Extending multiresolution time-domain (MRTD) technique to the simulation of high-frequency active devices

We present a new time-domain simulation approach for large-signal physical modeling of high-frequency semiconductor devices using wavelets. The proposed approach solves the complete hydrodynamic model, which describes the transport physics, on nonuniform self-adaptive grids. The nonuniform grids are obtained by applying wavelet transforms followed by hard thresholding. This allows forming fine and coarse grids in locations where variable solutions change rapidly and slowly, respectively. A general criterion is mathematically defined for grid updating within the simulation. In addition, an efficient thresholding formula is proposed and verified. The developed technique is validated by simulating a submicrometer FET. Different numerical examples are presented along with illustrative comparison graphs, showing over 75% reduction in CPU time, while maintaining the same degree of accuracy achieved using a uniform grid case. Tradeoffs between threshold values, CPU time, and accuracy are discussed. To our knowledge, this is the first time in the literature to implement and report a wavelet-based hydrodynamic model simulator. This study also represents a fundamental step toward applying wavelets to Maxwell's equations in conjunction with the hydrodynamic model for accurate modeling of high-frequency active devices aiming to reduce the simulation time, while maintaining the same degree of accuracy.     

RFCMOS Unit Width Optimization Technique

In this paper, we demonstrate a unit width ( W_f) optimization technique based on their unity short-circuit current gain frequency (f_T) unilateral power gain frequency (f_MAX)? and high-frequency (HF) noise for RFCMOS transistors. Our results show that the trend for the above figures-of-merit (FOMs) with respect to the W_f change is different; hence, some tradeoff is required to obtain the optimum W_f value. During the HF noise analysis, a new FOM is proposed to study the W_f effect on the HF noise performance. In our experiment, the flicker noise of the transistor is also measured and the result shows that the change in W_f does not affect the noise spectral density at the low-frequency range. This technique enables RF engineers to optimize the transistor's layout and helps to select the optimum Wf for transistors used in specific circuit design such as the low-noise amplifier, voltage-controlled oscillator, and mixer. Furthermore, by using layout optimized transistors in the RF circuit, the optimal circuit's performance can be easily achieved and, thus, greatly reduced the circuit development time. In the aspect of RF device modeling, by knowing the optimum W_f for a particular process or technology, the number of transistors to model is reduced and, hence, greatly shortens the RF modeling development time for existing and future technologies.     

Analysis of multifinger power HEMTs supported by effective 3-D device electrothermal simulation

The influence of the metallization layer geometry on the electrothermal behavior of the multifinger power high-electron mobility transistors (HEMTs) is studied. The analysis of thermal and electrical behavior is supported by effective 3-D electrothermal device simulation method developed for Synopsys TCAD Sentaurus environment using mixed-mode setup. The proposed methodology allows fast simulation of complex systems from individual semiconductor layers at a frontend up to package and cooling assemblies at a backend. More accurate electrothermal model of power HEMT is proposed and validated by finite element method (FEM) simulations. The analysis points on significant influence of metallization geometry design on electrothermal properties and reliability of the multifinger power HEMTs. The unique identification and visualization of the critical regions allows effective device optimization. Very good comparison between simulation results and experimental data demonstrate validity of the proposed simulation methodology and HEMT structures analysis.     

An improved method to estimate intrinsic small signal parameters of a GaAs MESFET from measured dc characteristics

This investigation offers a technique to predict the ac behavior of mm wavelength GaAs metal semiconductor field effect transistors by using dc characteristics. To predict the intrinsic equivalent circuit parameters of the device from dc data, the measured dc characteristics are first simulated by employing a nonlinear dc model. The effects of biasing on the device ac parameters are evaluated for its low-noise applications. An improvement greater than 10% in predicting the ac response of the device is observed. The concept of depletion layer modification caused by the transverse electric field inside the channel is introduced for accurate Miller's capacitor modeling. It is assumed that with increased device biasing there are more unbalanced positive charges in the gate depletion toward the drain-side of the Schottky barrier. The electric field lines originated by these uncompensated charges induce an opposite charge density in the gate electrode. This modifies the gate biasing and hence the Schottky barrier depletion. As a result, the values of intrinsic ac device parameters change. It is observed that an accurate dc modeling is key to predicting an accurate ac small signal equivalent circuit of a device.     

Rigorous modeling of high-speed semiconductor devices

We present a review of industrial heterostructure devices based on SiGe/Si and III–V compound semiconductors analyzed by means of numerical simulation. A comparison of device simulators and current transport models is given and critical modeling issues are addressed. Results from two-dimensional hydrodynamic analyses of heterojunction bipolar transistors (HBTs) are presented in good agreement with measured data. The examples are chosen to demonstrate technologically relevant issues which can be addressed by device simulation.     

A new efficient one-dimensional analysis program for junction device modeling

A new program for one-dimensional semiconductor device analysis is described and shown to be more general and more efficient than competitive methods. Numerical results from a study of high-frequency bipolar transistors are given with emphasis on the effect of Fermi statistics and velocity limitation in high-current density situations. A method for device modeling directly from the partial differential equation (PDE) solutions is described, and applied to the problem of simulating the performance of high-speed emitter-coupled logic circuits.     

Extraction technique for characterization of electric field distribution and drain current in VDMOS power transistor

This paper presents a methodology for modeling the electric field distribution in the vertical direction of VDMOS power transistors, considering the effects of cell spacing and drain voltage. An accurate and consistent extraction technique is developed to extract the values of various important parameters based on non-linear and multivariable regression techniques for the first time. The generalized form of electric field distribution enables the physical modeling of drain current at the onset of quasi-saturation considering the effect of non-uniform electron distribution in the n-epi region. Results so obtained are in good agreement with PISCES simulation over wide range of device parameters. The proposed model will be highly suitable for CAD (Computer Aided Design) tools in HVIC applications.     

Straightforward and accurate nonlinear device model parameter-estimation method based on vectorial large-signal measurements

To model nonlinear device behavior at microwave frequencies, accurate large-signal models are required. However, the standard procedure to estimate model parameters is often cumbersome, as it involves several measurement systems (DC, vector network analyzer, etc.). Therefore, we propose a new nonlinear modeling technique, which reduces the complexity of the model generation tremendously and only requires full two-port vectorial large-signal measurements. This paper reports on the results obtained with this new modeling technique applied to both empirical and artificial-neural-network device models. Experimental results are given for high electron-mobility transistors and MOSFETs. We also show that realistic signal excitations can easily be included in the optimization process.     

A High-Frequency MOS Transistor Model and its Effects on Radio-Frequency Circuits

Accurate modeling and efficient parameter extraction of a small signal equivalent circuit of MOS transistors for high-frequency operation are presented. The small-signal equivalent circuit is based on the quasi-static approximation which was found to be adequate up to 10 GHz for MOS transistors fabricated by a 20 GHz cutoff frequency technology. The extrinsic components and substrate coupling effects are properly included. Direct extraction is performed by Y-parameter analysis on the equivalent circuit in the linear and saturation regions of operation. A low-noise amplifier is used to illustrate the effects on circuit performance due to accurate inclusion of extrinsic components in the model. Good agreement between simulated results and measured data on high-frequency transistor characteristics has been achieved.     

Polysilicon TFT structures for kink-effect suppression

Experimental results and numerical simulations of asymmetric fingered polysilicon thin-film transistors (AF-TFTs) are analyzed in detail. In the AF-TFTs, the transistor channel region is split into two zones with different lengths separated by a floating n/sup +/ region. This structure allows an effective reduction of the kink effect depending on the relative length of the two subchannels, without introducing any additional series resistance. In addition, an appreciable reduction of the leakage current is also observed. The AF-TFTs characteristics have been analyzed by two-dimensional numerical simulation and by modeling the device with two transistors in series. This model clarifies the mechanisms of kink effect suppression in AF-TFT. On the basis of this analysis, two new modified device structures for kink-effect suppression are also proposed and discussed.     

Accurate modeling of AlGaAs/GaAs heterostructure bipolar transistors by two-dimensional computer simulation

An accurate modeling of Al0.3Ga0.7As/GaAs heterostructure bipolar transistors (HBT's) has been carried out using a two-dimensional numerical simulator. By comparing an HBT with a GaAs homotransistor, the potential advantages of the HBT, such as a high injection efficiency, a low voltage drop in the base region, and an improvement in high-frequency operation, were well confirmed, not only by the calculated transistor performance but also by the carrier and the potential distributions. It was also demonstrated that the injected electrons spread more and more conspicuously into the extrinsic base region, even for the HBT's with the increase in the collector current. Taking into account the near ballistic transport in the base region, it is possible to realize an abrupt HBT with a maximum cutoff frequency of above 130 GHz and current gains of up to 3200. The HBT was confirmed to be a promising device for the realization of ultrahigh-frequency integrated circuits.     

Physical/electromagnetic pHEMT modeling

An effective technique, which is based only on geometrical and physical data, is presented for the analysis of high-frequency FETs. The intrinsic part of this electron device is described by a quasi-two-dimensional hydrodynamic transport model, coupled to a numerical electromagnetic field time domain solver in three dimensions that analyzes the passive part of the FET. Such an analysis is entirely performed in the time domain, thus allowing linear and nonlinear operations. The obtained data give insights to some parameters affecting the signal distribution through the entire device structure; a comprehensive discussion of these is given for a test device. In order to prove the validity of the approach, the bias-dependent small-signal analysis is compared with the corresponding measurements up to 50 GHz for two 0.3-/spl mu/m gate-length AlGaAs-InGaAs-GaAs pseudomorphic high electron-mobility transistors, each having two gate fingers of 25-/spl mu/m and 100-/spl mu/m width, at bias points ranging from Idss to the pinchoff regime. The accuracy and the efficiency of the approach make it suitable for device optimization.     

A TCAD approach to the physics-based modeling of frequencyconversion and noise in semiconductor devices under large-signal forcedoperation

The paper presents a novel, unified technique to evaluate, through physics-based modeling, the frequency conversion and noise behavior of semiconductor devices operating in the large-signal periodic regime. Starting from the harmonic balance (HE) solution of the spatially discretized physics-based model under (quasi) periodic forced operation, frequency conversion at the device ports in the presence of additional input tones is simulated by application of the small-signal large-signal network approach to the model. Noise analysis under large-signal operation readily follows as a direct extension of classical approaches by application of the frequency conversion principle to the modulated microscopic noise sources and to the propagation of these to the external device terminals through a Green's function technique. An efficient numerical implementation is discussed within the framework of a drift-diffusion model and some examples are finally provided on the conversion and noise behavior of rf Si diodes     

Time-dependent thermal crosstalk in multifinger AlGaN/GaN HEMTs and implications on their electrical performance

The thermal dynamics of multifinger AlGaN/GaN high electron mobility transistors (HEMTs) with varying gate pitch was investigated using time-resolved Raman thermography. An identical temperature rise was measured in all gate fingers within the first ∼500 ns duration of electrical pulses. Gate finger temperature differences only emerge after 500 ns, due to the onset of thermal crosstalk. This thermal crosstalk onset delay is attributed to the finite rate of heat diffusion between gate fingers. Reducing the device gate pitch was found to increase the magnitude of transient thermal crosstalk. Implications of each gate finger within a multifinger HEMT having a different transient temperature are discussed in the context of device characteristics. The experimental results are compared to time-domain three-dimensional finite difference heat diffusion simulations.     

SBR image approach for radio wave propagation in tunnels with andwithout traffic

We propose a deterministic approach to model the radio propagation channels in tunnels with and without traffic. This technique applies the modified shooting and bouncing ray (SBR) method to find equivalent sources (images) in each launched ray tube and sums the receiving complex amplitude contributed by all images coherently. In addition, the vector effective antenna height (VEH) is introduced to consider the polarization-coupling effect resulting from the shape of the tunnels. We verify this approach by comparing the numerical results in two canonical examples where closed-form solutions exist. The good agreement indicates that our method can provide a good approximation of high-frequency radio propagation inside tunnels where reflection is dominant. We show that the propagation loss in tunnels can vary considerably according to the tunnel shapes and the traffic inside them. From the results we also find a “focusing” effect, which makes the power received in an arched tunnel higher than that in a rectangular tunnel. Besides, the deep fading that appears in a rectangular tunnel is absent in an arched tunnel. The major effect of the traffic is observed to be the fast fading due to the reflection/obstruction of vehicles. Additional considerations, such as time delay, wall roughness, and wedge diffraction of radio wave propagation in tunnels are left for future studies     

Industrial application of heterostructure device simulation

We give an overview of the state-of-the-art of heterostructure RF-device simulation for industrial application based on III-V compound semiconductors. The work includes a detailed comparison of device simulators and current transport models to be used, and addresses critical modeling issues. Results from two-dimensional hydrodynamic simulations of heterojunction bipolar transistors (HBTs) and high electron mobility transistors (HEMTs) with MINIMOS-NT are presented in good agreement with measured data. The simulation examples are chosen to demonstrate technologically important issues which can be addressed and solved by device simulation.     

A multiscale analytical correction technique for two-dimensional thermal models of AlGaN/GaN HEMTs

Two-dimensional approaches are widely used in the numerical thermal models of AlGaN/GaN high electron mobility transistors (HEMTs) to reduce the high computational cost of the three-dimensional approaches. The aforementioned simplified models predict inaccurate device temperatures with significant overestimation of the thermal resistance. In order to take advantage of the computational efficiency of the two-dimensional models, a correction procedure is necessary for the accurate representation of the actual device temperatures. In this paper, a novel correction method is introduced for this purpose. Correction technique presented in this study can be used to improve the accuracy of the multiscale numerical thermal device models.     

Identification and measurement of scaling-dependent parasiticcapacitances of small-geometry MOSFET's

The scaling dependence of the gate-to-source capacitance in strong accumulation C_gs^acc is investigated for the first time to study the parasitic capacitances for MOSFETs with different gate-lengths. Results show a gate-length L_gate dependent characteristic for C_gs^acc and is consistent with the results from numerical device simulation. The result is found to be different from the widely used assumption in the literature, and is believed to be reported for the first time. The gate-length dependent C _gs^acc characteristic is due to the top side capacitance, and is verified through careful device characterization and numerical device simulation. A measurement technique is further developed to determine C_top for small geometry MOSFETs. The technique is demonstrated on actual transistors with no special test devices required. Based on the technique, a sub-linear dependent relationship is found for the dependence of C_top on L_gate, and is in close agreement with the results predicted by theory. Results also indicate that C_top is about 8-13% of the measured total C_gs^acc, which corresponds to a nonnegligible portion of the total capacitance, and needs to be considered for future device design, characterization and modeling. The impact of C_top on CMOS inverter gate delay is also investigated. Results indicate C_top adversely impact the gate delay by as much as 5.5% as supply voltage is scaled down to 1 V     

Self-heating effects in a BiCMOS on SOI technology for RFIC applications

Self-heating in a 0.25 /spl mu/m BiCMOS technology with different isolation structures, including shallow and deep trenches on bulk and silicon-on-insulator (SOI) substrates, is characterized experimentally. Thermal resistance values for single- and multifinger emitter devices are extracted and compared to results obtained from two-dimensional, fully coupled electrothermal simulations. The difference in thermal resistance between the investigated isolation structures becomes more important for transistors with a small aspect ratio, i.e., short emitter length. The influence of thermal boundary conditions, including the substrate thermal resistance, the thermal resistance of the first metallization/via layer, and the simulation structure width is investigated. In the device with full dielectric isolation-deep polysilicon-filled trenches on an SOI substrate-accurate modeling of the heat flow in the metallization is found to be crucial. Furthermore, the simulated structure must be made wide enough to account for the large heat flow in the lateral direction.     

A simple four-terminal small-signal model of RF MOSFETs and its parameter extraction

After differences between the RF MOSFET and conventional high-frequency transistors, which make the proper modeling of RF MOSFET complicated and difficult, are addressed, a four-terminal small-signal model of an RF MOSFET with a very simple and accurate parameter extraction method is presented. This model includes the intrinsic and extrinsic elements important for RF AC simulation in the strong inversion operation region. Accuracy of the model and extraction method is verified with the measured data and the needs of the intrinsic body node are demonstrated to describe the gate bias dependence of the substrate-signal-coupling effect.