Guo, Yijin2025-01-172025-01-172025-01-16vt_gsexam:42282https://hdl.handle.net/10919/124236Power semiconductor device, as a significant contributor to power electronics industry, plays an indispensable role in energy conversion applications including electric vehicles, data centers, consumer electronics, power grids, etc. The evolution of power semiconductor materials has progressed from traditional silicon (Si) to wide-bandgap materials including silicon carbide (SiC) and gallium nitride (GaN). Benefitting from the wide bandgap, high electron mobility and good thermal conductivity of GaN, GaN-based power devices can achieve fast switching speed, high breakdown voltage, and small on-resistance. They have been deployed in numerous power electronics applications, outperforming the Si and SiC counterparts. Nevertheless, despite their inherent advantages, the commercialization of GaN devices, particularly high-electron-mobility transistors (HEMTs), has predominantly been confined to the low-voltage domain of typically below 650 volts. This limitation blocks GaN HEMTs for medium- and high-voltage applications such as electric vehicles, renewable energy processing, and power grids, which have a total market size over USD$15 billion. The challenge for GaN HEMTs to reach high-voltage applications arises primarily from the highly non-uniform electric field (E-field) distribution within the device structure, predisposing the device to premature breakdown and limiting its operational voltage range. Consequently, the quest for higher voltage capabilities in GaN HEMTs requires the fundamental understanding and effective mitigation of this non-uniform E-field distribution. In this work, the p-type GaN-based Reduced Surface Field (RESURF) structure is proposed to balance the net charge in the two-dimensional-electron gas (2DEG) channel in GaN HEMT. This design enables a uniform distribution of E-field, enabling the voltage upscaling in GaN HEMT up to 10,000 V (i.e., 10 kV), which is the milestone voltage class in unipolar power devices for high-power applications. The first part of this thesis introduces the history, background and mechanism of power semiconductor devices and provides solid reasons for GaN as a competitive participant in power electronics industry. It covers a basic introduction about GaN HEMT devices and their commercialization status and states the challenges GaN HEMTs are facing when dealing with mass production. An innovative RESURF structure is introduced to overcome the existing trade-off between on-resistance and breakdown voltage, and to achieve superior overall performance that would be beneficial for GaN HEMT to upscale the voltage classes. Secondly, the development of a 10 kV unidirectional GaN HEMTs is discussed in detail. An optimized fabrication process flow, including etching, metal deposition, contact formation and dielectric passivation, is established. The RESURF structure is formed through a two-step chlorine-based etching process, with an innovative introduction of sulfur hexafluoride (SF6) that enables a self-termination etch stop onto the AlGaN surface without damage to the 2DEG channel beneath. A controlled slow etch recipe has been developed as well, aiming for large-scale manufacturing with improved yields. A detailed analysis of the on-state and off-state I-V characterization of devices with various RESURF thickness and length provides an insight into the device breakdown mechanism, which has been verified with physics-based technology computer-aided design (TCAD) simulation. The third part of this work demonstrates a 3.3-kV monolithic bidirectional switch (MBDS), which a novel device concept that can significantly simplify the circuit design in alternative current (AC) power conversion. A symmetrical p-GaN junction termination extension (JTE) design is proposed for electric field management, and the lateral conduction of this GaN-based MBDS enables a state-of-the-art high-voltage bidirectional switch with low on-resistance, achieving considerable performance advantage compared to the conventional bidirectional switch implemented by discrete devices. In summary, this research work covers the design, fabrication, characterization, simulation, and breakdown mechanism analysis of GaN-based unidirectional and bidirectional transistors for multi-kilovolt power conversion applications. The extended p-GaN configuration (RESURF for unidirectional devices and JTE for bidirectional devices) offers a spatially-distributed E-field management, enhancing the breakdown voltage scaling capability of GaN HEMTs to exploit the full material advantages of GaN.ETDenIn CopyrightPower electronicssemiconductor devicewide-bandgapGallium Nitridetransistorhigh electron mobility transistorThesis