95-GHz scattering by terrain at near-grazing incidence

This study, consisting of three complimentary topics, examines the millimeter-wave backscattering behavior of terrain at incidence angles extending between 70 and 90°, corresponding to grazing angles of 20° to 0°. The first topic addresses the character of the statistical variability of the radar backscattering cross section per unit area σ_A. Based on an evaluation of an extensive data set acquired at 95 GHz, it was determined that the Rayleigh fading model (which predicts that σ_A is exponentially distributed) provides an excellent fit to the measured data for various types of terrain covers, including bare surfaces, grasses, trees, dry snow, and wet snow. The second topic relates to the angular variability and dynamic range of the backscattering coefficient σ⁰, particularly near grazing incidence. We provide a summary of data reported to date for each of several types of terrain covers. The last topic focuses on bare surfaces. A semi-empirical model for σ⁰ is presented for vertical (VV), horizontal (HH), and cross (HV) polarizations. The model parameters include the incidence angle &thetas;, the surface relative dielectric constant ϵ, and the surface roughness ks, where k=2π/λ and s is the surface root mean square (RMS) height     

Monte Carlo simulations of InGaAs nano-MOSFETs

The potential performance of sub-50 nm n-type implant free III-V MOSFETs with an In_0.75Ga_0.25As channel is studied using Monte Carlo (MC) device simulations. The simulated I_D-V_G characteristics of the In_0.75Ga_0.25As implant free MOSFETs are compared with equivalent In_0.3Ga_0.7As implant free MOSFETs and with a state-of-the-art silicon CMOS transistors. The study is based on careful calibration of the MC simulator against experimental data obtained from a δ-doped In_0.52Ga_0.48As/ In_0.53Ga_0.47As/In_0.75Ga_0.25As heterostructure with a high-κ gate dielectric. At 0.8 V supply voltage, the 30 nm gate length In_0.75Ga_0.25As implant free III-V MOSFET delivers a drive current of 1730 μA/μm as compared to the 1550 μA/μm obtained in the equivalent In_0.3Ga_0.7As implant free MOSFET. When this high indium channel transistor is scaled to 20 and 15 nm gate lengths the drive current at 0.8 V supply voltage increases to 2465 and 2745 μA/μm, respectively, making it a good candidate for high performance, low power digital applications at the 22 nm technology generation and beyond.     

Monte Carlo simulations of double-gate MOSFETs

A fullband Monte Carlo simulator has been used to analyze the performance of scaled n-channel double-gate (DG) MOSFETs. Size quantization in the channel is accounted for by using a quantum correction based on Schrodinger equation. Scattering induced by the oxide interface is included with a model calibrated with measurements for bulk devices. The detailed self-consistent treatment of quantum effects leads to several interesting observations. We observe that the sheet charge in DG devices does not decrease as much as expected in bulk devices when quantum-mechanical effects are included. The average carrier velocity in the channel is also somewhat reduced by quantum effects, as a second-order effect. In the test cases studied here, application of quantum effects causes a reduction in simulated current not exceeding 15%. In a DG structure, quantum effects tend to concentrate the charge density in the center of the channel, where transverse fields are lower. Because of this, interface scattering appears to be less pronounced when quantum effects are included.     

Advantages of collocation methods over finite differences in one-dimensional Monte Carlo simulations of submicron devices

Collocation methods are very useful when one-dimensional Monte Carlo simulations of semiconductor submicron devices require a very accurate solution of Poisson's equation. Potential and electric field may be solved simultaneously with better accuracy than using finite differences. The extension to two dimensions is also outlined. We present the results obtained for Monte Carlo simulation of submicron W/Si and AuGaAs Schottky barrier diodes under forward bias conditions. The accurate solution for the electric field at the ohmic contact boundary allows us to model the injected current and to account for depletion of carriers. Tunnelling effects across the barrier are also included in the simulation.     

Monte Carlo simulations of high-speed InSb-InAlSb FETs

Self consistent Monte Carlo simulations which include impact ionization are used to study the high-speed potential of InSb field-effect transistors. It is found that the impact ionization has a strong influence on the performance of InSb for high speed. The ionization leads to a high electron drift velocity and substrate bias can be used to extract the holes which are generated in the channel. Residual hole density within the channel, however, limits the maximum speed. The substrate bias and buffer doping are critical for extracting holes from the channel without inducing excess ionization. Simulations yield a peak cutoff frequency of 820 GHz with a 0.03125-/spl mu/m gate, a source to drain voltage of 0.58, and a sheet doping density of 1.7/spl times/10/sup 12/ cm/sup -2/.     

Monte Carlo simulations for tilted-channel electron multipliers

Microchannel electron multipliers with tilted structures are simulated using the Monte Carlo method. Gains of secondary electrons are calculated for different structures of the electron multiplier. For a short tilted cylindrical channel of the electron multiplier, a maximum gain is achieved greater than 10⁴ at a tilt angle near 25°. The maximum gain is about 10³ times larger than that of the nontilted channel. An explanation for the improvement of gain in tilted channel is suggested     

Monte Carlo computer technique for one-dimensional random media

Hochstim and Martens were the first to use Monte Carlo computations to study scattering characteristics of scalar waves from randomly fluctuating slabs with an exponential spatial correlation. This paper describes an alternative procedure which overcomes some of the difficiencies in their treatment and extends the study to physical circumstances which their treatment excludes (e.g., background inhomogeneity, crossover into cutoff regions, etc.). The two-point correlation function characterizing the medium fluctuations (more directly the spectral density) may be arbitrarily selected and is no longer restricted to an exponential. The correlation properties of the medium are virtually independent of the length of the elementary slabs comprising the overall slab region, whence the medium may be structured by equal sized slab realizations-a major advantage whenever a background profile is superimposed. Furthermore, the statistics associated with the reflection and transmission of both the coherent as well as the incoherent waves are calculated separately. The results so obtained yield solutions with which appropriate analytical theories can be conveniently compared and they provide information additional to that found in previous studies. As examples, the syntheses of Gaussian and exponential correlation functions are shown. For the exponential, propagation calculations for a homogeneous random slab were made using the new technique and compared with results using Hochstim and Martens' approach.     

Monte Carlo simulations of carrier transport in AlGaInP laserdiodes

A self-consistent ensemble Monte Carlo simulation of charge transport in AlGaInP quantum-well (QW) lasers has been developed in an effort to understand the temperature sensitivity of these devices. In particular, the lasing capability of a three-well design has been studied at 300 and 360 K. Although the electron and hole leakage currents are found to increase with the temperature, this does not explain a reduction in the emitted light hole emission time intensity for the particular device studied. Instead, the fall in the light output is due to increased emission from the QW's, since this reduces the net electron and hole capture efficiency     

Monte Carlo simulations of charge transport in high-speed lasers

A self-consistent ensemble Monte Carlo calculation of carrier transport in multiple-quantum-well lasers has been developed in an effort to understand the impact of picosecond carrier dynamics upon the modulation bandwidth. The model has been applied to InGaAsP-based devices which are designed to emit at 1550 nm. Results are discussed for structures with ungraded confinement layers of varied width and different numbers of QWs. Simulations have been carried out at fixed and also modulated bias, from which the intrinsic frequency response can be derived     

Monte Carlo simulations of homogeneous upconversion in erbium-dopedsilica glasses

Quenching of Er³⁺ ions by homogeneous energy-transfer upconversion in high-concentration erbium-doped silica glasses has been theoretically investigated, The results indicate that at Er³⁺ concentrations of 1.0-2.0·10²⁶ m^-3 or below, the kinetic limit of strong migration is not reached, and hence the widely accepted quadratic upconversion model is not generally valid. Nevertheless, the results offer an explanation of the experimental observations of quadratic upconversion. Furthermore, it has been shown that at a given population inversion, the quenching rate depends on the rate of exchange of the excited Er³⁺ ions by emission and absorption     

Monte Carlo surface scattering simulation in MOSFET structures

The Monte Carlo method has been applied to MOSFET devices with the gate lengths less than 1 µm. The electric field in the channel was obtained by an analytical approach. Since the classical situation is approached in the submicrometer gate device, the partial diffusive model is employed for surface scattering process. Transient phenomena such as velocity overshoot have been predicted with drain biases causing a large field gradient in the channel. Comparison of the results of the Monte Carlo simulation with those obtained by an analytical approach based on static mobility shows that the carrier transit time in the channel is shorter (as much as two times) than that predicted by the analytical approach for a 0.3 µm gate device.     

Noise characterization of block-iterative reconstruction algorithms: II. Monte Carlo simulations

In Soares et al. (2000), the ensemble statistical properties of the rescaled block-iterative expectation-maximization (RBI-EM) reconstruction algorithm and rescaled block-iterative simultaneous multiplicative algebraic reconstruction technique (RBI-SMART) were derived. Included in this analysis were the special cases of RBI-EM, maximum-likelihood EM (ML-EM) and ordered-subset EM (OS-EM), and the special case of RBI-SMART, SMART. Explicit expressions were found for the ensemble mean, covariance matrix, and probability density function of RBI reconstructed images, as a function of iteration number. The theoretical formulations relied on one approximation, namely that the noise in the reconstructed image was small compared to the mean image. In this paper, we evaluate the predictions of the theory by using Monte Carlo methods to calculate the sample statistical properties of each algorithm and then compare the results with the theoretical formulations. In addition, the validity of the approximation will be justified.     

Monte Carlo simulations of the bandwidth of InAlAs avalanche photodiodes

We present a Monte Carlo simulation of the bandwidth of an InAlAs avalanche photodiode with an undepleted absorber. The carrier velocities are simulated in the charge layer and the multiplication region. It is shown that the velocity overshoot effect is not as significant as simpler models have suggested. At high electric field intensity, the electron effective saturation velocity is only slightly higher when impact ionization is significant, compared with when impact ionization is absent. The simulated 3 dB bandwidth is consistent with experiments for gains up to 50.     

Quantifying spatial localization of optical mapping using Monte Carlo simulations

Optical mapping techniques used to study spatial distributions of cardiac activity can be divided into two categories. 1) Broad-field excitation method, in which hearts stained with voltage or calcium sensitive dyes are illuminated with broad-field excitation light and fluorescence is collected by image or photodiode arrays. 2) Laser scanning method, in which illumination uses a scanning laser and fluorescence is collected with a photomultiplier tube. The spatial localization of the fluorescence signal for these two methods is unknown and may depend upon light absorption and scattering at both excitation and emission wavelengths. We measured the absorption coefficients (micro a), scattering coefficients (micro s), and scattering anisotropy coefficients (g) at representative excitation and emission wavelengths in rabbit heart tissue stained with di-4-ANEPPS or co-stained with both Rh237 and Oregon Green 488 BAPTA 1. Monte Carlo models were then used to simulate absorption and scattering of excitation light and fluorescence emission light for both broad-field and laser methods in three-dimensional tissue. Contributions of local emissions throughout the tissue to fluorescence collected from the tissue surface were determined for both methods. Our results show that spatial localization depends on the light absorption and scattering in tissue and on the optical mapping method that is used. A tissue region larger than the laser beam or collecting area of the array element contributes to the optical recordings.     

Monte Carlo simulations of wave scattering from lossy dielectricrandom rough surfaces using the physics-based two-grid method and thecanonical-grid method

In using the method of moments to solve scattering by lossy dielectric surfaces, usually a single dense grid (SDG) with 30 points per wavelength is required for accurate results. A single coarse grid (SCG) of ten points per wavelength does not give accurate results. However, the central processing unit (CPU) and memory requirements of SDG are much larger than that of SCG. In a physics-based two-grid method (PBTG) two grids are used: a dense grid and a coarse grid. The method is based on the two observations: (1) Green's function of the lossy dielectric is attenuative and (2) the free-space Green's function is slowly varying on the dense grid. In this paper, the PBTG method is combined with the banded-matrix iterative approach/canonical grid method to solve rough surface scattering problem for both TE and TM cases and also for near grazing incidence. We studied cases of dielectric permittivities as high as (25+i)ϵ₀ and incidence angle up to 85°. Salient features of the numerical results are: (1) an SCG has poorer accuracy for TM case than TE case; (2) PBTG-banded-matrix iterative approach/canonical grid BMIA/CAG method speeds up CPU and preserves the accuracy; it has an accuracy comparable to single dense grid and yet has CPU comparable to single coarse grid; (3) PBTG-BMIA/CAG gives accurate results for emissivity calculations and also for low grazing backscattering problems (LGBA); and (4) the computational complexity and the memory requirements of the present algorithm are O(N log(N)) and O(N), respectively, where N is the number of surface unknowns on the coarse grid     

Monte Carlo simulation of scattering from a layer of verticalcylinders

Electromagnetic scattering by a collection of randomly distributed vertical cylinders over a half-space dielectric is considered, using two approaches. In the first approach, a Monte Carlo simulation that takes into account scattering terms up to second order is used. Closed-form expressions for the second-order scattering terms are derived for cylinders that are in each other's near field. The second approach is based on the radiative transfer equations, which are solved by an iterative method up to and including the second-order terms. Radar backscatter measurements at X-band for a collection of metallic cylinders over a conducting ground plane are compared with the Monte Carlo and radiative transfer solutions. The data were acquired polarimetrically from 144 independent spots of the cylinder layer at incidence angles ranging from 20° to 60°. The simulation results agree well with the measured data and are used to check the validity of the radiative transfer results for a medium with large particles. It is shown that both the phase function computed for the cylinders and the extinction matrix of the layer are overestimated in the radiative transfer solution     

Monte Carlo simulation of electromagnetic scattering fromtwo-dimensional random rough surfaces

The fast multipole method fast Fourier transform (FMM-FFT) method is developed to compute the scattering of an electromagnetic wave from a two-dimensional (2-D) rough surface. The resulting algorithm computes a matrix-vector multiply in O(N log N) operations. This algorithm is shown to be more efficient than another O(N log N) algorithm, the multilevel fast multipole algorithm (MLFMA), for surfaces of small height. For surfaces with larger roughness, the MLFMA is found to be more efficient. Using the MLFMA, Monte Carlo simulations are carried out to compute the statistical properties of the electromagnetic scattering from 2-D random rough surfaces using a workstation. For the rougher surface, backscattering enhancement is clearly observable as a pronounced peak in the backscattering direction of the computed bistatic scattering coefficient. For the smoother surface, the Monte Carlo results compare well with the results of the approximate Kirchhoff theory     

舰船尾流气泡多重散射偏振特性研究

舰船尾流气泡的多次散射效应对光尾流探测领域的研究具有重要意义.针对水中气泡多次散射对后向散射光强以及偏振度的影响规律问题,基于矢量Monte Carlo方法,采用欧拉矢量法对光子在水中气泡后向散射偏振态进行跟踪,对比分析了线偏振、圆偏振激光入射情况下,不同散射次数回波信号在强度和偏振度特性上的差异性,验证了欧拉矢量法用于光尾流探测仿真领域的可行性.     

A method to reduce small-angle scattering in Monte Carlo deviceanalysis

Ionized-impurity scattering is an anisotropic process showing a high preference for small scattering angles. In a Monte Carlo simulation of a semiconductor device many small angle scattering events have to be processed, although the contribution of these events to carrier momentum relaxation is small. A new method is presented which reduces the amount of small-angle scattering very effectively. In the simulation an isotropic process is used which yields the same momentum relaxation time as the anisotropic process. A theoretical analysis based on the Boltzmann equation is carried out. Monte Carlo calculations are performed over a wide range of doping concentrations, lattice temperatures and electric fields. No systematic difference is found in the results from the anisotropic and the isotropic scattering models. For a given accuracy, the reduction of needed scattering events and free flights can be more than one order of magnitude at low and medium doping concentrations     

Monte-Carlo simulations of large-scale problems of random roughsurface scattering and applications to grazing incidence with theBMIA/canonical grid method

Scattering of a TE incident wave from a perfectly conducting one-dimensional random rough surface is studied with the banded matrix iterative approach/canonical grid (BMIA/CAG) method. The BMIA/CAG is an improvement over the previous BMIA. The key idea of BMIA/CAG is that outside the near-field interaction, the rest of the interactions can be translated to a canonical grid by Taylor series expansion. The use of a flat surface as a canonical grid for a rough surface facilitates the use of the fast Fourier transform for nonnear field interaction. The method can be used for Monte-Carlo simulations of random rough surface problems with a large surface length including all the coherent wave interactions within the entire surface. We illustrate results up to a surface length of 2500 wavelengths with 25000 surface unknowns. The method is also applied to study scattering from random rough surfaces at near-grazing incidence. The numerical examples illustrate the importance of using a large surface length for some backscattering problems