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., Al_xGa_1−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 electrical characterization of nickel silicide Schottky contacts on silicon carbide (4H–SiC) is reported in this article. In spite of the nearly ideal behavior of the contact at room temperature $\text{(n=1.05),}$ the electrical behavior monitored in a wide temperature range exhibited a deviation from the ideality at lower temperatures, thus suggesting that an inhomogeneous barrier has actually formed. A description of the experimental results by the Tung’s model, i.e., considering an effective area of the inhomogeneous contact, provided a procedure for a correct determination of the Richardson’s constant $A^{\mathrm{**}}.$ An effective area lower than the geometric area of the diode is responsible for the commonly observed discrepancy in the experimental values of $A^{\mathrm{**}}$ from its theoretical value in silicon carbide. The same method was applied to Ti/4H–SiC contacts.

The temperature dependence of the electrical properties of $\mathrm{Pt}∕\mathrm{Ga}\mathrm{N}$ Schottky barrier was studied. In particular, a Schottky barrier height of $0.96\mathrm{eV}$ and an ideality factor of 1.16 were found after a postdeposition annealing at $400°\mathrm{C}$ . 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 $\mathrm{Pt}∕\mathrm{Ga}\mathrm{N}$ 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.