Giovanna Sozzi

Alkali-templated surface nanopatterning of chalcogenide thin films: a novel approach toward solar cells with enhanced efficiency.

Concepts of localized contacts and junctions through surface passivation layers are already advantageously applied in Si wafer-based photovoltaic technologies. For Cu(In,Ga)Se2 thin film solar cells, such concepts are generally not applied, especially at the heterojunction, because of the lack of a simple method yielding features with the required size and distribution. Here, we show a novel, innovative surface nanopatterning approach to form homogeneously distributed nanostructures (<30 nm) on the faceted, rough surface of polycrystalline chalcogenide thin films. The method, based on selective dissolution of self-assembled and well-defined alkali condensates in water, opens up new research opportunities toward development of thin film solar cells with enhanced efficiency.

Temperature-Dependent Characterization of AlGaN/GaN HEMTs: Thermal and Source/Drain Resistances

This paper shows the application of simple dc techniques to the temperature-dependent characterization of AlGaN/ GaN HEMTs in terms of the following: 1) thermal resistance and 2) ohmic series resistance (at low drain bias). Despite their simplicity, these measurement techniques are shown to give valuable information about the device behavior over a wide range of ambient/channel temperatures. The experimental results are validated by comparison with independent measurements and numerical simulations.

Electric-field-related reliability of AlGaAs/GaAs power HFETs: bias dependence and correlation with breakdown

This work shows experimental and simulated data of hot electron degradation of power AlGaAs/GaAs HFETs with different gate lengths and recess widths, and uses them to infer general indications on the bias and geometry dependence of the device high-field degradation, the meaningfulness of the breakdown voltage figure of merit from a reliability standpoint, and the physical phenomena taking place in the devices during the stress and leading to performance degradation. Possible formulations of a voltage-acceleration law for lifetime extrapolation are also tested.

A review of the use of electro-thermal simulations for the analysis of heterostructure FETs

Abstract This paper deals with using device-level numerical simulations for the investigation of the electro-thermal behavior of a GaAs-based heterostructure FET. We show a way of dealing with the software/hardware limitations related with the huge disproportion between the electrically active region and the volume relevant to heat outflow. We study very wide simplified structures to obtain guidelines for building up a reduced grid and proper boundary conditions for the complete simulation of the electro-thermal behavior of the FET. As an application example, we use this approach to simulate the military standard (MIL-STD) method for the measurement of the thermal resistance of GaAs FETs, thus discussing its accuracy and limitations. We also show that in multi-finger structures a single channel temperature such as that obtained by electrical thermal resistance extraction techniques cannot satisfactorily describe the FET’s thermal behavior. Finally, we briefly dwell on a comparison between 2D and 3D simulations.

Large-signal GaN HEMT electro-thermal model with 3D dynamic description of self-heating

This paper shows a physical approach to large-signal electro-thermal simulation of AlGaN/GaN HEMTs. The dynamic thermal behavior of the HEMT is described by a 3D network of thermal resistances and capacitances describing the physical structure of the HEMT, and including features such as the thermal boundary resistance between GaN and SiC, and the die-attach, as well as temperature non-uniformity along the gate finger. The thermal network is self-consistently coupled inside ADS with an electro-thermal large-signal model.

Hybrid Large-Signal/Lumped-Element Electro-Thermal Modeling of GaN-HEMTs

This paper shows a practical approach to GaN-based HEMT self-consistent electro-thermal simulation for circuit modeling and reliability estimation. A physical-level lumped element dynamic thermal network able to describe the 2-D device geometry is self-consistently coupled with a novel electro-thermal compact large-signal model. The results obtained with the lumped-element thermal network are compared with finite-element simulations and shown to provide valuable estimates of the thermal behavior of very large 2-D structures. Measured results taken at ambient temperatures between 200 and 400 K are shown to be well described by the model.

Analysis of the gate current as a suitable indicator for FET degradation under nonlinear dynamic regime

Abstract Electron device degradation, although not directly accounted for, represents a key issue in microwave circuit design. This is especially true when the particular applications involved (e.g., satellite, military, consumer) do not allow or strongly discourage any kind of maintenance. As a matter of fact, in order to account for device degradation in circuit design, a suitable electron device model is needed which is able to predict the performance degradation as a function of the actual electrical regime involved in the device operation. Such a kind of model is not available in literature. In this paper, quantitative results are provided for device degradation indicators which correlate DC and RF stress experiments. These results can be considered an important step toward the definition of a nonlinear model accounting for device degradation.

Physical investigation of trap-related effects in power HFETs and their reliability implications

This paper presents a detailed physical investigation of trapping effects in GaAs power HFETs. Two-dimensional numerical simulations, performed using a hydrodynamic model that includes impact ionization, are compared with experimental results of fresh as well as hot-carrier-stressed HFETs in order to gain insight of intertwined phenomena such as the kink in the dc output curves, the hot-carrier degradation of the drain current, and the impact-ionization-dominated reverse gate current. Thoroughly consistent results show that: 1) the kink effect is dominated by the traps at the source-gate recess surface; and 2) as far as the hot-carrier degradation is concerned, only a simultaneous increase of the trap density at the drain-gate recess surface and at the channel-buffer interface (again at the drain side of the channel) is able to account for the simultaneous decrease of the drain current and the increase of the impact-ionization-dominated reverse gate current.

Breakdown and degradation issues and the choice of a safe load line for power HFET operation

This work shows data of hot electron degradation of power AlGaAs/GaAs HFETs and uses them to infer general indications on the bias point dependence of the device degradation, the meaningfulness of the breakdown voltage figure of merit and the physical phenomena taking place in the devices during the stress.

Lumped element thermal modeling of GaN-Based HEMTs

This paper shows a practical approach to GaN-based HEMT self-consistent electro-thermal simulation for circuit modeling and reliability estimation. A physical-level lumped element dynamic thermal network able to describe the two-dimensional device geometry is self-consistently coupled with an electro-thermal compact large-signal model. The results obtained with the lumped-element thermal network are compared with finite-element simulations and shown to provide valuable estimates of the thermal behavior of very large 2D structures.