We report interface charge engineering in AlTiO/AlGaN/GaN metal-insulator-semiconductor (MIS) field-effect transistors (FETs), where aluminum titanium oxide Al_xTi_yO (AlTiO), an alloy of Al_2O_3 and TiO_2, is employed as a gate insulator. AlTiO is a versatile insulator whose properties can be modified for interface charge engineering, as well as energy gap and dielectric constant engineerings via its composition. It was found that AlTiO can effectively suppress a positive fixed charge at AlTiO/etched-AlGaN interface, leading to a positive slope in the relation between the threshold voltage and the AlTiO insulator thickness. As a result, we successfully obtained normally-off operations in partially-gate-recessed Al-TiO/AlGaN/GaN MIS-FETs with a rather thick remaining AlGaN layer, showing favorable performances, such as a threshold voltage of 1.7 V, an on-resistance of 9.5 Ω·mm, an output current of 450 mA/mm, a sub-threshold swing of 65 mV/decade, and a electron mobility of 730 cm~2/V.s.
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