Superior suppression of gate current leakage in Al2O3/Si3N4 bilayer-based AlGaN/GaN insulated gate heterostructure field-effect transistors

AlGaN/GaN-based metal-insulator-semiconductor heterostructure field-effect transistors (MIS-HFETs) with Al₂O₃/Si₃N₄ bilayer as insulator have been investigated in detail, and compared with the conventional HFET and Si₃N₄-based MIS-HFET devices. Al₂O₃/Si₃N₄ bilayer-based MIS-HFETs exhibited much lower gate current leakage than conventional HFET and Si₃N₄-based MIS devices under reverse gate bias, and leakage as low as 1×10⁻¹¹ A/mm at −15 V has been achieved in Al₂O₃/Si₃N₄-based MIS devices. By using ultrathin Al₂O₃/Si₃N₄ bilayer, very high maximum transconductance of more than 180 mS/mm with ultra-low gate leakage has been obtained in the MIS-HFET device with gate length of 1.5 μm, a reduction less than 5% in maximum transconductance compared with the conventional HFET device. This value was much smaller than the more than 30% reduction in the Si₃N₄-based MIS device, due to the employment of ultra-thin bilayer with large dielectric constant and the large conduction band offset between Al₂O₃ and nitrides. This work demonstrates that Al₂O₃/Si₃N₄ bilayer insulator is a superior candidate for nitrides-based MIS-HFET devices.