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We have systematically investigated effects of metal–semiconductor or insulator–semiconductor interfacial layers (ILs) in AlGaN/GaN devices, where AlO x , TiO x , or NiO x is employed as an IL. From capacitance–voltage characteristics of metal/IL/AlGaN/GaN devices with a metal–semiconductor IL between the gate metal and AlGaN, it is shown that the IL modulates the threshold voltage V th , attributed to the vacuum level step induced by the dipole of the IL. We find negative vacuum level steps for AlO x and TiO x ILs, and positive for NiO x , from which the IL dipole density is estimated for each IL material. The two-dimensional electron gas carrier concentration in the metal/IL/AlGaN/GaN devices is also modulated by the vacuum level step. On the other hand, from capacitance–voltage characteristics of metal/ Al 2 O 3 /IL/AlGaN/GaN devices with an insulator–semiconductor IL between Al 2 O 3 and AlGaN, the fixed charge density of the Al 2 O 3 /IL/AlGaN interface is evaluated by the Al 2 O 3 thickness dependence of V th . For AlO x and TiO x ILs, the fixed charge density is higher than that of the Al 2 O 3 /AlGaN interface with no IL, while lower for NiO x. The fixed charge density for an IL shows a positive correlation with the IL dipole density, suggesting that the fixed charge is related to the unbalanced IL dipole. Furthermore, using the conductance method, we find a low trap density of the Al 2 O 3 /IL/AlGaN interface for AlO x and NiO x ILs, in comparison with that of the Al 2 O 3 /AlGaN interface with no IL. [ABSTRACT FROM AUTHOR]

We report normally-off operations in partially-gate-recessed Al x Ti y O (AlTiO)/AlGaN/GaN metal-insulator-semiconductor (MIS) field-effect transistors (FETs), where aluminum titanium oxide AlTiO, an alloy of Al 2 O 3 and TiO 2 , is employed as a gate insulator. Since AlTiO is useful for interface charge engineering owing to a trend that the AlTiO/AlGaN interface fixed charge is suppressed in comparison with Al 2 O 3 , we investigated combining the interface charge engineering with a partial gate recess method for AlTiO/AlGaN/GaN MIS-FETs. For AlTiO with a composition of x / (x + y) = 0.73 , a suppressed positive interface fixed charge at the AlTiO/recessed-AlGaN interface leads 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 AlTiO/AlGaN/GaN MIS-FETs with 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 low sub-threshold swing of 65 mV/decade, and a rather high electron mobility of 730 cm 2 / V s . The results show that the interface charge engineering in combination with partial gate recess is effective for the GaN-based normally-off device technology. [ABSTRACT FROM AUTHOR]