Amirreza Ghadimi Avval | University of Arkansas (original) (raw)

Papers by Amirreza Ghadimi Avval

IEEE, 2024

In this work, we present the design and modeling of a new type of choke horn antenna. It incorpor... more In this work, we present the design and modeling of a new type of choke horn antenna. It incorporates a rectangular waveguide and a rectangular choke acting as a parasitic element. The four-sided geometry of the antenna is applicable to systems that utilize rectangular waveguides. Also, it can overcome the need for rectangular-to-circular transition of transmission line or mode conversion. The main objective of this paper is to develop a model that calculates the far field radiation characteristics of the proposed antenna (analytical part) and to incorporate a finite element method (FEM) solver that adds to the theoretical solution (empirical part), which finally leads to obtaining a hybrid model. The omnidirectional radiation property of the choke is demonstrated, which gives an insight into the influence of this parasitic element on the total radiated power. The same observations made on the rectangular choke can be translated to the circular choke as well. At an operating frequency of 2.45 GHz, the design is numerically and experimentally validated. Also, the demonstrated hybrid model can leverage the integration of supervised machine learning (ML) models by exporting radiation variables such as gain and half-power beamwidth (HPBW) and performing predictions based on the training data from the model. Therefore, we incorporate gradient boosting and neural network ML algorithms, which are tailored to the desired radiation pattern parameters. INDEX TERMS Analytical model, beamwidth enhancement, choke horn antenna, empirical model, gain enhancement, gradient boosting, neural network.

IEEE, 2023

Temperature-controlled disinfection using microwave and plasma energy is a novel method that has ... more Temperature-controlled disinfection using microwave and plasma energy is a novel method that has the potential to revolutionize the way we combat infectious diseases. This method presented here utilizes the combined power of microwave energy and plasma to effectively eliminate harmful microorganisms from treated specimens. One of the problems facing disinfection methods that use microwaves is that they utilize high temperatures that could alter or destroy the material which is addressed in this design. The design of the system is demonstrated, and the parts are explained. Experimental results to deactivate pathogens are also presented, where E. Coli bacteria samples were treated by the system using different combinations of microwave and plasma energy levels and exposure times to illustrate the effectiveness of the system. According to the results, the system is very effective at deactivating pathogens in a short period of time.

IEEE, 2023

This paper discusses the benefit of adding a second choke to a choke antenna. An analytical model... more This paper discusses the benefit of adding a second choke to a choke antenna. An analytical model that obtains the antenna far-field radiation components is developed. This model proves the advantage in using a second choke which introduces further phase delay in the formed complex currents. Therefore, the desired half power beamwidth (HPBW) or radiation maxima direction can be achieved with smaller antenna size. Analytical model validation is conducted using finite element method solver Ansys HFSS. Finally, the processing time of the analytical model and FEM simulation is shown and compared in CPU time.

IEEE, 2023

In this paper, a microwave heating system controlled by temperature is presented. Each system com... more In this paper, a microwave heating system controlled by temperature is presented. Each system component is described, and the system's design is provided. The system uses a solid-state microwave generator as the source. The power delivery method is also discussed, and the waveguide environment is shown. An infrared temperature sensor is used to collect the temperature of the sample which is then fed to a python program to control the operation of the microwave generator. A study of the temperature profile for three different power levels is shown with and without using the control system. Using a preset temperature range, the system has proven the ability to keep the sample in the desired temperature range.

IEEE, 2023

This paper discusses a simple choke antenna composed mainly of a circular waveguide surrounded by... more This paper discusses a simple choke antenna composed mainly of a circular waveguide surrounded by a choke. An analytical model that plots field patterns and currents of the choke is presented. Also, this model can obtain the total amplitude and phase of antenna far-field radiation characteristics to get the desired gain, half-power beamwidth, or radiation maxima direction for applications such radar systems and satellite communications. A comparison in CPU time between this model and the finite element method is introduced. At a selected operating frequency of 2.45 GHz, finite element method solver Ansys HFSS is utilized to validate the resulting radiation patterns of the proposed analytical model and to export the choke currents.

IEEE, 2023

In this work, a review of millimeter-wave transistors modeling is presented. The physics-based mo... more In this work, a review of millimeter-wave transistors modeling is presented. The physics-based modeling approaches are studied and their advantages over the empirical models are demonstrated. Physics-based models account for the physical phenomena that affect the performance of millimeter-wave devices at high frequencies. Electromagnetic-wave propagation effects and non-linear operations are incorporated in physicsbased models utilizing appropriate numerical strategies. This makes the modeling approaches predictive over a broad frequency range, under both small-and large-signal analyses and regardless of the device size. Conventional electrode configurations for millimeter-wave transistors are also studied in this work to illustrate the importance of pad layout design in attaining suitable functionality. The necessity of accounting for the device width limitations is also discussed in this work and the transmission line theory is emphasized as a useful tool to address the challenges at higher operating frequencies.

IEEE, 2022

A simple choke antenna composed mainly of a circular waveguide surrounded by a ring is discussed.... more A simple choke antenna composed mainly of a circular waveguide surrounded by a ring is discussed. The main objective of this paper is developing an analytical model that obtains a wider, yet variable half power beamwidth (HPBW) of the antenna for different applications. By modeling the ring as a parasitic element and the circular waveguide as the feed, high-frequency field-based method Geometrical Theory of Diffraction (GTD) is proposed. Due to the curved geometry of the ring, caustics are present and introduce singularities in the far field radiation pattern solutions. A correcting factor is imported to overcome these discontinuities. At a selected operating frequency of 2.45 GHz, finite element method solver Ansys HFSS is utilized to validate the resulting radiation patterns of GTD. Antenna radiated power and tangential components constant are used to calibrate between GTD and HFSS.

IEEE, 2022

The high-frequency operability of millimeter-wave devices is investigated in this paper. An AlGaN... more The high-frequency operability of millimeter-wave devices is investigated in this paper. An AlGaN/GaN HEMT device is studied, and the results obtained from the distributed modeling approach is compared with measurements. To validate the performance of the modeling approach, non-linear operation of the model under large signal analysis is demonstrated. According to the distributed model, the signal behavior on the input and output electrodes are observed for various operating frequencies. The main reasons for the discrepancies in the output power and gain of the device are explained and the limitations arisen from the device width in higher frequency bands can be exploited. Finally, a solution is provided to demonstrate alleviating the frequency restrictions and nonlinear frequency-gain relations for devices configured with the current fabrication technologies.

IEEE, 2021

The effects of wave propagations on gain and cut-off frequency of millimeter-wave transistors are... more The effects of wave propagations on gain and cut-off frequency of millimeter-wave transistors are studied based on a new modeling approach. This model is developed based on the electron transport in semiconductor layers and transmission line properties of the electrodes in high-frequency transistors. In this approach, no small- or large-signal equivalent circuit is required for the analysis of the device. The electrodes of the transistor are considered as coupled multi-conductor transmission lines to represent the effect of non-uniform voltage and current distributions along the gate and drain conductors. A finite-difference time-domain solver is utilized to solve the governing equations of the distributed model. The achieved excellent agreement between the simulation and measurement results for a fabricated GaN-HEMT over a broad frequency range validates the accurate performance of the wave-electron-transport model in finding the figures of merit in W-band and beyond.

IEEE, 2021

The effects of pad layout variations on highfrequency characteristics of millimeter-wave transist... more The effects of pad layout variations on highfrequency characteristics of millimeter-wave transistors are studied in this work. Extrinsic parameters of the device are extracted using a conformal mapping technique merely based on the physical structure of the device. A wave-electrontransport model is utilized for the analysis. A device solver and an electromagnetic solver are included to solve the governing electron-transport and transmission line equations, respectively. Comparison results for two different pad configurations of the transistor are provided and measurement results for a GaN HEMT device is used for this purpose.

IEEE, 2021

A distributed-model-based approach is developed for small- and large-signal analyses of millimete... more A distributed-model-based approach is developed for small- and large-signal analyses of millimeter-wave transistors. To obtain the equivalent circuit model parameters, a structure based extraction technique is utilized. The method employs the physical geometry and structural topology of the device for finding the parameter values based on conformal mapping concepts and incremental inductance rule. The significant contribution of this approach is to incorporate the electromagnetic wave propagation effects in designing the distributed circuit which ensures achieving a universally accurate model over a very broad frequency range. This technique is applied to analyze a GaN MISHEMT device. The simulation results for the gain and output power under the small- and large-signal conditions are obtained for different device widths at various bias points and operating frequencies. To validate the performance of the proposed modeling approach, the results are compared with the measurement.

IEEE, 2020

A novel distributed modeling approach is proposed in this paper that predicts the device behavior... more A novel distributed modeling approach is proposed in this paper that predicts the device behavior over a broad frequency range. The detailed device dimensions along with the 19-element equivalent circuit model are presented and the boundary conditions to be incorporated in the model are provided. In order to validate the approach, the simulation results are compared with the measurement results obtained from a N-polar GaN HEMT device over a 0.25-67 GHz frequency band. The results are also generated for the hypothetical case of a wider transistor at higher operating frequencies in order to explain the importance of considering the wave propagation effects in the modeling and demonstrate how the results converge. Additionally, a novel design approach is proposed for redesigning the device electrodes so as to achieve matched phase velocities on the input and output ports of the transistor. The new device with the optimized electrode configurations yields considerably higher gain values compared to the conventional device.

IEEE, 2020

A fully distributed modeling approach is proposed in this paper that incorporates the wave propag... more A fully distributed modeling approach is proposed in this paper that incorporates the wave propagation effects in developing the equivalent circuit and works as a simulation tool for analyzing high-frequency transistors such as GaN high electron mobility devices. The details regarding the device arrangement are discussed and it is explained how the parameter extraction in the proposed modeling approach is developed merely based on the physical structure of the device along with the electrode dimensions and geometry. The governing equations for obtaining the currents and voltages at each time interval and spatial distance are analyzed based on a finite difference time domain method, where an implicit scheme is developed to ensure the unconditional stability of the technique along with reducing the computation time of the developed solution. The obtained results for the current gain and maximum available gain are verified by the measurement results from a 0.1-μm nitrogen-polar oriented GaN HEMT at 0.25-67 GHz frequency range. The results are also generated for wider cases and over higher frequency ranges to show the importance of using a fully distributed modeling technique and demonstrate the convergence of the results. An experiment is also developed regarding the thermal analysis of the high-frequency HEMT device and to discuss the heat transfer phenomenon over the device width and its effects on the distributed modeling approach are analyzed.

IEEE, 2020

A new distributed small-signal modeling approach is proposed for high-frequency analysis of trans... more A new distributed small-signal modeling approach is proposed for high-frequency analysis of transistors, where the extraction methods for each parameter are defined based on solely the structure of the device. An unconditionally stable implicit scheme is presented to analyze the time domain behavior of the device and the obtained results are compared with the measured results of a 0.1-µm N-polar GaN MISHEMT over a broad frequency range. Another simulation is conducted for a device with a larger width to show the distributed effects of the proposed technique and the reliability of the method in higher operating frequencies.

IEEE, 2019

A new modelling approach is presented to obtain the 19-element active and passive equivalent circ... more A new modelling approach is presented to obtain the 19-element active and passive equivalent circuit element values of high electron mobility transistors over a broad frequency range. In this method, a new technique is introduced to acquire the appropriate extrinsic values of the model suitable for the frequency of operation. The bias-dependent intrinsic values are also obtained using the extracted Y-parameters from a simulation scheme developed in SILVACO. The proposed method addresses the issues and limitations with existing extraction methods regarding their suitability for high-frequency ranges. A finite-difference scheme is also presented to analyze the device which is characterized by its small-signal equivalent circuit. Keywords-active device, small-signal equivalent circuit, high electron mobility transistor, extrinsic and intrinsic, finite difference methods

International Journal of Numerical Modeling, 2017

An explicit finite difference time domain method is proposed for modelling of composite right-/le... more An explicit finite difference time domain method is proposed for modelling of composite right-/left-handed active multiconductor transmission lines in time domain. For the first time, the concept of active composite right-/left-handed transmission lines is considered in field effect transistor (FET) modelling. In this work, based on the distributed model, the FET is considered as an active multiconductor transmission line in the mm/wave frequency range. In this modelling technique, the FET is divided into 2 different parts, the active part, represents the intrinsic equivalent circuit of the transistor, while the passive part symbolizes the device electrodes. The finite-difference time-domain scheme is proposed for the analysis, and the results of this technique are compared with the ones obtained from simulations. Furthermore, the stability issue is investigated and the sufficient condition for stability is derived. KEYWORDS active multiconductor, CRLH TL, FDTD method, FET modelling, millimetre-wave, stability 1 | INTRODUCTION In traditional materials, both electric and magnetic fields along with wave vector form a right-handed triad, with vector is along the direction of the Poynting vector. In contrast, in metamaterials, because the Poynting vector and wave vector have opposite directions, they are considered as left-handed (LH) materials. 1 Such materials have both negative permeability and permittivity and exhibit some attractive specifications for electrodynamic applications. 2-8 The concept of composite right-/left-handed transmission lines (CRLH TLs) has been developed 9,10 and widely used to design microwave devices. 11-13 Afrooz and Abdipour have presented 2 explicit finite-difference time-domain (FDTD) algorithms for time domain analysis of metamaterial TLs, and the results of the proposed method are confirmed with numerical examples. 14 Also, a time-domain analysis of the CRLH TL has been already investigated using numerical methods by Gomez-Diaz et al. 15 Additionally, the idea of active CRLH TL has already been applied to some devices and circuits. 16,17 Modern communication system performance strongly depends on active elements, among which field effect transistors (FETs) are largely present. However, their behaviour in the millimetre-wave frequency range is still perfectible. Analysing transmission lines in the millimetre-wave range will result in some complicated problems such as radiation and unwanted coupling between circuit elements. Therefore, a full wave approach needs to be used when the size of the electrodes are proportional to the wavelength. 18 However, full-wave analysis are usually based on highly complicated numerical techniques that require huge CPU time and computational resources. 18-21 In the proposed modelling technique, the FET is considered in the form of 3 active TLs. Based on this modelling procedure, the device will be split into an infinite number of sections. Each section is composed of 3 multiconductor transmission lines (referring to gate, drain, and source) in which each electrode is supposed to be composed of metamaterial and a traditional

Elsevier, 2018

Due to high power and high speed applications, high electron mobility transistors (HEMT) have bee... more Due to high power and high speed applications, high electron mobility transistors (HEMT) have been the area of focus for many research groups in recent years. Compared to conventional field effect transistors (FET), HEMT devices have demonstrated superior performance considering the operational frequency, output power density, and power added efficiency. An overview of GaN/AlGaN HEMT structures is presented in this chapter and the graphical illustrations are included. The metallic and semiconductor layers along with the Schottky and ohmic contacts are explained and the growth methods are discussed. The latest research on GaN HEMT structures have been brought into attention and the modifications are described. Furthermore, various modelling procedures of HEMT devices are introduced and compared so as to illustrate the advantages and disadvantages in different analysis methodologies. A brief discussion on HEMT applications is presented in the final chapter.

IEEE, 2018

The recent upsurge in wireless communications demands devices working at higher frequencies with ... more The recent upsurge in wireless communications demands devices working at higher frequencies with higher output densities. In general, wide-bandgap materials seem to be the reliable choice for these applications, specifically GaN HEMT that has shown great advantage over its previous counterparts. A holistic optimization technique is proposed to define the stages that a high frequency, high power device is designed. An issue with the thermal conductivity of the substrates for these devices is also addressed and a fabrication technique is proposed to solve it.

IEEE, 2024

In this work, we present the design and modeling of a new type of choke horn antenna. It incorpor... more In this work, we present the design and modeling of a new type of choke horn antenna. It incorporates a rectangular waveguide and a rectangular choke acting as a parasitic element. The four-sided geometry of the antenna is applicable to systems that utilize rectangular waveguides. Also, it can overcome the need for rectangular-to-circular transition of transmission line or mode conversion. The main objective of this paper is to develop a model that calculates the far field radiation characteristics of the proposed antenna (analytical part) and to incorporate a finite element method (FEM) solver that adds to the theoretical solution (empirical part), which finally leads to obtaining a hybrid model. The omnidirectional radiation property of the choke is demonstrated, which gives an insight into the influence of this parasitic element on the total radiated power. The same observations made on the rectangular choke can be translated to the circular choke as well. At an operating frequency of 2.45 GHz, the design is numerically and experimentally validated. Also, the demonstrated hybrid model can leverage the integration of supervised machine learning (ML) models by exporting radiation variables such as gain and half-power beamwidth (HPBW) and performing predictions based on the training data from the model. Therefore, we incorporate gradient boosting and neural network ML algorithms, which are tailored to the desired radiation pattern parameters. INDEX TERMS Analytical model, beamwidth enhancement, choke horn antenna, empirical model, gain enhancement, gradient boosting, neural network.

IEEE, 2023

Temperature-controlled disinfection using microwave and plasma energy is a novel method that has ... more Temperature-controlled disinfection using microwave and plasma energy is a novel method that has the potential to revolutionize the way we combat infectious diseases. This method presented here utilizes the combined power of microwave energy and plasma to effectively eliminate harmful microorganisms from treated specimens. One of the problems facing disinfection methods that use microwaves is that they utilize high temperatures that could alter or destroy the material which is addressed in this design. The design of the system is demonstrated, and the parts are explained. Experimental results to deactivate pathogens are also presented, where E. Coli bacteria samples were treated by the system using different combinations of microwave and plasma energy levels and exposure times to illustrate the effectiveness of the system. According to the results, the system is very effective at deactivating pathogens in a short period of time.

IEEE, 2023

This paper discusses the benefit of adding a second choke to a choke antenna. An analytical model... more This paper discusses the benefit of adding a second choke to a choke antenna. An analytical model that obtains the antenna far-field radiation components is developed. This model proves the advantage in using a second choke which introduces further phase delay in the formed complex currents. Therefore, the desired half power beamwidth (HPBW) or radiation maxima direction can be achieved with smaller antenna size. Analytical model validation is conducted using finite element method solver Ansys HFSS. Finally, the processing time of the analytical model and FEM simulation is shown and compared in CPU time.

IEEE, 2023

In this paper, a microwave heating system controlled by temperature is presented. Each system com... more In this paper, a microwave heating system controlled by temperature is presented. Each system component is described, and the system's design is provided. The system uses a solid-state microwave generator as the source. The power delivery method is also discussed, and the waveguide environment is shown. An infrared temperature sensor is used to collect the temperature of the sample which is then fed to a python program to control the operation of the microwave generator. A study of the temperature profile for three different power levels is shown with and without using the control system. Using a preset temperature range, the system has proven the ability to keep the sample in the desired temperature range.

IEEE, 2023

This paper discusses a simple choke antenna composed mainly of a circular waveguide surrounded by... more This paper discusses a simple choke antenna composed mainly of a circular waveguide surrounded by a choke. An analytical model that plots field patterns and currents of the choke is presented. Also, this model can obtain the total amplitude and phase of antenna far-field radiation characteristics to get the desired gain, half-power beamwidth, or radiation maxima direction for applications such radar systems and satellite communications. A comparison in CPU time between this model and the finite element method is introduced. At a selected operating frequency of 2.45 GHz, finite element method solver Ansys HFSS is utilized to validate the resulting radiation patterns of the proposed analytical model and to export the choke currents.

IEEE, 2023

In this work, a review of millimeter-wave transistors modeling is presented. The physics-based mo... more In this work, a review of millimeter-wave transistors modeling is presented. The physics-based modeling approaches are studied and their advantages over the empirical models are demonstrated. Physics-based models account for the physical phenomena that affect the performance of millimeter-wave devices at high frequencies. Electromagnetic-wave propagation effects and non-linear operations are incorporated in physicsbased models utilizing appropriate numerical strategies. This makes the modeling approaches predictive over a broad frequency range, under both small-and large-signal analyses and regardless of the device size. Conventional electrode configurations for millimeter-wave transistors are also studied in this work to illustrate the importance of pad layout design in attaining suitable functionality. The necessity of accounting for the device width limitations is also discussed in this work and the transmission line theory is emphasized as a useful tool to address the challenges at higher operating frequencies.

IEEE, 2022

A simple choke antenna composed mainly of a circular waveguide surrounded by a ring is discussed.... more A simple choke antenna composed mainly of a circular waveguide surrounded by a ring is discussed. The main objective of this paper is developing an analytical model that obtains a wider, yet variable half power beamwidth (HPBW) of the antenna for different applications. By modeling the ring as a parasitic element and the circular waveguide as the feed, high-frequency field-based method Geometrical Theory of Diffraction (GTD) is proposed. Due to the curved geometry of the ring, caustics are present and introduce singularities in the far field radiation pattern solutions. A correcting factor is imported to overcome these discontinuities. At a selected operating frequency of 2.45 GHz, finite element method solver Ansys HFSS is utilized to validate the resulting radiation patterns of GTD. Antenna radiated power and tangential components constant are used to calibrate between GTD and HFSS.

IEEE, 2022

The high-frequency operability of millimeter-wave devices is investigated in this paper. An AlGaN... more The high-frequency operability of millimeter-wave devices is investigated in this paper. An AlGaN/GaN HEMT device is studied, and the results obtained from the distributed modeling approach is compared with measurements. To validate the performance of the modeling approach, non-linear operation of the model under large signal analysis is demonstrated. According to the distributed model, the signal behavior on the input and output electrodes are observed for various operating frequencies. The main reasons for the discrepancies in the output power and gain of the device are explained and the limitations arisen from the device width in higher frequency bands can be exploited. Finally, a solution is provided to demonstrate alleviating the frequency restrictions and nonlinear frequency-gain relations for devices configured with the current fabrication technologies.

IEEE, 2021

The effects of wave propagations on gain and cut-off frequency of millimeter-wave transistors are... more The effects of wave propagations on gain and cut-off frequency of millimeter-wave transistors are studied based on a new modeling approach. This model is developed based on the electron transport in semiconductor layers and transmission line properties of the electrodes in high-frequency transistors. In this approach, no small- or large-signal equivalent circuit is required for the analysis of the device. The electrodes of the transistor are considered as coupled multi-conductor transmission lines to represent the effect of non-uniform voltage and current distributions along the gate and drain conductors. A finite-difference time-domain solver is utilized to solve the governing equations of the distributed model. The achieved excellent agreement between the simulation and measurement results for a fabricated GaN-HEMT over a broad frequency range validates the accurate performance of the wave-electron-transport model in finding the figures of merit in W-band and beyond.

IEEE, 2021

The effects of pad layout variations on highfrequency characteristics of millimeter-wave transist... more The effects of pad layout variations on highfrequency characteristics of millimeter-wave transistors are studied in this work. Extrinsic parameters of the device are extracted using a conformal mapping technique merely based on the physical structure of the device. A wave-electrontransport model is utilized for the analysis. A device solver and an electromagnetic solver are included to solve the governing electron-transport and transmission line equations, respectively. Comparison results for two different pad configurations of the transistor are provided and measurement results for a GaN HEMT device is used for this purpose.

IEEE, 2021

A distributed-model-based approach is developed for small- and large-signal analyses of millimete... more A distributed-model-based approach is developed for small- and large-signal analyses of millimeter-wave transistors. To obtain the equivalent circuit model parameters, a structure based extraction technique is utilized. The method employs the physical geometry and structural topology of the device for finding the parameter values based on conformal mapping concepts and incremental inductance rule. The significant contribution of this approach is to incorporate the electromagnetic wave propagation effects in designing the distributed circuit which ensures achieving a universally accurate model over a very broad frequency range. This technique is applied to analyze a GaN MISHEMT device. The simulation results for the gain and output power under the small- and large-signal conditions are obtained for different device widths at various bias points and operating frequencies. To validate the performance of the proposed modeling approach, the results are compared with the measurement.

IEEE, 2020

A novel distributed modeling approach is proposed in this paper that predicts the device behavior... more A novel distributed modeling approach is proposed in this paper that predicts the device behavior over a broad frequency range. The detailed device dimensions along with the 19-element equivalent circuit model are presented and the boundary conditions to be incorporated in the model are provided. In order to validate the approach, the simulation results are compared with the measurement results obtained from a N-polar GaN HEMT device over a 0.25-67 GHz frequency band. The results are also generated for the hypothetical case of a wider transistor at higher operating frequencies in order to explain the importance of considering the wave propagation effects in the modeling and demonstrate how the results converge. Additionally, a novel design approach is proposed for redesigning the device electrodes so as to achieve matched phase velocities on the input and output ports of the transistor. The new device with the optimized electrode configurations yields considerably higher gain values compared to the conventional device.

IEEE, 2020

A fully distributed modeling approach is proposed in this paper that incorporates the wave propag... more A fully distributed modeling approach is proposed in this paper that incorporates the wave propagation effects in developing the equivalent circuit and works as a simulation tool for analyzing high-frequency transistors such as GaN high electron mobility devices. The details regarding the device arrangement are discussed and it is explained how the parameter extraction in the proposed modeling approach is developed merely based on the physical structure of the device along with the electrode dimensions and geometry. The governing equations for obtaining the currents and voltages at each time interval and spatial distance are analyzed based on a finite difference time domain method, where an implicit scheme is developed to ensure the unconditional stability of the technique along with reducing the computation time of the developed solution. The obtained results for the current gain and maximum available gain are verified by the measurement results from a 0.1-μm nitrogen-polar oriented GaN HEMT at 0.25-67 GHz frequency range. The results are also generated for wider cases and over higher frequency ranges to show the importance of using a fully distributed modeling technique and demonstrate the convergence of the results. An experiment is also developed regarding the thermal analysis of the high-frequency HEMT device and to discuss the heat transfer phenomenon over the device width and its effects on the distributed modeling approach are analyzed.

IEEE, 2020

A new distributed small-signal modeling approach is proposed for high-frequency analysis of trans... more A new distributed small-signal modeling approach is proposed for high-frequency analysis of transistors, where the extraction methods for each parameter are defined based on solely the structure of the device. An unconditionally stable implicit scheme is presented to analyze the time domain behavior of the device and the obtained results are compared with the measured results of a 0.1-µm N-polar GaN MISHEMT over a broad frequency range. Another simulation is conducted for a device with a larger width to show the distributed effects of the proposed technique and the reliability of the method in higher operating frequencies.

IEEE, 2019

A new modelling approach is presented to obtain the 19-element active and passive equivalent circ... more A new modelling approach is presented to obtain the 19-element active and passive equivalent circuit element values of high electron mobility transistors over a broad frequency range. In this method, a new technique is introduced to acquire the appropriate extrinsic values of the model suitable for the frequency of operation. The bias-dependent intrinsic values are also obtained using the extracted Y-parameters from a simulation scheme developed in SILVACO. The proposed method addresses the issues and limitations with existing extraction methods regarding their suitability for high-frequency ranges. A finite-difference scheme is also presented to analyze the device which is characterized by its small-signal equivalent circuit. Keywords-active device, small-signal equivalent circuit, high electron mobility transistor, extrinsic and intrinsic, finite difference methods

International Journal of Numerical Modeling, 2017

An explicit finite difference time domain method is proposed for modelling of composite right-/le... more An explicit finite difference time domain method is proposed for modelling of composite right-/left-handed active multiconductor transmission lines in time domain. For the first time, the concept of active composite right-/left-handed transmission lines is considered in field effect transistor (FET) modelling. In this work, based on the distributed model, the FET is considered as an active multiconductor transmission line in the mm/wave frequency range. In this modelling technique, the FET is divided into 2 different parts, the active part, represents the intrinsic equivalent circuit of the transistor, while the passive part symbolizes the device electrodes. The finite-difference time-domain scheme is proposed for the analysis, and the results of this technique are compared with the ones obtained from simulations. Furthermore, the stability issue is investigated and the sufficient condition for stability is derived. KEYWORDS active multiconductor, CRLH TL, FDTD method, FET modelling, millimetre-wave, stability 1 | INTRODUCTION In traditional materials, both electric and magnetic fields along with wave vector form a right-handed triad, with vector is along the direction of the Poynting vector. In contrast, in metamaterials, because the Poynting vector and wave vector have opposite directions, they are considered as left-handed (LH) materials. 1 Such materials have both negative permeability and permittivity and exhibit some attractive specifications for electrodynamic applications. 2-8 The concept of composite right-/left-handed transmission lines (CRLH TLs) has been developed 9,10 and widely used to design microwave devices. 11-13 Afrooz and Abdipour have presented 2 explicit finite-difference time-domain (FDTD) algorithms for time domain analysis of metamaterial TLs, and the results of the proposed method are confirmed with numerical examples. 14 Also, a time-domain analysis of the CRLH TL has been already investigated using numerical methods by Gomez-Diaz et al. 15 Additionally, the idea of active CRLH TL has already been applied to some devices and circuits. 16,17 Modern communication system performance strongly depends on active elements, among which field effect transistors (FETs) are largely present. However, their behaviour in the millimetre-wave frequency range is still perfectible. Analysing transmission lines in the millimetre-wave range will result in some complicated problems such as radiation and unwanted coupling between circuit elements. Therefore, a full wave approach needs to be used when the size of the electrodes are proportional to the wavelength. 18 However, full-wave analysis are usually based on highly complicated numerical techniques that require huge CPU time and computational resources. 18-21 In the proposed modelling technique, the FET is considered in the form of 3 active TLs. Based on this modelling procedure, the device will be split into an infinite number of sections. Each section is composed of 3 multiconductor transmission lines (referring to gate, drain, and source) in which each electrode is supposed to be composed of metamaterial and a traditional

Elsevier, 2018

Due to high power and high speed applications, high electron mobility transistors (HEMT) have bee... more Due to high power and high speed applications, high electron mobility transistors (HEMT) have been the area of focus for many research groups in recent years. Compared to conventional field effect transistors (FET), HEMT devices have demonstrated superior performance considering the operational frequency, output power density, and power added efficiency. An overview of GaN/AlGaN HEMT structures is presented in this chapter and the graphical illustrations are included. The metallic and semiconductor layers along with the Schottky and ohmic contacts are explained and the growth methods are discussed. The latest research on GaN HEMT structures have been brought into attention and the modifications are described. Furthermore, various modelling procedures of HEMT devices are introduced and compared so as to illustrate the advantages and disadvantages in different analysis methodologies. A brief discussion on HEMT applications is presented in the final chapter.

IEEE, 2018

The recent upsurge in wireless communications demands devices working at higher frequencies with ... more The recent upsurge in wireless communications demands devices working at higher frequencies with higher output densities. In general, wide-bandgap materials seem to be the reliable choice for these applications, specifically GaN HEMT that has shown great advantage over its previous counterparts. A holistic optimization technique is proposed to define the stages that a high frequency, high power device is designed. An issue with the thermal conductivity of the substrates for these devices is also addressed and a fabrication technique is proposed to solve it.