Gallium Oxide (Ga2O3) is an ultra-wide bandgap semiconductor with a bandgap of 4.5–4.9 eV, which is higher than the bandgap of Silicon (Si), Silicon Carbide (SiC), and Gallium Nitride (GaN). A benefit of this wide-bandgap is the high critical electric field of Ga2O3, which is estimated to be from 5 MV/cm to 9 MV/cm. This allows a higher Baliga’s figure of merit (BFOM), i.e., unipolar Ga2O3 devices potentially possess a smaller specific on-resistance (Ron,sp) as compared to the Si, SiC, and GaN devices with the same breakdown voltage (BV). This prospect makes Ga2O3 devices promising candidates for next-generation power electronics.
This thesis explores the design, fabrication, and packaging of vertical Ga2O3 Schottky barrier diodes (SBDs). The power SBD allows for a small forward voltage and a fast switching speed; thus, it is ubiquitously utilized in power electronics systems. It is also a building block for many advanced power transistors. Hence, the study of Ga2O3 SBDs is expected to pave the way for developing a series of Ga2O3 power devices.
In this work, a vertical β-Ga2O3 SBD with a novel edge termination, which is the small-angle beveled field plate (SABFP), is fabricated on thinned Ga2O3 substrates. This SABFP structure decreases the peak electric field (Epeak) at the triple point when the Ga2O3 SBD is reverse biased, resulting in a BV of 1.1 kV and an Epeak of 3.5 MV/cm. This device demonstrates a BFOM of 0.6 GW/cm2, which is among the highest in β-Ga2O3 power devices and is comparable to the state-of-the-art vertical GaN SBDs.
The high-temperature characteristics of Ga2O3 SBDs with a 45o beveled angle sidewall edge termination are studied at temperatures up to 600 K. As compared to the state-of-the-art SiC and GaN SBDs with a similar blocking voltage, the vertical Ga2O3 SBDs are capable of operating at higher temperatures and show a smaller leakage current increase with temperature. The leakage current mechanisms were also revealed at various temperatures and reverse biases.
A new fabrication method of a dielectric field plate and Ga2O3 mesa of a medium angle (10o~30o) is achieved by controlling the adhesion between the photoresist (PR) and the dielectric surface. As compared to the small-angle termination, this medium-angle edge termination can allow a superior yield and uniformity in device fabrication, at the same time maintaining the major functionalities of beveled edge termination. Good surface morphology of the field plates and Ga2O3 mesa of the medium angle 10o~30o sidewall angle is verified by atomic force microscopy.
Finally, large-area Ga2O3 SBDs are fabricated and packaged using silver sintering as the die attach. The sintered silver joint has higher thermal conductivity and better reliability as compared to the solder joint. The metal finish on the anode and cathode has been optimized for silver sintering. Large-area, packaged Ga2O3 SBDs with an anode size of 3×3 mm2 are prototyped. They show a forward current of over 5 A, a current on/off ratio of ~109, and a BV of 190 V. To the best of the author’s knowledge, this is the first experimental demonstration of a large-area, packaged Ga2O3 power device. / M.S. / Power electronics is the processing of electric energy using solid-state electronics. It is ubiquitously used in consumer electronics, data centers, electric vehicles, electricity grids, and renewable energy systems. Advanced power device technologies are paramount to improving the performance of power electronic systems. Power device design centers on the concurrent realization of low on-resistance (RON), high breakdown voltage (BV), and small turn-on/turn-off power losses. The performance of power devices hinges on semiconductor material properties. Over the last several years, power devices based on wide-bandgap semiconductors like Silicon Carbide (SiC) and Gallium Nitride (GaN) have enabled tremendous performance advancements in power electronic systems.
Gallium Oxide (Ga2O3) is an ultra-wide bandgap semiconductor with a bandgap of 4.5–4.9 eV, which is higher than the bandgap of Silicon (Si), SiC, and GaN. As a benefit of this wide bandgap, the theoretical performance of Ga2O3 devices is superior to the Si, SiC, and GaN counterparts. Hence, Ga2O3 devices are regarded as promising candidates for next-generation power electronics.
This thesis explores the design, fabrication, and packaging of vertical Ga2O3 Schottky barrier diodes (SBDs). The power SBD allows a small forward voltage and a fast switching speed; thus, it is extensively utilized in power electronics systems. It is also a building block for many advanced power transistors.
First, a vertical β-Ga2O3 SBD with a novel edge termination is fabricated. This edge termination structure reduces the peak electric field (Epeak) in the device and enhances the BV. The fabricated device shows one of the highest figure of merits in β-Ga2O3 power devices. Next, the high-temperature characteristics of the fabricated Ga2O3 SBDs are studied at temperatures up to 600 K. The leakage current mechanisms were also revealed at various temperatures and reverse biases. Finally, large-area Ga2O3 SBDs are fabricated and packaged using silver sintering as the die attach. The sintered silver joint has higher thermal conductivity and better reliability as compared to the conventional solder joint. The packaged Ga2O3 SBDs show a forward current of over 5 A and a BV of 190 V. To the best of the author’s knowledge, this is the first experimental demonstration of a large-area, packaged Ga2O3 power device.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110757 |
| Date | 17 December 2021 |
| Creators | Wang, Boyan |
| Contributors | Electrical and Computer Engineering, Zhang, Yuhao, Lu, Guo-Quan, DiMarino, Christina |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf, application/pdf |
| Rights | Creative Commons Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
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