High power electronics using wide bandgap materials are maturing rapidly, and significant market growth is expected in a near future. Ultra wide bandgap materials, which have an even larger bandgap than GaN (3.4 eV), represent an attractive choice of materials to further push the performance limits of power devices. In this work, we report on the fabrication of AlN/AlGaN/AlN high-electron mobility transistors (HEMTs) using 50% Al-content on the AlGaN channel, which has a much wider bandgap than the commonly used GaN channel. The structure was grown by metalorganic chemical vapor deposition (MOCVD) on AlN/sapphire templates. A buffer breakdown field as high as 5.5 MV/cm was reported for short contact distances. Furthermore, transistors have been successfully fabricated on this heterostructure, with low leakage current and low on-resistance. A remarkable three-terminal breakdown voltage above 4 kV with an off-state leakage current below 1 μA/mm was achieved. A regrown ohmic contact was used to reduce the source/drain ohmic contact resistance, yielding a drain current density of about 0.1 A/mm.

In this work, two series of AlGaN/GaN/AlN high electron mobility transistor (HEMT) heterostructures have been grown by molecular beam epitaxy on AlN bulk substrates. The effects of reduction in the GaN channel thickness as well as the AlGaN barrier thickness and composition on structural and electrical properties of the heterostructures have been studied. The material analysis involved high-resolution x-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy. In a first series of HEMT structures grown with an aluminum content of 30% in the AlGaN barrier, the channel downscaling results in a reduction in the GaN strain relaxation rate but at the expense of degradation in the mean crystal quality and in the electron mobility with a noticeable increase in the sheet resistance. An opposite trend is noticed for the three-terminal breakdown voltage of transistors, so that a trade-off is obtained for a 50 nm width GaN channel HEMT, which exhibits a sheet resistance of 1700 Ω/sq. with transistors demonstrating three-terminal breakdown voltage up to 1400 V for 40 μm gate to drain spacing with static on resistance R_on= 32 mΩ cm². On the other hand, a second series of HEMT structures with high aluminum content AlGaN barriers and sub-10 nm GaN channels have been grown perfectly strained with high sheet carrier densities allowing to preserve sheet resistances in the range of 880–1050 Ω/sq.

We report on the fabrication and electrical characterization of AlGaN/GaN normally off transistors on silicon designed for high-voltage operation. The normally off configuration was achieved with a p-gallium nitride (p-GaN) cap layer below the gate, enabling a positive threshold voltage higher than +1 V. The buffer structure was based on AlN/GaN superlattices (SLs), delivering a vertical breakdown voltage close to 1.5 kV with a low leakage current all the way to 1200 V. With the grounded substrate, the hard breakdown voltage transistors at V_GS = 0 V is 1.45 kV, corresponding to an outstanding average vertical breakdown field higher than 2.4 MV/cm. High-voltage characterizations revealed a state-of-the-art combination of breakdown voltage at V_GS = 0 V together with low buffer electron trapping effects up to 1.4 kV, as assessed by means of substrate ramp measurements.

Nitrogen-polar (N-polar) AlGaN-channel high electron mobility transistors (HEMTs) offer a promising pathway to overcome the limitations of equivalent metal-polar (M-polar) devices, particularly the high contact resistance associated with increased aluminum content. In this work, we report the growth and fabrication of N-polar AlGaN-channel HEMTs on a silicon substrate by ammonia molecular beam epitaxy (NH3-MBE). N-polarity is achieved using an epitaxial NbN polarity-inversion layer, enabling the growth of a buffer stack with adequate structural and electrical quality. Two heterostructure designs are investigated. Capacitance–voltage and Hall measurements confirm the formation of a high-density two-dimensional electron gas with sheet carrier densities up to 2.4×1013 cm−2. Owing to the elimination of the Al-rich barrier between the surface and the channel (present in M-polar configuration) and improving surface morphology, encouraging low-resistance Ohmic contacts are achieved, with contact resistance reduced below 1 Ω mm on the Al0.2Ga0.8N channel. These results demonstrate the viability of N-polar AlGaN-channel HEMTs on silicon and highlight their potential for next-generation ultra-wide-bandgap power devices on Si.