Threshold current density in strained layerInxGa1-xAs-GaAs quantum-well heterostructurelasers

The authors consider the transparency carrier density in ideal and practical strained layer In_xGa_1-xAs-GaAs quantum-well heterostructure lasers. The transparency carrier density in practical structures is then related to transparency current density using realistic values for spontaneous recombination rates. These parameters are incorporated with representative structural parameters into a nonlinear model for gain in a quantum-well laser, in order to provide a complete model for the laser threshold current density in strained layer In_xGa_1-xAs-GaAs quantum-well heterostructure lasers. These results are then compared and contrasted with experimental laser results from several laboratories     

Reconstruction of the constitutive parameters for an /spl Omega/ material in a rectangular waveguide

The inverse problem of determining the constitutive parameters of an /spl Omega/ material is considered. The dispersive bianisotropic /spl Omega/ sample is placed in a rectangular waveguide. All the constitutive parameters except one are reconstructed using the reflection and transmission data for some TE/sub m0/ and TE/sub 0n/ modes. The remaining one can be obtained, e.g., from reflection of normally incident plane waves. Numerical results for the reconstruction are presented.<>     

Very low rate speech coding using temporal decomposition

A method for encoding the spectral characteristics of speech, at rates below 180 bit/s, using hierarchical temporal decomposition (HTD) is proposed. A set of the log-area-ratio (LAR) parameters, extracted from a given block of speech, are approximated through Gaussian interpolation between the most-steady frames detected by the HTD. This results in a smaller set of parameters which are encoded using vector quantisation. It is shown that the same spectral distortion is obtained with the new coder at a rate of 180 bit/s as that using a scalar quantisation, TD-based coder, at 600 bit/s     

Super-resolution texture synthesis using stochastic PAR/NL model

Super-resolution texture synthesis using a locally-adaptive stochastic signal model is investigated in this work. The 2D random texture is modeled by a piecewise auto-regressive (PAR) process whose parameters are determined by a non-local (NL) training procedure and, consequently, it is called the PAR/NL model. Unlike previous work that applies the NL scheme to image pixels directly, the proposed PAR/NL scheme applies the NL scheme to PAR model parameters by assuming that these parameters are self-similar. Furthermore, we describe a probabilistic method for PAR/NL model computation using the maximum a posteriori (MAP) and the expectation–maximization (EM) principles. Experimental results are given to demonstrate the synthesis performance of the proposed PAR/NL technique, which can boost texture detail and eliminate the blurring artifact perceptually.     

Carrier concentration optimization in semiconductor thermoelements

This paper considers the problem of carrier concentration optimization giving attention to the temperature dependence of the material parameters. The equations to be satisfied by the optimum constant and optimum variable carrier concentrations, in order to obtain maximum "average parameters" figure of merit are derived. The equations are obtained in general form so that they can be applied to any semiconductor model. In particular, the equations are solved for a nondegenerate extrinsic semiconductor. A comparison is made between the maximum "average parameters figures of merit" which can be obtained using optimum constant and optimum variable carrier concentrations, and the results interpreted in terms of available materials. A method is presented for the calculation of the optimum constant carrier concentration in a material when the thermoelectric parameters, as function of temperature, are known for a single sample with arbitrary constant carrier concentration. The results of the analysis are applied to the case ofnandptype cast lead telluride. The optimum constant carrier concentration determined from the equations of the analysis and the one determined from the measurement of the materials parameters are compared in order to show the error to be expected by using the postulated model for the semiconductor material.     

Analysis of device parameters for pnp-type AlGaAs/GaAs HBTsincluding high-injection using new direct parameter extraction

Device parameters of the small-signal T equivalent circuit for pnp-type AlGaAs/GaAs heterojunction bipolar transistors (HBTs) are obtained using a new direct parameter extraction technique. These parameters are analyzed not only under the low-current conditions but also under high-current conditions so as to understand the RF-performance fall-off after base pushout occurs. In this analysis, the intrinsic and extrinsic small-signal parameters which affect RF performance are directly determined using several steps without numerical optimization in order to properly analyze device parameters. The T equivalent circuit model determined by the method shows excellent agreement with the mean errors of 3.5-6.9% under both low-and high-current conditions. The analysis showed that the intrinsic transit time, which is the sum of the base transit time (τ_b) and the collector depletion layer transit time (τ_c), small-signal emitter resistance (r_e), small-signal base resistance (r_b) and collector-base capacitance (C_BC) all increase under high-current conditions. In addition, we found that the intrinsic transit time is the dominant parameter for the fall-off of the cut-off frequency (f_t) under high-current conditions, and there is little effect of r_b and C_BC in the fall-off of the maximum oscillation frequency (f_max) under high-current conditions. Judging from these results, device parameters are successfully obtained under a wide current range by a new parameter extraction technique and circuit modeling for HBTs under a wide current range can be achieved using the small-signal T-equivalent circuit     

The inaccuracy of Kubicek's one-cylinder model in thoracicimpedance cardiography

The validity of a one- and a two-cylinder model, underlying thoracic impedance cardiography (TIC), was investigated by studying the length dependence of the impedance parameters Z₀, (dZ/dt)_min, and stroke volume (SV). It can be shown that, within a one-cylinder model, all parameters are directly proportional to the length, whereas, if the volume conduction of the thorax and the neck are modeled separately, Z₀ and (dZ/dt)_min are expected to be linear dependent and SV will he nonlinear dependent upon the length. The expectations were compared to results from in vivo measurements. Two electrode arrays were studied, in which the caudal recording electrode position was varied; SV was calculated using Kubicek's equation. Except for small distances, the results showed a nearly linear relation between the parameters and the length. Regression analysis of the linear part revealed statistically significant intercepts (p<0.05). Neither the intercept nor the nonlinear part can be explained by a one-cylinder model, whereas a model consisting of two cylinders serially connected describes the experimental results accurately. Thus SV estimation based on a one-cylinder model is biased due to the invalid one-cylinder model. Corrections for the Kubicek-equation need to be developed in future research using this two-cylinder model     

增强型InGaP/AlGaAs/InGaAs PHEMT小信号等效电路参数的提取

介绍了增强型InGaP／AlGaAs／InGaAs PHEMT小信号等效电路中元件参数值的提取方法，并利用IC-CAP软件EEHEMT1模型提取了参数．利用ADS软件验证了提参结果，ADS仿真的直流I-V曲线和S参数与实测结果基本吻合．结果表明EEHEMT1模型可以用于提取增强型PHEMT参数，并且具有可操作性．     

增强型InGaP/AlGaAs/InGaAs PHEMT小信号等效电路参数的提取

介绍了增强型InGaP/AlGaAs/InGaAs PHEMT小信号等效电路中元件参数值的提取方法,并利用IC-CAP软件EEHEMT1模型提取了参数.利用ADS软件验证了提参结果,ADS仿真的直流I-V曲线和S参数与实测结果基本吻合.结果表明EEHEMT1模型可以用于提取增强型PHEMT参数,并且具有可操作性.     

An approach to determine small-signal model parameters for InP-based heterojunction bipolar transistors

A new method for the extraction of the small-signal model parameters of InP-based heterojunction bipolar transistors (HBT) is proposed. The approach is based on the combination of the analytical and optimization technology. The initial values of the parasitic pad capacitances are extracted by using a set of closed-form expressions derived from cutoff mode S-parameters without any test structure, and the intrinsic elements determined by using the analytical method are described as functions of the parasitic elements. An advanced design system is then used to optimize only the parasitic parameters with very small dispersion of initial values. Good agreement is obtained between simulated and measured results for an InP HBT with 5/spl times/5 /spl mu/m/sup 2/ emitter area over a wide range of bias points up to 40 GHz.     

Frequency and bias-dependent modeling of correlated base and collector current RF noise in SiGe HBTs using quasi-static equivalent circuit

This paper presents modeling of correlated RF noise in the intrinsic base and collector currents of SiGe heterojunction bipolar transistors using quasi-static equivalent circuits. The noises are first extracted from measured noise parameters using standard noise circuit analysis. Using the extraction results, model equations are proposed to describe both the frequency and bias dependence of the correlated noise sources using a single set of model parameters. The model is demonstrated using noise data from both measurement and microscopic noise simulation. The model is shown to work at frequencies up to at least half of the peak f/sub T/ and at biasing currents below high injection f/sub T/ rolloff.     

A simple and analytical parameter-extraction method of a microwaveMOSFET

A simple and accurate parameter-extraction method of a high-frequency small-signal MOSFET model including the substrate-related parameters and nonreciprocal capacitors is proposed. Direct extraction of each parameter using a linear regression approach is performed by Y-parameter analysis on the proposed equivalent circuit of the MOSFET for high-frequency operation. The extracted results are physically meaningful and good agreement has been obtained between the simulation results of the equivalent circuit and measured data without any optimization. Also, the extracted parameters, such as g_m and g_ds, match very well with those obtained by DC measurement     

On the Use of a Microstrip Three-Line System as a Six-Port Reflectometer

The scattering parameters for a coupled symmetrical three-Iine system in an inhomogeneous dielectric medium (e.g., microstrip) are derived directly in terms of a set of three orthogonal modes. The obtained results show that the condition for isolation of nonadjacent ports (e.g., ports 1 and 3 in Fig. 1) does not result from putting the corresponding per unit length immittance parameters equal to zero (i.e., z/sub 13/ =y/sub 13/ = 0). The use of such a three-line system as a six-port reflectometer is analyzed in terms of the derived scattering parameters. The reflectometer discussed in this paper allows an unknown impedance to be measured using a standard impedance.     

D-shaped optical fibre design for methane gas sensing

The authors demonstrate how the sensitivity of D-fibre based sensors depends on its parameters (refractive index difference, core diameter and flat surface-core distance). The optimal fibre parameters are predicted using the authors' finite difference/perturbation theory model, and similar results were obtained using a ray optics method for the planar waveguide to model the D fibre. It is concluded that the D fibre within its optimum architecture exhibits relatively low sensitivity. This sensitivity can however be improved using a suitable overcoating.<>     

Performance degradation due to code tracking errors inspread-spectrum code-division multiple-access systems

The influence of code synchronization errors on the performance of direct-sequence spread-spectrum (DS/SS) communication systems is investigated. Insight is gained in the degradation of some basic performance parameters due to the tracking bias of a noncoherent delay lock loop (DLL). The performance parameters investigated are the bit-error probability, throughput and delay. Numerical results show that for receivers with an early-late spacing of d=1, using a noncoherent DLL, the results of a system performance analysis in a fast fading environment can be much too optimistic if the tracking errors are ignored     

Transport and Mobility Properties of Bulk Indium Nitride (InN) and a Two-Dimensional Electron Gas in an InGaN/GaN Quantum Well

We studied the transport and low-field mobility properties of bulk InN and a two-dimensional electron gas confined in an InGaN/GaN quantum well with regard to various parameters such as well width and interface roughness as a function of temperature. Since new material parameters for InN have been suggested by recent studies, the traditionally accepted and recently published parameter values for InN are used in our simulations and the results are compared. Mobility values in two and three dimensions are found from the steady-state drift velocities of carriers calculated using an ensemble Monte Carlo technique. Electron transport properties of bulk GaN and AlN are also presented and compared with bulk InN and InGaN/GaN quantum wells. The mobility of carriers in two dimensions is about 10,000 cm²/V s for low temperatures and in bulk InN increases significantly to a value of about 6,450 cm²/V s at room temperature when recently established material parameters are used.     

A compact nonlinear equivalent circuit model and parameter extraction method for packaged high-speed VCSELs

A compact nonlinear circuit model for the input of packaged high-speed vertical-cavity surface-emitting lasers (VCSELs) is presented in this paper. The model includes the thermal effects as well as the parasitics, due to the various levels of the packaging hierarchy, to ensure a realistic representation of the input of the VCSELs. The values of the model parameters are extracted from dc current-light-voltage characteristics and S/sub 11/ vector measurements using a two-step parameter extraction procedure. Extraction of the model parameters and comparison between measured and simulated results have been performed for two different commercially available VCSELs operating at 2.5 Gb/s. The achieved agreement between the measured and simulated results is very satisfactory for the dc as well for the S/sub 11/ curves in the frequency range from 3 MHz to 3 GHz.     

Estimation of time-varying velocities of moving objects by time-frequency representations

An approach to the estimation of motion parameters of moving objects in a video-sequence, by using the SLIDE (subspace-based line detection algorithm) algorithm, is considered. The proposed procedure projects video-frames to the coordinate axes, in order to obtain synthetic images containing information about the motion parameters. These synthetic images are mapped to the FM signals by using constant /spl mu/-propagation. The problem of velocity estimation is reduced to the instantaneous frequency (IF) estimation. IF estimators, based on time-frequency (TF) representations, are used. Three TF representations: spectrogram (SPEC), Wigner distribution (WD), and S-method (SM), are used and compared to this aim. A tradeoff between concentration of the TF representation (velocity estimation accuracy) and reduction of the cross-terms (possibility for estimation of the multiple objects parameters) is achieved by the SM. A performance analysis of the algorithm is done. Theoretical results are illustrated on several numerical examples.     

毫米波AlGaN / GaN HEMT 交流特性研究

毫米波频段已经成为AlGaN/GaN HEMT研究的一个发展趋势。利用器件仿真软件TCAD,对AlGaN/GaN HEMT交流特性进行了研究。从势垒层的Al组分和厚度两个参数分析了器件特征频率变化趋势。用TCAD仿真得到的AlGaN/GaN HEMT器件本征S参数,在ADS中添加器件的非本征参数,得到器件仿真的频率特性。在器件设计的基础上,进行了器件版图设计和流片,并测量了器件频率特性。测试和仿真结果的对比表明两者较为一致,表明器件仿真的有效性和指导意义。     

A Novel Technique for the Identification of ARMA Systems Under Very Low Levels of SNR

In this paper, a novel technique for the identification of minimum-phase autoregressive moving average (ARMA) systems from the output observations in the presence of heavy noise is presented. First, starting from the conventional correlation estimator, a simple and accurate ARMA correlation (ARMAC) model in terms of the poles of the ARMA system is presented in a unified manner for white noise and impulse-train excitations. The AR parameters of the ARMA system are then obtained from the noisy observations by developing and using a residue-based least-squares correlation-fitting optimization technique that employs the proposed ARMAC model. As for the estimation of the MA parameters, it is preceded by the application of a new technique intended to reduce the noise present in the residual signal that is obtained by filtering the noisy ARMA signal via the estimated AR parameters. A scheme is then devised whereby the task of MA parameter estimation is transformed into a problem of correlation-fitting of the inverse autocorrelation function corresponding to the noise-compensated residual signal. In order to demonstrate the effectiveness of the proposed method, extensive simulations are performed by considering synthetic ARMA systems of different orders in the presence of additive white noise and the results are compared with those of some of the existing methods. It is shown that the proposed method is capable of estimating the ARMA parameters accurately and consistently with guaranteed stability for signal-to-noise ratio (SNR) levels as low as $-{5}~{hbox {dB}}$ . Simulation results are also provided for the identification of a human vocal-tract system using natural speech signals showing a superior performance of the proposed technique in terms of the power spectral density of the synthesized speech signal.