For improvement of reliability and device performance in GaN-based FETs, insulated-gate and surface-passivation structures were investigated. Serious stoichiometry disorder and N deficiency were found at the AlGaN surfaces processed by high-temperature annealing, H_2-plasma cleaning, dry etching in CH_4/H_2/Ar plasma and deposition of SiO_2 The N deficiency could introduce a localized deep donor level related to N vacancy (V_N) at AlGaN surfaces. The results obtained indicate that it becomes a key to control V_N-related defects for reliability improvement of GaN-based electronic devices. The SiN_x passivation structure prepared by ECR-CVD with the N_2-plasma pre-treatment showed good interface properties. However, excess leakage currents based on the Fowler-Nordheim tunneling were observed in the SiNx/Al_(0.3)Ga_(0.7)N structure, due to a relatively small conduction band offset, ΔE_C, of 0.7 eV between SiN_x and Al_(0.3)Ga_(0.7)N. A novel Al_2O_3-based passivation structure was successfully formed on the AlGaN surface by molecular beam deposition of Al and the subsequent ECR O_2-plasma oxidation. In-situ XPS analysis showed a bandgap of 7.0 eV for the formed Al_2O_3 layer with a thickness of 3.5 nm and a sufficiently large ΔE_C of 2.1 eV between Al_2O_3 and Al_(0.3)Ga_(0.7)N. The AlGaN/GaN insulated-gate HFETs having the Al_2O_3-based passivation structure showed a good gate control of drain currents and the suppression of current collapse. These results indicate a remarkable advantage of the present Al_2O_3-based passivation structure for GaN/AlGaN HFETs, leading to the reliability improvement of AlGaN/GaN HFETs.
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