Deep acceptor doping of β-Ga₂O₃ with Mg and N was demonstrated by implantation of the impurity ions into n-type bulk substrates. Systematic physical and electrical characterizations were performed to demonstrate recovery of the implantation-damaged crystals and electrical activation of the dopant atoms by thermal annealing at 1000–1200 °C in an N₂ atmosphere. N was found to exhibit much lower thermal diffusivity than Mg, thus enabling the use of higher annealing temperatures to maximize N activation efficiency without significantly altering the impurity profile. Consequently, an n-Ga₂O₃/Ga₂O₃:N/n-Ga₂O₃ structure was capable of sustaining a much larger voltage across its end terminals than its Mg-doped counterpart. The development of an ion implantation technology for acceptor doping of β-Ga₂O₃ creates unique opportunities for designing and engineering a variety of high-voltage β-Ga₂O₃ devices.

A guard ring (GR) was employed to improve the breakdown voltage ( ${V}_{\text {br}}$ ) of vertical Ga ₂ O ₃ Schottky barrier diodes (SBDs) with or without a field-plate (FP) by eliminating electric field concentration at the edges of anode and FP electrodes. The GR structure was formed by nitrogen (N)-ion implantation. Four types of vertical SBD structures with: 1) neither a GR nor a FP; 2) a GR; 3) a FP; and 4) both a GR and a FP were fabricated on the same substrate. The SBDs with a GR [structures 2) and 4)] showed larger ${V}_{\text {br}}$ values than their GR-free counterparts [structures 1) and 3)]. Considering the trade-off relationship between ${V}_{\text {br}}$ and specific on-resistance ( ${R}_{\text {on}}$ ), a ${V}_{\text {br}}/{R}_{\text {on}}$ combination of 1.43 kV/ $4.7~\text {m}\Omega \cdot \text {cm}^{2}$ for the GR/FP-SBD corresponds to one of the best balanced data for Ga ₂ O ₃ SBDs.

Depletion-mode vertical Ga2O3 metal-oxidesemiconductor field-effect transistors featuring a current aperture were developed on a halide vapor phase epitaxial drift layer grown on a bulk β-Ga2O3 (001) substrate. Three ion implantation steps were employed to fabricate the n++ source regions, lateral n channel, and p current blocking layers, where Si and N were selected as the donor and deep acceptor dopant species, respectively. The transistors delivered a drain current density of 0.42 kA/cm2, a specific on-resistance of 31.5 mΩ·cm2, and an output current on/off ratio of over 108. High-voltage performance of the present devices was hampered by a large gate oxide field in the off-state causing high gate leakage, a limitation that can be readily overcome through optimized doping schemes and an improved gate dielectric. The demonstration of a planar-gate vertical Ga2O3 transistor based on a highly manufacturable all-ionimplanted process greatly enhances the prospects for Ga2O3-based power electronics.

With projected performance advantages over silicon and incumbent wide-bandgap compound semiconductors, gallium oxide (Ga ₂ O ₃ ) has garnered worldwide attention as an ultrawide-bandgap semiconductor material suitable for high-voltage, high-temperature, and radiation-hard electronics. Thanks to recent breakthroughs in crystal growth and device processing technologies, the research and development of vertically oriented Ga ₂ O ₃ power transistors has made rapid strides. In this article, we review the important engineering achievements and performance milestones of the two major types of vertical Ga ₂ O ₃ transistors-current aperture vertical metal-oxide-semiconductor field-effect transistors (MOSFETs) and vertical fin-channel MOSFETs. Challenges underlying the unique processing approaches to these devices and their implications on device reliability are also discussed.