(Invited) Beta-Gallium Oxide-Based Devices Towards Grid-Scale Electronics
Authors: Krishnamoorthy Sriram , Roy Saurav , Bhattacharyya Arkka , Peterson Carl , Liu Yizheng 
Published: December 2024

Gallium Oxide is an emerging ultra-wide band gap semiconductor with promising material properties relevant for high performance power electronic devices. The availability of large area bulk substrates using melt growth techniques
and the wide range of tunable n-type conductivity makes it a compelling semiconductor material platform for next-generation power devices.

Lateral Devices: Homoepitaxial Si-doped channel layers are grown using metal-organic vapor phase epitaxy (MOVPE) on NCT and Synoptics (010) Ga2O3 bulk substrates after chemical cleaning to minimize impurities at the substrate-epitaxial layer interface, necessary for pinching off the lateral field effect transistors. Excellent electron mobilities, contact resistance values, breakdown voltages and power figures of merit are demonstrated in β-Ga2O3 epitaxial films and devices. MOVPE-grown β-Ga2O3 thin films with electron mobility values close to the predicted theoretical maximum (~ 200 cm2/Vs) are achieved at low (600°C) and high (800°C) growth temperatures. A low-temperature MOVPE masked ohmic contact regrowth technique is developed with a low contact resistance value. Multi-kilovolt (up to 4.5 kV) class β-Ga2O3 transistors are demonstrated with state-of-the-art power figures of merit exceeding several times the theoretical maximum of Silicon. A channel-buffer stack engineering is demonstrated that enabled record-high electron mobility values in doped β-Ga2O3 films. High-current (> 100 mA/mm) and VBR (up to ~ 3kV) β-Ga2O3 MOSFETs are realized on an engineered β-Ga2O3 /SiC composite substrate – for enhanced bottom-side device cooling. Tri-Gate β-Ga2O3 MESFETs with record high power figure of merit (~1 GW/cm2) are demonstrated. We report on the demonstration of kV-class β-Ga2O3 MOSFETs fabricated on 1-in. bulk substrates using metalorganic vapor phase epitaxy (MOVPE) with disilane (Si2H6) as the silicon precursor. In this talk, we will highlight some of these critical advances in β-Ga2O3 epitaxy/devices, pushing the boundary of β-Ga2O3device performance.

Vertical Devices: Gallium Oxide devices are very attractive for next-generation grid-scale power rectifiers and transistors. We report a vertical β-Ga2O3 Schottky barrier diode (SBD) with BaTiO3 as field plate oxide on a low doped thick epitaxial layer exhibiting 2.1 kV breakdown voltage. A thick drift layer of 11 μm with a low effective doping concentration of 8 × 1015 cm–3 is used to achieve high breakdown voltage. Using the high-k dielectric with a dielectric constant of 248, the breakdown voltage increases from 816 V for the non-field-plated SBD to 2.1 kV (>2× improvement) for the field-plated SBD without compromising the on-state performance.
We introduce vertical Schottky barrier diodes (SBDs) based on β-Ga2O3 with trench architecture, featuring a high-permittivity dielectric RESURF structure. These diodes are designed for application demanding high voltage and current capacities while maintaining ultra-low reverse leakage currents. The trench design plays a pivotal role in reducing the electric field at the metal–semiconductor junction, thereby yielding minimal reverse leakage attributed to field emission under high reverse bias conditions. Additionally, the incorporation of a high-k dielectric helps to suppress leakage through the trench bottom corner dielectric layer. The incorporation of a trench geometry, coupled with the high-permittivity dielectric RESURF, effectively reduces the surface electric field at the metal-semiconductor junction. This reduction facilitates the use of a lower work-function anode contact, further diminishing the turn-on voltage. The use of high-k dielectric also significantly reduces the dielectric leakage contribution to the overall reverse leakage of the diode. Large-area SBDs with areas of 1mm2 and 4mm2 exhibit pulsed forward current of 6A/20A and a catastrophic breakdown voltage of 1.74kV/1.4kV, surpassing other β-Ga2O3-based large-area power diodes, while small-area SBDs (200×200 µm2) do not exhibit catastrophic breakdown until 3kV. The large-area SBDs also demonstrate lower capacitance, stored charge, and stored energy compared to commercial SiC SBDs, indicating potential for superior switching performance. The combination of lower stored charge and a low forward voltage drop results in an excellent trade-off between conduction and switching power loss, yielding a QCVF figure of merit comparable to commercial bare die SiC SBDs.
We report on the fabrication and characterization of a NiOx/β-Ga2O3 heterojunction diode (HJD) that uses a metallic Nickel (Ni) target to deposit NiOx layers via reactive RF magnetron sputtering and lift-off processing with >3 kV breakdown voltage, record-low reverse current leakage under high reverse bias, and high junction electric fields (4 MV/cm). The breakdown voltage was measured to be greater than 3 kV with a noise floor-level reverse leakage current density (~10-6 A/cm2) until 3 kV. Such rapid advances in Gallium Oxide device engineering will unleash the potential for these devices for next-generation grid-scale power electronics. We acknowledge funding from the Coherent/II–VI Foundation and AFOSR.