*3.5. Gallium Oxide Power Transistors*

A power MOSFET fabrication process generally includes a number of technological steps including either gate dielectrics, surface passivation, drain/source ohmic contacts, implant doping, isolation, mesa etch, or in combination. Due to the large bandgap of Ga2O3, the most suitable gate insulators are those with enough (conduction and valence) band-offsets to avoid current injection through the gate (e.g., SiO<sup>2</sup> and Al2O<sup>3</sup> and perhaps other oxides such as Y2O3, MgO, and Mg2AlO4). While balancing the dielectric constant to achieve more gate capacitance and more carriers in the conductive channel [166]. Defining a contact region by implantation, such as in Si, SiC, and GaN power MOSFET technologies, is a usual choice [167], in Ga2O<sup>3</sup> this is typically n<sup>+</sup> Si-ion implantation. While other techniques have been suggested to further decrease the contact resistivity, such as formation of surface states [168] or the adoption of a TCO as a metallic interface [169].

As in, the more mature, AlGaN/GaN HEMT technology, Ohmic contacts are typically made with a multilayer metal stack consisting of an adhesion layer (e.g., Ti, Ta), an overlayer (Al), a barrier layer (e.g., Ni, Ti, Mo), and a capping of Au [170,171]. Nevertheless, it has been argued that simpler metal structures, such as Ti/Ga2O3, are also efficient if there is an oxygen deficient Ga2O<sup>3</sup> surface [172] (a double charged oxygen vacancy is a well-known intrinsic donor in oxides [107]). Indeed, Yao et al. [132] suggested that the surface states appear to have a more dominant role in the transformation from a Schottky to an Ohmic interface than the choice of metal.

As with power SBDs, power MOSFETs can be defined in a vertical Ga2O<sup>3</sup> homoepitaxial structure (typical of SiC power MOSFETs) and lateral structure (typical of AlGaN/GaN power HEMTs) which can be either homoepitaxial or heteroepitaxial (Figure 6). Ga2O<sup>3</sup> power MOSFETs are mostly unipolar *n*-type and operate in depletion mode (D-mode or normally-on) but a number of techniques have been reported to make enhancement mode (E-mode or normally-off) Ga2O<sup>3</sup> devices. For example, Chabak et al. [173] reported an enhancement-mode *β*-Ga2O<sup>3</sup> MOSFETs on a Si-doped homoepitaxial channel grown by molecular beam epitaxy and, using a gate recess process to partially remove the epitaxial channel under the 1-µm gated region to fully deplete at zero gate bias. With a breakdown voltage of 505 V (8 mm source-drain spacing), a maximum current density of 40 mA mm−<sup>1</sup> , and an on/off ratio of 10<sup>9</sup> . Hu et al. [174] achieved (in 2018) a larger blocking voltage (1.075 kV), a larger threshold voltage (1.2–2.2 V), and a larger output current (~500 A cm−<sup>2</sup> ) in a first demonstration of vertical E-mode MOSFET with significatively larger FOM (~80 MW cm−<sup>2</sup> ).

**Figure 6.** Schematics of (**a**) a vertical Ga2O<sup>3</sup> power transistor (VFET) and (**b**) a lateral transistor (LFET). (**c**) Baliga's FOM for selected LFETs and VFETs from the literature. (**d**) Prospects of Ga2O<sup>3</sup> devices as UV PDs, D\* refers to specific detectivity; dots symbols referrer to diodes (either SBD or MSM), while square symbols denote transistors (data adapted from Wu et al. [131]). Adapted with permission from [12] © 2021 copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

The E-mode was accomplished by doping profiling in a FinFET design (a type of 3D, non-planar transistor which has become the usual layout for the smallest CMOS 14 nm, 10 nm, and 7 nm nodes). This kind of E-mode vertical power device was later optimized to sustain up to a blocking voltage of 1.6kV [175], a threshold voltage of 2.66 kV, a maximum current density of 25.2 mWcm<sup>2</sup> , and a record FOM of 280 MW cm−<sup>2</sup> [176]. Among Dmode devices, the ones reported by Lv et al. [177] stand out for exhibiting a particularly large FOM. They reported (in 2019) [177] source-FP *β*-Ga2O<sup>3</sup> MOSFETs on a Si-doped/Fedoped semi-insulating *β*-Ga2O<sup>3</sup> substrate exhibiting 222 mA mm−<sup>1</sup> (18 mm source-drain spacing) with on-resistance of 11.7 mΩcm<sup>2</sup> , a Vbr of 680 V and an FOM of 50.4 MWcm−<sup>2</sup> . Later (in 2020) [178], they adopted a T-shaped gate and source connected FP structure to increase the Vbr up to 1.4 kV/2.9 kV (for 4.8 µm/17.8 µm source-drain spacing), with a specific on-resistances of 7.08 mΩcm2/46.2 mΩcm<sup>2</sup> . These yielded a record high FOM of

277 MW cm−<sup>2</sup> , together with negligible gate or drain pulsed current collapse and a drain current on/off ratio of 10<sup>9</sup> .

Other lateral D-mode devices with high FOM were reported by Tetzner et al. [179]. By using sub-µm gate lengths (combined with gate recess) and optimization of compensationdoped high-quality crystals, implantation based inter-device isolation, and SiNx-passivation, breakdown voltages of 1.8 kV and an FOM of 155 MW cm−<sup>2</sup> were achieved. In 2020, Sharma et al. [180] reported Ga2O<sup>3</sup> lateral D-mode field-plated MOSFETs exhibiting an ultra-high Vbr of 8.03 kV (70 mm) by using polymer SU8 passivation. The current was rather low, however, due to plasma-induced damage of channel and access regions resulting in an impractical FOM of 7.73 kW cm−<sup>2</sup> (i.e., not above the silicon limit). As reported by Kalarickal et al. [164], ultra-high-*k* ferroelectric dielectrics, such as BaTiO3, can, in principle, provide an efficient field management strategy by improving the uniformity of electric field profile in the gate-drain region of lateral FETs. High average breakdown fields of 1.5 MV/cm (918 V) and 4 MVcm−<sup>1</sup> (201 V) were demonstrated for gate-drain spacings of 6µm and 0.6 <sup>µ</sup>m, respectively, in *<sup>β</sup>*-Ga2O3, at a high channel sheet charge density of 1.8×10<sup>13</sup> cm−<sup>2</sup> . An elevated sheet charge density together with a high breakdown field enabled a record power FOM of 376 MWcm−<sup>2</sup> at a gate-drain spacing of 3 µm (Figure 6c). As in the case of SBDs, these performances for the Ga2O<sup>3</sup> devices are already impressive and well beyond the silicon limit but still lag behind the best (much more mature) GaN devices in their respective power ratings [181,182].

All the above power MOSFET devices are unipolar *n*-type. These devices are sometimes referred as MISFETs so as to distinguish them from the conventional p-n junction based MOSFETs, since there are no *p*-regions in these MISFETs [175]. As mentioned in the previous sections, there are, however, several reports of *p*-type Ga2O3in nominally undoped, H-doped and N-doped *β*-Ga2O3. In particular, Wuetal. [131] proposed a growth mechanism of multistep structural phase transitions from hexagonal P63mc GaN to rhombohedral R3c *<sup>α</sup>*-GaNxO3(1−x)/2,and finally to monolithic C2/m N-doped *<sup>β</sup>*-Ga2O3. This improves the crystalline quality, facilitates acceptor doping, increases the acceptor activation efficiency, and thus enhances the *p*-type conductivity (acceptor ionization energy of 0.165 eV, Hall resistivity of 17.0 Ωcm, Hall hole mobility of 23.6 cm2V −1 s −1 , hole concentration of 1.56×10<sup>16</sup> cm−<sup>3</sup> ). P-type *β*-Ga2O<sup>3</sup> films-based lateral MOSFET deep-ultraviolet (DUV) PDs were fabricated with extremely high responsivity (5.1×10<sup>3</sup> A/W) and detectivity (1.0×1016Jones) under 250 nm light illumination (40 <sup>µ</sup>W/cm<sup>2</sup> ) conditions. Figure 6d shows the responsivity and detectivity (D\*) for state-of-the-art DUV PDs based on various WBG materials (adapted from [131]), in which it can be seen how *β*-Ga2O<sup>3</sup> surpasses conventional Si-, SiC-, and AlGaN-based devices in terms of responsivity and detectivity.

#### **4. Other Emerging Oxide Semiconductors for Power Electronics**

Ga2O3phase engineering: Owing to the nonpolar nature of β-Ga2O<sup>3</sup> crystals, modulationdoped heterostructure is one of the possible approaches to realize Ga2O3-based FETs [183]. Analogously, *p*-type semiconductors (e.g., *p*-type nitrides such as GaN) may be introduced to yield normally-off β-Ga2O<sup>3</sup> field-effect transistors with tunable positive threshold voltages [184]. Other phases of Ga2O<sup>3</sup> have also received attention due to potentially favorable growth characteristics, and to the possibility of polarization engineering made possible by the polar nature of their crystal structures. In principle, this polarization could be utilized to produce Ga2O<sup>3</sup> two-dimensional electron gases (2DEGs) in analogy with GaN/AlN-based transistors [185].

Ga2O3alloy engineering: The aluminum gallium oxide, AlxGa1-xO3, is a ternary alloy of Al2O<sup>3</sup> and Ga2O3. It was already noted by Roy [186] in 1952 that the gallium ion closely resembles the aluminum ion and substitutes for it in several structures. Because *β*-(AlGa)2O<sup>3</sup> is not the energetically favored crystalline phase for large Al compositions, the crystal converts to competing structural phases when grown on β-Ga2O<sup>3</sup> substrates [187]. Thus, it has been difficult to obtain gallium oxide UWBG materials exceeding the bandgap of ~6 eV which is available to the materials in the nitride family in AlN. Very recently however, it was found that single-crystalline layers of α-(AlGa)2O<sup>3</sup> alloys spanning bandgaps of 5.4–8.6 eV can be grown by molecular beam epitaxy [188]. By varying the alloy composition, bandgap energies from ~5.4 up to 8.6 eV with a bowing parameter of 1.1 eV are achieved, making α-(AlxGa1−x)2O<sup>3</sup> the largest bandgap epitaxial material family to date. If these layers can be controllably doped, it would pave the way for α-(AlxGa1−x)2O3–based high-power heterostructure electronic and photonic devices at bandgaps far beyond all materials available today [189].

Spinel electronics: The spinel zinc gallate, ZnGa2O4, is a nearly stoichiometric mixed oxide made of Ga2O<sup>3</sup> and ZnO.A potential advantage of spinel ZnGa2O<sup>4</sup> is its great dopability prospects owing to the spinel's inherent diversity in cation coordination possibilities [106]. Normal spinels have all A cations in the tetrahedral site and all B cations in the octahedral site, e.g., Zn-tetrahedral site Zn2+(Td) and Ga-octahedral site Ga3+(Oh), so that normal ZnGa2O<sup>4</sup> is Zn(2+[Td])Ga2( 3+[Oh])O4( <sup>2</sup>−). The spinel's off-stoichiometry, from the ideal 1:2:4 proportions, or the creation of cation antisite defects are known routes for doping these compounds. Dominant defects in spinels are antisite donors (e.g., ZnGa) or donor-like Ga3+(Oh)-on-T<sup>d</sup> and antisite acceptors (e.g., GaZn) with acceptor-like Zn2+(Td)-on-O<sup>h</sup> antisite defects resulting in an intrinsic bipolar power semiconductor [190]. ZnGa2O<sup>4</sup> is therefore a potential outstanding UWBG (~5 eV) oxide semiconductor but is only one among the many possible spinel oxides. There are over 1000 compounds that are known to crystalize in the spinel structure. The sub-family of spinel oxides is a large and important class of multi-functional oxide semiconductors with many optoelectronics applications in areas such as batteries, fuel cells, catalysis, photonics (phosphors, bio-imaging, photodetectors), spintronics (magnets, bio-magnets), or thermoelectricity [191]. Other magnesium-based Ga-spinels, such as MgGa2O4and Zn1-xMgxGa2O4, are related oxides that are currently being investigated [192,193].

#### **5. Conclusions**

The rational use of electrical energy and information are central themes in the greatest climatic challenge of the 21st century. UWBG oxides, such as Ga2O<sup>3</sup> and related materials, are promising power electronic candidates since their critical electric field is large compared to beyond silicon WBG (i.e., SiC and GaN), while still yielding a moderate mobility, high quality epi-layers, and large bulk single crystals (more than 6-inch) using low cost and scalable fabrication approaches. During the last decade, the Ga2O<sup>3</sup> power diode and transistor progress has been impressive, with devices now approaching the frontier of the field. The material system also opens new optoelectronics avenues (owing its UVC spanning bandgap), and new electronics perspectives based on stabile interfaces and a natural integration with extremely high-*k* functional oxides. The advances offered by Ga2O<sup>3</sup> are also opening the door to many more UWBG oxides (the largest family of wide bandgap semiconductors), such as the spinel, ZnGa2O4, along with many more that are anticipated. Therefore, the ever-increasing family of UWBG oxides is at the very frontier of a more efficient energy electronics which is adapted to tackle the 21st century climatic targets, although there still is a lot of room for performance improvements, technical innovation, and new discoveries.

**Author Contributions:** Conceptualization A.P.-T., E.C., M.R.J.; writing—original draft preparation, A.P.-T., Z.C., J.J.A.; writing—review and editing, A.P.-T., Z.C., J.J.A.; visualization, A.P.-T.; supervision, A.P.-T.; project administration, A.P.-T.; funding acquisition, E.C., A.P.-T., M.R.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the French National Research Agency ANR, "Accelerating the demonstration of Gallium Oxide's outstanding potential for Energy application" (GOPOWER), grant number Project-ANR-21-CE50-0015.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Authors acknowledge IRP -"GALLIA", CNRS, France. The ICN2 is funded by the CERCA programme/Generalitat de Catalunya. The ICN2 is supported by the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (AEI, grant no. SEV-2017-0706).

**Conflicts of Interest:** The authors declare no conflict of interest.
