Recent years have witnessed GaN-based devices delivering their promise of unprecedented power and frequency levels and demonstrating their capability as an able replacement for Si-based devices. High-electron-mobility transistors (HEMTs), a key representative architecture of GaN-based devices, are well-suited for high-power and high frequency device applications, owing to highly desirable III-nitride physical properties. However, these devices are still hounded by issues not previously encountered in their more established Si-and GaAs-based devices counterparts. Metal-insulator-semiconductor (MIS) structures are usually employed with varying degrees of success in sidestepping the major problematic issues such as excessive leakage current and current instability. While different insulator materials have been applied to GaN-based transistors, the properties of insulator/III-N interfaces are still not fully understood. This is mainly due to the difficulty of characterizing insulator/AlGaN interfaces in a MIS HEMT because of the two resulting interfaces: insulator/AlGaN and AlGaN/GaN, making the potential modulation rather complicated. Although there have been many reports of low interface-trap densities in HEMT MIS capacitors, several papers have incorrectly evaluated their capacitance-voltage (C-V) characteristics. A HEMT MIS structure typically shows a 2-step C-V behavior. However, several groups reported C-V curves without the characteristic step at the forward bias regime, which is likely to the high-density states at the insulator/ AlGaN interface impeding the potential control of the AlGaN surface by the gate bias. In this review paper, first we describe critical issues and problems including leakage current, current collapse and threshold voltage instability in AlGaN/GaN HEMTs. Then we present interface properties, focusing on interface states, of GaN MIS systems using oxides, nitrides and high-κ dielectrics. Next, the properties of a variety of AlGaN/GaN MIS structures as well as different characterization methods, including our own photo-assisted C-V technique, essential for understanding and developing successful surface passivation and interface control schemes, are given in the subsequent section. Finally we highlight the important progress in GaN MIS interfaces that have recently pushed the frontier of nitride-based device technology.

We have investigated the relationship between improved electrical properties of Al₂O₃/AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) and electronic state densities at the Al₂O₃/AlGaN interface evaluated from the same structures as the MOS-HEMTs. To evaluate Al₂O₃/AlGaN interface state densities of the MOS-HEMTs, two types of capacitance-voltage (C-V) measurement techniques were employed: the photo-assisted C-V measurement for the near-midgap states and the frequency dependent C-V characteristics for the states near the conduction-band edge. To reduce the interface states, an N₂O-radical treatment was applied to the AlGaN surface just prior to the deposition of the Al₂O₃ insulator. As compared to the sample without the treatment, the N₂O-radical treated Al₂O₃/AlGaN/GaN structure showed smaller frequency dispersion of the C-V curves in the positive gate bias range. The state densities at the Al₂O₃/AlGaN interface were estimated to be 1 × 10¹² cm⁻² eV⁻¹ or less around the midgap and 8 × 10¹² cm⁻² eV⁻¹ near the conduction-band edge. In addition, we observed higher maximum drain current at the positive gate bias and suppressed threshold voltage instability under the negative gate bias stress even at 150 °C. Results presented in this paper indicated that the N₂O-radical treatment is effective both in reducing the interface states and improving the electrical properties of the Al₂O₃/AlGaN/GaN MOS-HEMTs.

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.

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 × 106 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 1012 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 °C in air for 3 h. The reverse-bias-annealed sample showed state densities less than 1 × 1011 cm−1 eV−1 and small shifts of flat-band voltage. In addition, the C–V curve measured at 200 °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.