The impacts of SiN/Al 2 O 3 bi-layer passivation on the carrier transport characteristics in GaN-based metal-insulator-semiconductor high electron mobility transistors (MISHEMTs) were studied. Various mechanical stresses, as measured by micro-Ramam spectroscopy, were introduced on the GaN channel according to the different passivation systems. The SiN dielectric layer deposited by plasma enhanced chemical vapor deposition on top of the GaN capping layer resulted in compressive stress. On the other hand, the Al 2 O 3 passivation layer deposited by atomic layer deposition on SiN layer generated tensile stress, which compensated the compressive stress produced by the SiN layer. The correlation between the applied mechanical stress induced by the deposited dielectric layers and device performance of the GaN-based HEMT was also investigated. When a slight tensile stress was applied on the GaN channel through the bi-layer passivation, the carrier transfer characteristics were improved in terms of carrier concentration at the AlGaN/GaN interface, as well as carrier mobility and sheet resistance compared to the high compressive stress condition. These results show that the mechanical stress engineering via optimized passivation process is a promising technique for the improvement of the device performance in GaN-based MISHEMTs.
Thermally stable, low-resistance PdGe-based ohmic contacts to high–low doped n-GaAs have been developed. The lowest contact resistance obtained is two times lower than that of previously reported PdGe ohmic contacts. The contacts are thermally stable even after isothermal annealing for 5 h at 400 °C under atmosphere ambient. X-ray diffraction results and Auger depth profiles show that the good PdGe-based ohmic contact is due to the formation of both AuGa and TiO compounds. The AuGa compound enhances the creation of more Ga vacancies, followed by the incorporation of Ge into Ga vacancies, and the TiO compound suppresses As outdiffusion from the GaAs substrate, respectively.
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