The wide spectrum of power electronics applications, including their role in renewable energy conversion and energy saving, require the innovation from conventional Silicon (Si) technology into new materials and architectures that allow the fabrication of increasingly lightweight, compact, efficient and reliable devices. However, the trade-off between long lifetime, high performance and low cost in the emerging technologies represents a huge limitation that has gained the attention of different research groups in the last years. Gallium Nitride (GaN) is a wide-bandgap semiconductor (WBGS) that constitutes an excellent candidate for high-power and high-frequency applications due to its remarkable features such as high operating temperature, high dielectric strength, high current density, high switching speed, and low on-resistance. Compared with its Silicon counterpart, GaN is superior in terms of high breakdown field ( 3 MV/cm), exceptional carrier mobility, and power dissipation. By taking into account other WBG materials such as SiC, GaN grown on Si substrates promises similar performance but at a much lower cost in the low to mid power and high-frequency range. Since GaN allows size and weight device reduction due to a better relationship between on-resistance and breakdown voltage, it is suitable for a variety of applications such as RF power amplifiers, power switching systems, sensors, detectors, etc. Especially, in the field of energy efficiency, GaN technology appears as a future successor of Si in power conversion circuits. However, some drawbacks related to technology cost, integration, and long-term reliability have to be overcome for its wide adoption in the power applications market. One of the worst inconveniences of AlGaN/GaN High Electron Mobility Transistors (HEMTs) is the normally-ON operation. Since a two-dimensional electron gas (2DEG) channel is formed at the AlGaN/GaN interface due to inherent material properties, a negative bias has to be applied at the gate to switch the device off. Among the proposed solutions to fabricate normally-OFF devices, the metaloxide/ insulator-semiconductor (MOS/MIS) structure with different insulators has shown remarkable improvements in gate leakage reduction and drain current increase. Also in AlGaN/GaN Schottky Barrier Diodes (SBDs), the introduction of a MOS structure to create a gated edge termination (GET) at the anode area has resulted in significant improvements in reverse diode leakage and forward diode voltage. Nevertheless, the improvement in the device performance by the introduction of a dielectric could seriously affect the device long-term reliability since additional degradation in this layer and at its interfaces with AlGaN or GaN occurs. In the case of conversion systems, power devices are continuously switched from an OFF-state condition at high drain bias to an ON-state condition at large drain current. Therefore, the reliability of GaN-based devices has to be proven for the complete ON/OFF operation. This dissertation focuses on providing a more comprehensive analysis of two main reliability issues related to the dielectric insertion under the gate/anode stacks by analyzing the use of different dielectric materials and device architectures. The first issue is the positive bias temperature instability (PBTI), which is related to the degradation of electrical parameters when high gate voltages and temperatures are applied and is especially observed during the ON-state operation of the transistor. By using MOS-HEMT structures with different gate dielectrics (SiO2, Al2O3, and AlN/Al2O3), the impact of the stress voltage, recovery voltage and temperature on the device reliability is analyzed including the role of oxide traps and the interface states to provide physical insights into this mechanism. The second phenomenon discussed in this thesis is the time-dependent dielectric breakdown (TDDB) observed on GET-SBDs during its OFF-operation. The percolation model and Weibull distribution are used to understand this degradation mechanism. As a result, it has been demonstrated that the time to breakdown tBD is influenced by the GET structure (single vs. double), the passivation thickness, the preclean process at the anode region before the GET dielectric deposition and the capping layer. Finally, by using 2D TCAD simulations, the long-term reliability improvement has been related to the reduction of the electric fie Dottorato in Tecnologie dell’informazione e della comunicazione, Ciclo XXXI University of Calabria