Characterization, Modeling and Reliability of Lateral and Vertical GaN-based Power Devices

The increasing demand for efficient power conversion led to the study of new semiconductors for the realization of the active components adopted in power converters. Among all, gallium nitride (GaN) based devices are showing good performance in the final applications, achieving higher efficiency than silicon-based devices. In this context, this thesis work is focused on two different aspects: (i) the study of the threshold voltage and on-resistance instabilities affecting the performance of lateral-based GaN transistors in on-state stress, off-state stress and switching (soft and hard) stress; (ii) the study of reliability and threshold/flatband voltage instabilities in vertical GaN devices, needed to further push the voltage limitations present with lateral-based GaN transistors. In the lateral transistors’ framework, we studied the trade-off between the gate leakage current and the threshold voltage stability, in p-GaN gate HEMTs, highlighting that a reasonable amount of gate current is beneficial for the device performance. In addition, we investigated the hard switching behavior of such devices, being able to extract for the first time the threshold voltage instabilities associated to this switching condition (as well as the on-resistance behavior). In the vertical transistors’ framework, we demonstrated that a bilayer dielectric composition is beneficial for the MOS gate stack of a vertical GaN MOSFET. Furthermore, we demonstrated also the reliability of MOS capacitors with different dielectric and doping compositions. Finally, threshold voltage instabilities studies demonstrated a significant electron trapping processes in oxide traps and the existence of impact ionization (and its interaction with electron trapping) in SiO2 based MOS structure realized on GaN. This thesis work resulted in a significant improvement in the knowledge of GaN-based power devices physics.