Characterization of electronic states at insulator/(Al)GaN interfaces for improved insulated gate and surface passivation structures of GaN-based transistors

This paper presents a systematic characterization of electronic states at insulators/(Al)GaN interfaces, particularly focusing on insulator/AlGaN/GaN structures. First, we review important results reported for GaN metal–insulator–semiconductor (MIS) structures. SiO 2 is an attractive material for MIS transistor applications due to its large bandgap and high chemical stability. In-situ SiN x is effective for improving the operation stability of high electron mobility transistors (HEMTs). Meanwhile, Al 2 O 3 /GaN structures have high band offsets and low interface state densities, which are also desirable for insulated gate applications. We have proposed a calculation method for describing capacitance–voltage ( C – V ) characteristics of HEMT MIS structures for evaluating electronic state properties at the insulator/AlGaN interfaces. To evaluate near-midgap states at insulator/AlGaN interfaces, a photo-assisted C – V technique using photon energies less than the bandgap of GaN has been developed. Using the calculation in conjunction with the photo-assisted C – V technique, we estimate interface state density distributions at the Al 2 O 3 /AlGaN interfaces.