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Today, the introduction of wide band gap (WBG) semiconductors in power electronics has become mandatory to improve the energy efficiency of devices and modules and to reduce the overall electric power consumption in the world. Due to its excellent properties, gallium nitride (GaN) and related alloys (e.g., AlxGa1−xN) are promising semiconductors for the next generation of high-power and high-frequency devices. However, there are still several technological concerns hindering the complete exploitation of these materials. As an example, high electron mobility transistors (HEMTs) based on AlGaN/GaN heterostructures are inherently normally-on devices. However, normally-off operation is often desired in many power electronics applications. This review paper will give a brief overview on some scientific and technological aspects related to the current normally-off GaN HEMTs technology. A special focus will be put on the p-GaN gate and on the recessed gate hybrid metal insulator semiconductor high electron mobility transistor (MISHEMT), discussing the role of the metal on the p-GaN gate and of the insulator in the recessed MISHEMT region. Finally, the advantages and disadvantages in the processing and performances of the most common technological solutions for normally-off GaN transistors will be summarized.

The temperature dependence of the electrical properties of Pt∕GaN Schottky barrier was studied. In particular, a Schottky barrier height of 0.96eV and an ideality factor of 1.16 were found after a postdeposition annealing at 400°C. Nanoscale electrical characterization was carried out by the conductive biased tip of an atomic force microscope both on the bare GaN surface and on the Pt∕GaN contacts. The presence of a lateral inhomogeneity of the Schottky barrier, with a Gaussian distribution of the barrier height values, was demonstrated. Moreover, GaN surface defects were demonstrated to act as local preferential paths for the current conduction. The temperature dependent electrical characteristics of the diodes were discussed in terms of the existing models on inhomogeneous barriers and correlated to the nanoscale electrical characterization of the barrier. In this way, the anomalous electrical behavior of the ideality factor and of the Schottky barrier and the low experimental value of the Richardson’s constant were explained.

In this work, CO 2 adsorption on a laboratory-synthesized polymeric copper(II) benzene-1,3,5-tricarboxylate (Cu-BTC) metal-organic framework was modeled by means of the semiempirical Sips equation in order to obtain parameters of engineering interest. Produced Cu-BTC samples were characterized by X-ray diffraction, thermogravimetry, and microporosimetric analysis; high crystallinity and very high specific surface area and pore volume were found. CO 2 adsorption isotherms on Cu-BTC were evaluated at T ) (283, 293, 318, and 343) K for p e 1 bar by means of a volumetric technique. In order to establish a comparison, CO 2 adsorption isotherms on samples of commercial 13X zeolite were determined under the same experimental conditions and then modeled in the same way as those for Cu-BTC. The modeling and experimental results indicated that relative to 13X zeolite, Cu-BTC showed higher CO 2 adsorption capacities at near-ambient temperature and a lower heat release during the adsorption phase.

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The role of carbon related traps in GaN-based ungated HEMT structures has been investigated both experimentally and by means of numerical simulations. A clear quantitative correlation between experimental data and numerical simulations has been obtained. The observed current decrease in the tested structure during backgating measurements has been explained simply by means of a thermally activated hole-emission process with E A =0.9eV, corresponding to the distance of the acceptor-like hole-trap level from the GaN valence band. Moreover, it has been demonstrated by means of electrical measurements and numerical simulations that only a low percentage of the nominal Carbon doping levels induces the observed current reduction when negative substrate bias are applied to the tested structure.