Wide bandgap semiconductors have recently attracted the interest of academic researchers and the semiconductor industry. In fact, their superior material parameters with respect to silicon allow for a series of significant improvements. In particular, composite GaN/AlGaN semiconductors offer a large breakdown field and a good thermal conductivity together with a high electron mobility, for which they have become the best candidates for power electronics and high frequency switching applications. Possible targets that would benefit of the advantages of GaN–based devices include efficient power supplies, DC/DC converters and AC/DC adapters, as well as the field of radars and telecommunications.
Nevertheless, several issues still constitute a challenge to the development of a mature GaN–based technology. Besides the difficulties of growing a high–quality GaN material, point defects at interfaces play a major role in terms of reliability. In the present thesis, we investigate the threshold voltage instability under forward gate bias of GaN–based transistors, which is due to the defects located at the III–N/dielectric interface. Our goal is to characterize these trap levels and propose a valid physical model to explain the behavior observed on our test structures. We perform electrical measurements as well as a combination of electrical and optical experiments, by integrating an existing setup with a system capable of exposing the device under test to monochromatic light.
The main results of this work are the development of characterization methods and their application to various GaN/AlGaN/SiN transistors. In the first place, we find that most of the established characterization techniques for interface traps, particularly those based on the analysis of the impedance characteristics, are inappropriate for GaN/AlGaN devices. As a consequence, we develop an experimental method that allows to evaluate the amount of threshold voltage shift during the application of a positive gate bias stress accurately and precisely. The data measured in this way is interpreted according to a physical model for charge exchange based on non–radiative multiphonon transitions. The experimental results show that the standard AlGaN/SiN interface has a density of traps per unit area and energy of about 1013/(cm2 eV), and the activation energy for electron trapping at these defects is distributed at least from 0.1 eV to 0.8 eV. This can be explained with the disordered nature of the AlGaN/SiN interface. On the other hand, a completely different trapping behavior is observed in devices whose AlGaN surface has been cleaned with a fluorine plasma process before the deposition of the dielectric. In this case, we find defects with a very narrow distribution of activation energy around 0.32 eV, and a density of 8 × 1013/(cm2 eV). This result implies that the fluorine radical plays an active role in the passivation of the native defects, substituting them with a new trap level having very different properties.