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Interface characterization was carried out on Al2O3/GaN structures using epitaxial n-GaN layers grown on free-standing GaN substrates with relatively low dislocation density (<3 x 10(6) cm(-2)). The Al2O3 layer was prepared by atomic layer deposition. The as-deposited metal-oxide-semiconductor (MOS) sample showed a significant frequency dispersion and a bump-like feature in capacitance-voltage (C-V) curves at reverse bias, showing high-density interface states in the range of 10(12) cm(-1) eV(-1). On the other hand, excellent C-V characteristics with negligible frequency dispersion were observed from the MOS sample after annealing under a reverse bias at 300 degrees C in air for 3 h. The reverse-bias-annealed sample showed state densities less than 1 x 10(11) cm(-1) eV(-1) and small shifts of flat-band voltage. In addition, the C-V curve measured at 200 degrees C remained essentially similar compared with the room-temperature C-V curves. These results indicate that the present process realizes a stable Al2O3/GaN interface with low interface state densities

In this paper, we give an overview of the recent progress in GaN-based high-electron-mobility transistors (HEMTs) developed for mainstream acceptance in the power electronics field. The comprehensive investigation of AlGaN/GaN HEMTs fabricated on a free-standing semi-insulating GaN substrate reveals that an extracted effective lateral breakdown field of approximately 1 MV/cm is likely limited by the premature device breakdown originating from the insufficient structural and electrical quality of GaN buffer layers and/or the GaN substrate itself. The effective lateral breakdown field is increased to 2 MV/cm by using a highly resistive GaN substrate achieved by heavy Fe doping. Various issues relevant to current collapse are also discussed in the latter half of this paper, where a more pronounced reduction in current collapse is achieved by combining two different schemes (i.e., a prepassivation oxygen plasma treatment and a field plate structure) for intensifying the mitigating effect against current collapse. Finally, a novel approach to suppress current collapse is presented by introducing a three-dimensional field plate (3DFP) in AlGaN/GaN HEMTs, and its possibility of realizing true collapse-free operation is described.

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.

University. His research interests included surface passivation and device processing technologies for III-V compound semiconductors. Currently, his major activity expanded into characterization and control of surfaces and interfaces of GaN and related materials and their application to GaN-based electron devices. He has authored or co-authored over 160 papers in scientific and technical journals.

Gallium nitride (GaN) is one of the front-runner materials among the so-called wide bandgap semiconductors that can provide devices having high breakdown voltages and are capable of performing efficiently even at high temperatures. The wide bandgap, however, naturally leads to a high density of surface states on bare GaN-based devices or interface states along insulator/semiconductor interfaces distributed over a wide energy range. These electronic states can lead to instabilities and other problems when not appropriately managed. In this Tutorial, we intend to provide a pedagogical presentation of the models of electronic states, their effects on device performance, and the presently accepted approaches to minimize their effects such as surface passivation and insulated gate technologies. We also re-evaluate standard characterization methods and discuss their possible pitfalls and current limitations in probing electronic states located deep within the bandgap. We then introduce our own photo-assisted capacitance–voltage (C–V) technique, which is capable of identifying and examining near mid-gap interface states. Finally, we attempt to propose some directions to which some audience can venture for future development.

Dramatic reduction of thermal resistance was achieved in AlGaN/GaN Multi-Mesa-Channel (MMC) high electron mobility transistors (HEMTs) on sapphire substrates. Compared with the conventional planar device, the MMC HEMT exhibits much less negative slope of the I-D-V-DS curves at high V-DS regime, indicating less self-heating. Using a method proposed by Menozzi and co-workers, we obtained a thermal resistance of 4.8 K-mm/W at ambient temperature of similar to 350K and power dissipation of similar to 9W/mm. This value compares well to 4.1 K-mm/W, which is the thermal resistance of AlGaN/GaN HEMTs on expensive single crystal diamond substrates and the lowest reported value in literature

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