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Link visio: https://univ-grenoble-alpes-fr.zoom.us/j/93418770084?pwd=SGR6V0N6dWVPL254MjlHOUQ0c05zdz09
Meeting ID: 934 1877 0084 / Passcode: 055513
The defence will be in French.
Abstract: The increasing demand for energy requires the development of new and suitable infrastructure, and the advancement of power electronics plays a pivotal role in addressing this need. Utilizing wide-bandgap semiconductors, due to their superior physical properties compared to silicon, emerges as the most promising avenue for designing high-performance components. While high electron mobility transistors (HEMT) based on GaN on silicon are already commercialized, they exhibit substantial performance degradation at elevated temperatures. HEMTs with AlGaN channels on silicon offer the potential to overcome these limitations while also offering lower production costs compared to SiC devices.
Initially, an AlGaN/GaN/AlN heterostructure with an ultrathin channel was investigated. The objective was to enhance the breakdown voltage of the structure by minimizing the impact of the GaN channel. An analysis of the transport properties in this heterostructure using Hall effect measurements revealed relatively low electron mobility, coupled with anisotropy depending on the crystal orientation. This finding was substantiated by a structural analysis of the layer stack and the detection of electrically active defects, further supporting the conclusions drawn from the Hall effect measurements. Furthermore, charge carrier diffusion models indicated that interface roughness constituted the primary limiting mechanism for electron mobility.
Subsequently, an analysis of the electronic transport properties in AlGaN channel heterostructures was performed. Based on physical models, the optimal Al compositions for the AlGaN layers were determined. The impact of the aluminum fraction in the AlGaN channel on electronic mobility was experimentally studied through Hall effect measurements. These experimental results were compared to physical models to identify the limiting factors of mobility. Alloy disorder is the primary limiting mechanism in these structures, but its impact is less pronounced than expected. Observations also indicate a milder degradation of transport properties compared to GaN channel heterostructures, thus demonstrating superior thermal stability. However, passivating structures on Si substrates with a SiN layer appear to limit the stability of transport properties at high temperatures.
A study of electrically active defects through deep level transient spectroscopy (DLTS) in an epitaxial AlN layer on a Si (111) substrate was conducted to assess the quality of this layer, which influences the overall quality of the heterostructure. Two distinct groups of traps were detected, suggesting they correspond to different regions in the AlN/Si structure. Defect analysis was also conducted on an Al-rich AlGaN channel heterostructure grown on a silicon substrate. Two energy levels attributed to a single trap were detected and are responsible for the reduction in electron density in the 2DEG at low temperatures.
Finally, the degradation of transistor performance with temperature was assessed based on the aluminum fraction in the AlGaN channel, substrate choice (Si and AlN), and gate structure. Increasing the aluminum fraction in the channel not only led to a reduction in on-state current but also reduced off-state leakage current, resulting in improved thermal stability of performance. The mechanisms responsible for gate leakage currents were identified by comparing models with experimental results, revealing that increasing the aluminum fraction effectively reduces the intensity of these mechanisms.
In conclusion, AlGaN channel HEMTs on Si (111) substrates exhibit superior thermal stability of on-state performance and a significant enhancement of off-state blocking characteristics. These devices thus hold excellent potential for high-voltage and high-temperature applications.
