*3.4. Gallium Oxide Power Rectifiers*

Once the device-grade epitaxial layers have been grown either homo- (bulk Ga2O3) hetero- (e.g., sapphire, silicon), or both, the simplest electronic devices one can define are rectifiers. In a Schottky rectifier, the counter-electrode (cathode) is processed to allow low resistance Ohmic contact while the anode contact is intended as a Schottky junction over a lightly doped epitaxy; it conducts electrons in the forward mode while sustaining large electric fields (by the creation of a depletion space charge region) in the reverse mode. As mentioned previously, devices using Ga2O<sup>3</sup> are primarily limited to unipolar devices and Schottky diodes are made, in general, on *n*-type semiconductor layers as electrons are lighter than holes. However, it is also important to consider the appropriate metal contacts to Ga2O<sup>3</sup> as they are responsible for connecting the semiconductor to the surrounding electrical circuit/system and parameters such as the Schottky barrier height are crucial. For different contacts to Ga2O3, such as in GaN and AlGaN, which utilize stacks of different metals [132], this decision can make an important difference to the nature of the contact. Regarding Schottky contacts to Ga2O3,Ni/Au is a common choice (see Table 2). Other Schottky contacts investigated include Pt, Ni, Cu, W, Ir, TiN/Au, Pt/Ti/Au, Ni/Au, ndPt/Au [133–136]. Very recently, an ultra-large Schottky barrier of ~1.8 eV was extracted for all-oxide PdCoO2/*β*-Ga2O<sup>3</sup> Schottky diodes [137]. The polar layered structure of PdCoO<sup>2</sup> generates electric dipoles, realizing a large Schottky barrier height of ~1.8 eV (well beyond the 0.7 eV expected from the basal Schottky–Mott relation) along with a large on/off ratio approaching 10<sup>8</sup> , even at a high temperature of 350 ◦C (Figure 5c). As there are a number of polar oxides, this is a promising approach to increase the reverse blocking voltage of Ga2O<sup>3</sup> diodes [138].

**Figure 5.** Schematics of (**a**) vertical Ga2O<sup>3</sup> Schottky diodes and (**b**) *p*-*n* heterojunction diodes. (**c**) A PdCoC2/Ga2O<sup>3</sup> exhibiting the ultra-large Schottky barrier of 1.8 eV. (**d**) Baliga's FOM for selected Schottky and p-n HJ diodes from the literature. Panel (**c**) adapted with permission from Harada et al. [137] © 2022 AAAS 4.0 (CC BY-NC). Adapted with permission from [12] © 2021 copyright Society of Photo-Optical Instrumentation Engineers (SPIE).



In the counter-electrode, highly doped regions beneath the metallization are deployed to assist ohmicity of the contacts [139]. The dopants for this have previously been discussed. Another approach to this is using thin films of highly-conducting oxides [140].

Ohmic contacts to *β*-Ga2O<sup>3</sup> are commonly based on Ti/Au, however other metal contacts have been utilized, such as In, Ti, Ti/Al/Au, In/Au, and Ti/Al/Ni/Au. Besides, there are other metals which have exhibited pseudo Ohmic behavior including Zr, Ag, and Sn [132]. This pseudo nature meant that, initially, ohmicity was observed but, after annealing, rectifying behavior became dominant. Therefore, the Schottky/Ohmic nature is also dependent upon the Ga2O<sup>3</sup> ′ s surface/interface states together with the exact choice of metal stack, explaining, in turn, the varying contact resistivity of certain metals. While delivering low contact resistance, it is worth mentioning that Au is not considered a CMOS-compatible metal. This is an issue shared with GaN-based technology [148].

For the continued development of high voltage *β*-Ga2O3devices, edge termination is an important aspect as it is with its Si, GaN, and 4H-SiCcounterparts. Edge termination in *β*-Ga2O3is being explored and focused specifically on field plates (FP), imparted edge termination (ET), guard ring field plates, thermally oxidized termination, beveled mesas, and trench. These techniques are all deployed to further manage the electrical field to reduce the electric field crowding at the diode edges to increase its blocking capabilities. SBD devices can be made with either a vertical architecture, using homoepitaxial Ga2O<sup>3</sup> or with a lateral architecture using either homo- or heteroepitaxial (e.g., on sapphire) Ga2O3. In general, the vertical structure is preferred as the device pitch is reduced and the encapsulation is simpler. Hu et al. [141] demonstrated a field-plated lateral β-Ga2O<sup>3</sup> SBD on a sapphire substrate with a reverse blocking voltage of more than 3 kV, an *Ron*

of 24.3 mΩcm<sup>2</sup> (anode–cathode spacing 24 µm), and an FOM >0.37 GWcm−<sup>2</sup> (while an FOM of ~500 GWcm−<sup>2</sup> was achieved as the anode-cathode spacing (and *Vbr*) was reduced). Zhou et al. [149] implemented a Mg implanted ET device on a vertical β-Ga2O<sup>3</sup> SBD with a reverse blocking voltage of 1.55 kV and a low specific on-resistance of 5.1 mΩcm<sup>2</sup> (epi thickness 10 µm) and an FOM of 0.47 GWcm−<sup>2</sup> . Analogously, Lin et al. [150] implemented a guard ring with or without an FP on vertical SBDs. The terminated devices exhibited a specific on-resistance of 4.7 mΩcm<sup>2</sup> and a Vbr of 1.43 kV. Wang et al. [151] implemented a thermally oxidized termination on a vertical SBD with a Vbr of 940 V, a specific on-resistance of 3.0 mΩcm<sup>2</sup> , and an FOM of 0.295 GWcm−<sup>2</sup> . Allen et al. [152] implemented a small-angle beveled field plate (SABFP), on thinned Ga2O<sup>3</sup> substrates and a non-punch-through vertical SBD design rendering a Vbr of 1100 V, a peak electric field of 3.5 MVcm−<sup>1</sup> , and an FOM of 0.6 GWcm−<sup>2</sup> .

Somehow the state of the art is given by Li et al. [153]. They demonstrated an FP vertical Ga2O<sup>3</sup> trench SBDs with a Vbr of 2.89 kV (which is ~500 V higher than those without FPs). The trench SBDs exhibited a differential specific on-resistance of 10.5 (8.8) mΩcm<sup>2</sup> from DC (pulsed) measurements leading to an FOM of 0.80 (0.95) GWcm−<sup>2</sup> . This Baliga's power FOM is approaching that for the best vertical SBD GaN devices (e.g., 1.7 GWcm−<sup>2</sup> [154]) but is still several times smaller than lateral AlGaN/GaN SBD (e.g., 3.6 GWcm−<sup>2</sup> [155]) and bipolar *p*-*n* vertical GaN diodes (e.g., ~4.6 GWcm−<sup>2</sup> [156]). Both, the 2D gas formed at the AlGaN/GaN interface and the bipolar injection are effective ways of further reducing the on-resistance in these devices while keeping the breakdown voltage high. The lack of low resistivity *p*-type layer for the anode has to date, prevented a competitive homojunction p-n Ga2O<sup>3</sup> diode, but *p*-*n* heterojunction diodes have been realized by integrating *n*-type Ga2O<sup>3</sup> with *p*-type semiconductors, such as CuO (1.49 kV) [157] and NiO (1.06 kV/1.86kV) [158,159]. Nickel oxide as the *p*-type blocking layer in heterojunction power diodes resulted in a particularly promising approach with this NiO/Ga2O3device [160] yielding a Baliga's FOM of 0.33 GWcm−<sup>2</sup> (Figure 5c,d).

Recently, extremely high-*k* dielectrics have been explored for electric field management in WBG semiconductor-based lateral and vertical device structures [160–164]. According to the TCAD simulations of Roy et al. [165], a super-dielectric Ga2O<sup>3</sup> SBD with practically achievable device dimensions with extremely high FOM should be possible; e.g., 20kVcanbeachievedforan *Ron* of 10 mΩ-cm<sup>2</sup> with a dielectric constant of 300, a Ga2O<sup>3</sup> width/dielectric width ratio of 0.2, and an aspect ratio (drift layer length (anode to cathode spacing)/drift layer width ratio) of 10 resulting in a PFOM of 40 GWcm−<sup>2</sup> (surpassing the theoretical unipolar FOM of *β*-Ga2O3SBD by four times).
