Insulated-gate GaN-based transistors can fulfill the emerging demands for the future generation of highly efficient electronics for high-frequency, high-power and high-temperature applications. However, in contrast to Si-based devices, the introduction of an insulator on (Al)GaN is complicated by the absence of a high-quality native oxide for GaN. Trap states located at the insulator/(Al)GaN interface and within the dielectric can strongly affect the device performance. In particular, although AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) provide superior properties in terms of gate leakage currents compared to Schottky-gate HEMTs, the presence of an additional dielectric can induce threshold voltage instabilities. Similarly, the presence of trap states can be detrimental for the operational stability and reliability of other architectures of GaN devices employing a dielectric layer, such as hybrid MIS-FETs, trench MIS-FETs and vertical FinFETs. In this regard, the minimization of trap states is of critical importance to the advent of different insulated-gate GaN-based devices. Among the various dielectrics, aluminum oxide (Al2O3) is very attractive as a gate dielectric due to its large bandgap and band offsets to (Al)GaN, relatively high dielectric constant, high breakdown electric field as well as thermal and chemical stability against (Al)GaN. Additionally, although significant amounts of trap states are still present in the bulk Al2O3 and at the Al2O3/(Al)GaN interface, the current technological progress in the atomic layer deposition (ALD) process has already enabled the deposition of promising high-quality, uniform and conformal Al2O3 films to gate structures in GaN transistors. In this context, this paper first reviews the current status of gate dielectric technology using Al2O3 for GaN-based devices, focusing on the recent progress in engineering high-quality ALD-Al2O3/(Al)GaN interfaces and on the performance of Al2O3-gated GaN-based MIS-HEMTs for power switching applications. Afterwards, novel emerging concepts using the Al2O3-based gate dielectric technology are introduced. Finally, the recent status of nitride-based materials emerging as other gate dielectrics is briefly reviewed.

Trap states at the dielectric/GaN interface of AlGaN/GaN-based metalinsulator-semiconductor high electron mobility transistors (MIS-HEMTs) can cause threshold voltage (V th) instability especially under positive gate bias stress. Herein, the influence of O 2 plasma surface preconditioning (SPC) before the atomic layer deposition of the Al 2 O 3 gate dielectric and of N 2 postmetallization anneal (PMA) after gate metallization on the Al 2 O 3 /GaN interface quality is investigated. The interface is characterized by multifrequency capacitancevoltage measurements which show a smaller frequency dispersion after the employment of SPC and PMA treatments with a reduction of the interface trap density D it to a value in the order of 2 Â 10 12 cm À2 eV À1 near the conduction band edge. The effectiveness of SPC and PMA is demonstrated in Al 2 O 3 /AlGaN/ GaN MIS-HEMTs by pulsed current-voltage measurements which reveal improved V th stability.

Gold-free Ta/Al-based ohmic contacts fabricated by sputtering on AlGaN/GaN heterostructures and annealed at low temperature were investigated. The presence of a thin AlN spacer layer at the AlGaN/GaN heterojunction is demonstrated to prevent the ohmic contact formation as shown by rectifying behavior after annealing. Ta as an additional capping layer on Al leads to a severe morphology degradation and subsequent deterioration of the metal stack after annealing at 600 • C due to strong Ta-Al alloying verified by transmission electron microscopy. Using the compound metal TiN as capping layer circumvents the alloy formation, thereby making the contacts much more stable under annealing. A low contact resistance of 0.8 Ω mm was obtained using Ta/Al/TiN metal layers annealed at only 500 • C. The nature of the current transport was investigated by analyzing the temperature dependence of the specific contact resistance, which points towards a current path through the AlGaN barrier by thermionic field emission.