**3. Gallium Oxide (Ga2O3)**

Ga2O<sup>3</sup> has, at least, six polymorphs of which only one is thermodynamically stable at high temperatures (*β* phase, monoclinic), while the others are metastable and tend to convert to *β* upon high-temperature treatments including the phases α, corundum, δ, cubic, and ε, hexagonal, γ, defective-spinel, and orthorhombic κ polymorph [18]. The basic principles of polymorphism in crystals are clear: the lattices adapt to the minimum energy with respect to the temperature and pressure. Nearly all Ga2O3-containing devices utilize the monoclinic *β* phase, the most stable and best-characterized polymorph. As a well-known representative of a binary metal-oxide, gallium oxide cannot therefore be regarded as a new material, but as a revisited and rejuvenated one. For example, early crystallographic studies for single crystals [19] together with diverse luminescence studies of doped *β*-Ga2O<sup>3</sup> were reported as early as the1960s [20]. Lorenz et al. [21] already published in 1966 that *n*-type Ga2O<sup>3</sup> exhibits mobilities in the range of 100 cm2V −1 s <sup>−</sup><sup>1</sup> and an adequate device doping of 10<sup>18</sup> cm−<sup>3</sup> can be achieved just by controlling the native oxygen vacancies' density. Its deep-ultraviolet intrinsic bandgap of around 4.5–4.9 eV and excellent photoconductivity are also well-known from early contemporary studies [22]. It was not until this decade that the potential of Ga2O<sup>3</sup> for a certain class of extreme or power electronics was realized due to further availability of large-area single crystals with high quality and the control of doping. In the past, Ga2O<sup>3</sup> was somehow ignored as an ultra-wide bandgap material, as it was eclipsed by the potential of diamond which has never been fully realized [23].

Previously, SiC and GaN were the wide bandgap materials of choice [6]. However, from an ultra-high energy electronics perspective, Ga2O<sup>3</sup> transistors and diodes exhibit the potential of delivering outstanding performances in the form of high breakdown voltage, high power and low losses because of superior material properties, thus extending the power handling limits given by the SiC and GaN integration into the mainstream [4]. Indeed, an ultra-large breakdown electric field, (which is usually assumed to be of the order of *Ec*~8 MVcm−<sup>1</sup> ), is a prime material advantage of Ga2O3. However, this value may be well underestimated; it was very recently suggested that the critical electric field of Ga2O<sup>3</sup> could be as large as 13.2 MVcm−<sup>1</sup> , if the residual donors are efficiently removed [24].

A high critical field crucially promotes the suitability of a semiconductor material for power devices that would be able to manage a large amount of electrical energy per unit area. Baliga's figure of merit [25] for power electronics is proportional to *E<sup>c</sup> 3* , whilst only being linearly proportional to the bulk electron mobility (*µ*). Although Ga2O<sup>3</sup> presents a similar conduction band dispersion (i.e., effective mass) than GaN, a relatively small bound limit of *µ* ~300 cm2V −1 s −1 is frequently given [26]. This is due to a massive Fröhlich interaction which is common to many conducting oxides. Balancing critical field and mobility, the on-state losses can be still an order of magnitude lower than those for SiC and GaN for a given breakdown voltage (Figure 3). Comparing these values to other power semiconductors (see Figure 3), *β*-Ga2O<sup>3</sup> appears favorable, surpassing SiC and GaN. A major additional technological advantage of the *β*-Ga2O<sup>3</sup> is that the single crystal structure can be synthesized via several standard melt growth methods including the Czochralski (CZ) technique [27]. This, in practice, would imply SiC performances (or better ones) at a fraction of cost.


**Figure 3.** A summary of the main power device figure of merit (or Baliga's figure of merit. BFOM) parameters of the most popular wide bandgap semiconductors. Gallium oxide has a particularly poor thermal conductivity. However, when integrated into devices, heterojunctions with other better suited heat sinks (such as silicon carbide) area way to circumvent that limitation. As shown in the bottom panels, the simulate lattice temperature is lower on SiC (**b**) when compared with Ga2O<sup>3</sup> substrates (**a**). Furthermore, thinning the Ga2O<sup>3</sup> active film helps thermal performances. Adapted with permission from [11] © 2018 COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE).

There are certain applications, such as maritime and air transport, that are difficult to electrify as the power ratings are generally larger than, say, urban electric cars (Figure 1d,e).

For electric cars, devices delivering at or below the 1.2 kV perform well as rapid chargers or drive converters. These power ratings are well covered with "conventional" WBG, such as SiC and GaN. As the critical electric field of Ga2O<sup>3</sup> has been reported to be at least two times, (or even four times larger), than that of these WBGs, the blocking voltage range of single electronics devices may be significantly extended in the future beyond what is theoretically possible today. These promises will impact directly on the size and weight of planes and ships resulting in less energy and emissions. As energy and transportation represents a major portion of the current CO<sup>2</sup> emissions contributing to global warming, it is expected that UWBG such as Ga2O<sup>3</sup> may open new opportunities in sectors that are now difficult to decarbonize. Other prominent examples where the advantage of ultrawide bandgap semiconductors can be exploited are as more solar-blind (UV transparent) transparent conducting electrodes [11] and electron (or hole) transport layers within solar cells or photodiodes [28].
