Ali M. Darwish

Bio: Ali M. Darwish is an academic researcher from United States Army Research Laboratory. The author has contributed to research in topics: Amplifier & Monolithic microwave integrated circuit. The author has an hindex of 16, co-authored 96 publications receiving 1235 citations. Previous affiliations of Ali M. Darwish include Massachusetts Institute of Technology & University of Maryland, College Park.

Thermal resistance calculation of AlGaN-GaN devices

Abstract: We present an original accurate closed-form expression for the thermal resistance of a multifinger AlGaN-GaN high electron-mobility transistor (HEMT) device on a variety of host substrates including SiC, Si, and sapphire, as well as the case of a single-crystal GaN wafer. The model takes into account the thickness of GaN and host substrate layers, the gate pitch, length, width, and thermal conductivity of GaN, and host substrate. The model's validity is verified by comparing it with experimental observations. In addition, the model compares favorably with the results of numerical simulations for many different devices; very close (1%-2%) agreement is observed. Having an analytical expression for the channel temperature is of great importance for designers of power devices and monolithic microwave integrated circuits. In addition, it facilitates a number of investigations that are not practical or possible using time-consuming numerical simulations. The closed-form expression facilitates the concurrent optimization of electrical and thermal properties using standard computer-aided design tools.

Femtosecond index nonlinearities in InGaAsP optical amplifiers

Abstract: We present the first femtosecond measurements of refractive index nonlinearities in diode laser amplifiers at 1.5 μm. Our results are obtained with a novel measurement technique, based on a time domain interferometer with heterodyne detection that allows the study of polarization anisotropy in the refractive index nonlinearities. We observe index changes due to carrier heating and stimulated transitions, as well as an instantaneous refractive index change similar to that observed in AlGaAs devices.

Accurate determination of thermal resistance of FETs

Abstract: The accurate determination of the channel temperature in field-effect transistors (FETs) and monolithic microwave integrated circuits is critical for reliability. An original accurate closed-form expression is presented for the thermal resistance of multifinger FET structures. The model is based on the solution of Laplace's equations in prolate spheroidal coordinates and elliptical cylinder coordinates. The model's validity is verified by comparing the results with finite-element simulations, and experimental observations from liquid-crystal measurements and spatially resolved photoluminescence measurements. Very close agreement is observed in all cases.

Channel Temperature Analysis of GaN HEMTs With Nonlinear Thermal Conductivity

Abstract: This paper presents an enhanced, closed-form expression for the thermal resistance, and thus, the channel temperature of AlGaN/gallium nitride (GaN) HEMTs, including the effect of the temperature-dependent thermal conductivity of GaN and SiC or Si substrates. In addition, the expression accounts for temperature increase across the die-attach. The model’s validity is verified by comparing it with experimental observations. The model results also compare favorably with those from finite-element numerical simulations across the various device geometric and material parameters. The model provides a more accurate channel temperature than that from a constant thermal conductivity assumption; this is particularly significant for GaN/Si HEMTs where the temperature rise is higher than in GaN/SiC. The model is especially useful for device and monolithic microwave integrated circuit designers in the thermal assessment of their device design iterations against required performance for their specific applications.

GaN-Based RF Power Devices and Amplifiers

Abstract: The rapid development of the RF power electronics requires the introduction of wide bandgap material due to its potential in high output power density, high operation voltage and high input impedance GaN-based RF power devices have made substantial progresses in the last decade This paper attempts to review the latest developments of the GaN HEMT technologies, including material growth, processing technologies, device epitaxial structures and MMIC designs, to achieve the state-of-the-art microwave and millimeter-wave performance The reliability and manufacturing challenges are also discussed

Energy dissipation and transport in nanoscale devices

Abstract: Understanding energy dissipation and transport in nanoscale structures is of great importance for the design of energy-efficient circuits and energy-conversion systems. This is also a rich domain for fundamental discoveries at the intersection of electron, lattice (phonon), and optical (photon) interactions. This review presents recent progress in understanding and manipulation of energy dissipation and transport in nanoscale solid-state structures. First, the landscape of power usage from nanoscale transistors (∼10−8 W) to massive data centers (∼109 W) is surveyed. Then, focus is given to energy dissipation in nanoscale circuits, silicon transistors, carbon nanostructures, and semiconductor nanowires. Concepts of steady-state and transient thermal transport are also reviewed in the context of nanoscale devices with sub-nanosecond switching times. Finally, recent directions regarding energy transport are reviewed, including electrical and thermal conductivity of nanostructures, thermal rectification, and the role of ubiquitous material interfaces. Open image in new window

A Review of GaN on SiC High Electron-Mobility Power Transistors and MMICs

Abstract: Gallium-nitride power transistor (GaN HEMT) and integrated circuit technologies have matured dramatically over the last few years, and many hundreds of thousands of devices have been manufactured and fielded in applications ranging from pulsed radars and counter-IED jammers to CATV modules and fourth-generation infrastructure base-stations. GaN HEMT devices, exhibiting high power densities coupled with high breakdown voltages, have opened up the possibilities for highly efficient power amplifiers (PAs) exploiting the principles of waveform engineered designs. This paper summarizes the unique advantages of GaN HEMTs compared to other power transistor technologies, with examples of where such features have been exploited. Since RF power densities of GaN HEMTs are many times higher than other technologies, much attention has also been given to thermal management-examples of both commercial “off-the-shelf” packaging as well as custom heat-sinks are described. The very desirable feature of having accurate large-signal models for both discrete transistors and monolithic microwave integrated circuit foundry are emphasized with a number of circuit design examples. GaN HEMT technology has been a major enabler for both very broadband high-PAs and very high-efficiency designs. This paper describes examples of broadband amplifiers, as well as several of the main areas of high-efficiency amplifier design-notably Class-D, Class-E, Class-F, and Class-J approaches, Doherty PAs, envelope-tracking techniques, and Chireix outphasing.

Energy Dissipation and Transport in Nanoscale Devices

Abstract: Understanding energy dissipation and transport in nanoscale structures is of great importance for the design of energy-efficient circuits and energy-conversion systems. This is also a rich domain for fundamental discoveries at the intersection of electron, lattice (phonon), and optical (photon) interactions. This review presents recent progress in understanding and manipulation of energy dissipation and transport in nanoscale solid-state structures. First, the landscape of power usage from nanoscale transistors (~10^-8 W) to massive data centers (~10^9 W) is surveyed. Then, focus is given to energy dissipation in nanoscale circuits, silicon transistors, carbon nanostructures, and semiconductor nanowires. Concepts of steady-state and transient thermal transport are also reviewed in the context of nanoscale devices with sub-nanosecond switching times. Finally, recent directions regarding energy transport are reviewed, including electrical and thermal conductivity of nanostructures, thermal rectification, and the role of ubiquitous material interfaces.

Nonlinear Optics for High-Speed Digital Information Processing.

Abstract: Recent advances in developing nonlinear optical techniques for processing serial digital information at high speed are reviewed. The field has been transformed by the advent of semiconductor nonlinear devices capable of operation at 100 gigabits per second and higher, well beyond the current speed limits of commercial electronics. These devices are expected to become important in future high-capacity communications networks by allowing digital regeneration and other processing functions to be performed on data signals “on the fly” in the optical domain.