Application and integration of GaN power transistors for high-frequency and high-efficiency power conversion

The demand for power electronics technology for higher efficiency and increased power density is driven by increasing energy consumption and environmental issues. GaN transistors are promising candidates for next-generation power conversion systems due to their superior material properties, such as a high critical electric field and high electron mobility, which enable low specific on-resistance, low parasitic capacitance, and high breakdown voltage, thereby reducing system losses, size, and cost. This thesis presents a study on GaN HEMTs with the aim of enabling high frequency application and monolithic integration by investigating three key areas: simulation methodology, gate loop optimization, and the evaluation of a novel GaN-on-AlN/SiC transistor platform. Due to the fast switching characteristics of GaN HEMTs, the impact of parasitic components becomes significantly more pronounced due to high dv/dt and di/dt. These parasitics can cause voltage and current overshoots, spurious turn-on, electromagnetic interference, and reliability issues. This brings challenges to simulation accuracy and layout design. To address these challenges, simulation methodology using 3D finite element method tools to extract key parasitic components is developed. Circuit-level simulations combining SPICE-based transistor models with these extracted parasitics are also evaluated. Furthermore, this work investigates gate loop layout optimization with a focus on minimizing gate loop inductance and common source inductance. Design guidelines for optimized gate routing are presented. A improved chip pad layout that effectively reduces the common source inductance and improves switching performance is proposed. Depending on the adopted gate technology, some GaN transistors feature a non-insulating gate, such as gate injection transistors and Schottky-gate GaN transistors. Driving GaN transistors with a non-insulating gate is challenging, as a high gate current is required for rapid switching transitions, and a low continuous gate current is needed to maintain the device in on-state and minimize losses. The conventional RC-type drive circuit for non-insulating gate GaN transistors suffers from unstable turn-off voltage and inconsistent operating performance depending on the switching frequency and duty cycle. To address these issues, this work proposes an improved drive circuit designed with an active discharge path. It can drive non-insulating gate GaN transistors in the MHz range, allowing fast switching in a wide range of duty cycles without performance degradation. The configuration and operating principle of the drive circuit are introduced, and their effectiveness is experimentally verified for two types of non-insulating-gate transistors at multi-megahertz switching frequencies, demonstrating the suitability of the proposed circuit for fast switching and high-frequency applications. A novel GaN-on-AlN/SiC transistor platform is further investigated to achieve higher breakdown voltage and enhanced thermal performance, while mitigating substrate biasing effects that are inherent in the monolithic integration of conventional lateral GaN-on-Si transistors due to the use of a common substrate. Discrete GaN power HEMTs are systematically characterized in terms of both static and switching behavior, with a particular focus on the influence of substrate potential. Device modeling of this novel transistor structure is also presented. Furthermore, monolithic GaN-on-AlN/SiC half-bridge and bidirectional switches are systematically evaluated, with a focus on the impact of substrate termination and the crosstalk between integrated devices—specifically, between high-side and low-side transistors in the monolithic integrated half-bridge, and between the two gate terminals independently responsible for bidirectional blocking and conduction in the monolithic bidirectional switch. In addition, the performance of monolithically integrated devices is compared with discrete transistors to highlight the advantages and challenges associated with monolithic integration. Both discrete and integrated devices demonstrate stable, fast switching and exhibit satisfactory immunity to substrate-related effects. Prototype converters operating in the MHz range further validate the proposed technology, demonstrating its potential for compact, high-frequency, and high-efficiency power conversion systems.