moving dipole calculations to localize EEG sources in realistic, boundary element (integral equation) type of head models is presented. This method makes use of a rapid method of solving the forward problem of calculating the EEG's produced by a dipole in a realistic head model. This rapid forward calculation method allows the use of standard Simplex search techniques to solve the inverse problem of localizing electrical sources in the brain from EEG's measured on the scalp     

Two-dimensional energy-dependent models for the simulation ofsubstrate current in submicron MOSFET's

Two-dimensional energy-dependent substrate current models are described for NMOS and PMOS devices that have been developed using a multi-contour approach. The new models offer a significant improvement in the calculation of substrate current due to a more accurate calculation of the average energy as compared to the local-field model. The models are implemented in a post-processing manner by applying a one-dimensional energy conservation equation to each of many current contours in order to generate a two-dimensional representation of average energy and impact ionization rate, that is then integrated to calculate the substrate current. The new models have been compared to substrate current characteristics of a variety of NMOS and PMOS devices for a wide range of bias conditions and channel lengths, and very good agreement has been obtained with a single set of model parameters. An additional significance of this work is the enhancement of the standard multi-contour model by an energy-sink term that results in an improved prediction of the impact ionization process in PMOSFET's     

Accurate and efficient models for the simulation of neuron MOS integrated circuits

In this paper, we have simulated some neuron MOS analogue and digital integrated circuits by the proposed macromodels of neuron MOS transistor and complementary neuron MOS used in SPICE-based computer simulators. Both models take into account all the geometrical and electrical parameters of the studied device structure, and they are applicable to DC and transient simulations. Simulation results are presented and compared with recent experimental data.     

Comparison among mathematical models of the photovoltaic cell for computer simulation purposes

This paper presents a comparison among mathematical models used in the simulation of solar photovoltaic modules that can be easily integrated with power electronic converters. In order to perform the analysis, three models available in literature and also the physical model of the module in software PSIM® are used. Some results regarding the respective I × V and P × V curves are presented, while some advantages and eventual limitations are discussed. Besides, a DC–DC buck converter performs maximum power point tracking by using perturb and observe method, while the performance of each one of the aforementioned models is investigated.     

Investigation into the performance of turbulence models for fluid flow and heat transfer phenomena in electronic applications

Heat is extracted away from an electronic package by convection, conduction, and/or radiation. The amount of heat extracted by forced convection using air is highly dependent on the characteristics of the airflow around the package which includes its velocity and direction. Turbulence in the air is also important and is required to be modeled accurately in thermal design codes that use computational fluid dynamics (CFD). During air cooling the flow can be classified as laminar, transitional, or turbulent. In electronics systems, the flow around the packages is usually in the transition region, which lies between laminar and turbulent flow. This requires a low-Reynolds number numerical model to fully capture the impact of turbulence on the fluid flow calculations. This paper provides comparisons between a number of turbulence models with experimental data. These models included the distance from the nearest wall and the local velocity (LVEL), Wolfshtein, Norris and Reynolds, k-/spl epsiv/, k-/spl omega/, shear-stress transport (SST), and k/spl epsiv//kl models. Results show that in terms of the fluid flow calculations most of the models capture the difficult wake recirculation region behind the package reasonably well, although for packages whose heights cause a high degree of recirculation behind the package the SST model appears to struggle. The paper also demonstrates the sensitivity of the models to changes in the mesh density; this study is aimed specifically at thermal design engineers as mesh independent simulations are rarely conducted in an industrial environment.     

Implicit deductive fault simulation for complex delay fault models

This paper introduces an implicit version of the well-known deductive fault simulation technique suitable to delay fault models with an exponential number of faults. The proposed method calculates the fault coverage by generating lists of entities for each line during a single topological circuit traversal. Each stored entity only contains a number and a subset of the test vectors. No delay faults are stored, and no special data structures are required. There are significant differences between the presented implicit method and fault coverage using deductive fault simulation. The method is shown to be effective for delay the path and segment delay fault models.