GaN heterojunction power transistors have shown great promise to replace silicon power MOSFETs in next-generation power converters, owing to their remarkable performance in terms of ON-resistance (R_ON), breakdown voltage, switching speed, and high-temperature operation, breaking the so-called “Si limit”. Despite the advantages resulting from superior material properties such as high carrier density, high electron mobility and saturation velocity of the 2-D electron gas (2DEG) channel, large critical breakdown electric field, and extremely low intrinsic carrier density, it still remains challenging for GaN devices to reach their performance limit. This is mainly caused by a transient increase in R_ON under switching conditions, which is commonly known as dynamic R_ON or current collapse th...[ Read more ]

GaN heterojunction power transistors have shown great promise to replace silicon power MOSFETs in next-generation power converters, owing to their remarkable performance in terms of ON-resistance (R_ON), breakdown voltage, switching speed, and high-temperature operation, breaking the so-called “Si limit”. Despite the advantages resulting from superior material properties such as high carrier density, high electron mobility and saturation velocity of the 2-D electron gas (2DEG) channel, large critical breakdown electric field, and extremely low intrinsic carrier density, it still remains challenging for GaN devices to reach their performance limit. This is mainly caused by a transient increase in R_ON under switching conditions, which is commonly known as dynamic R_ON or current collapse that would lead to lower conversion efficiency and instability issues in a power conversion system.

This thesis focuses on the development of passivation technologies for suppressing surface-state-induced current collapse in GaN high-voltage power transistors. An effective passivation technique based on compensation of interface traps with polarization charges in AlN thin film has been recently demonstrated for depletion mode (D-mode) AlGaN/GaN high electron mobility transistors (HEMTs). A more practical and cost-effective solution using an AlN/SiN_x stack structure that features higher resistance to moisture and oxygen and compatibility with field plate structures is further designed and developed for 600 V AlGaN/GaN HEMTs that exhibit low dynamic R_ON and low OFF-state leakage simultaneously. Detailed analysis of the AlN/SiN_x-passivated HEMT in support of the passivation mechanism is carried out with temperature-dependent pulsed I–V and high-voltage switching characterization, electroluminescence (EL) microscopy, and TCAD simulations.

Since enhancement mode (E-mode) operation is highly desirable for power switches from fail-safe point of view and a metal-insulator-semiconductor (MIS) gate has advantages of lower gate leakage and larger gate swing over the Schottky gate of HEMTs, AlN passivation is incorporated in the fabrication of 600 V E-mode MIS-HEMTs. Moreover, in order to provide peripheral functional blocks and enable more reliable and compact switched-mode power supplies, a technology platform for monolithic integration of 600 V E/D-mode GaN MIS-HEMTs is also developed. These devices exhibit high drive current, high breakdown voltage, large gate swing, low ON-resistance, low OFF-state leakage, and low current collapse.