Abstract
Gallium nitride or GaN material owns exceptional properties, like excellent electron mobility, saturation velocity, electric field, and thermal durability. This enables GaN-based high-electron-mobility transistors (HEMTs) to outperform silicon transistors, allowing them to function at higher voltages, currents, frequencies, and temperatures. Therefore, GaN HEMTs are ideal for efficient, new-generation power electronic applications, including chargers for electric vehicles, consumer electronics, and IoT systems. Normally-off transistors are necessary for switching applications to ensure safety. p-GaN gate HEMT technology, with its positive threshold voltage, stands out as the most promising candidate among various normally-off HEMT options. However, challenges such as instability of threshold voltage, low gate reliability, poor gate breakdown and dynamic ON-Resistance persist in p-GaN gate technology.
This research investigates and realises high-performance p-GaN (gate)/AlGaN/GaN Lateral E-Mode HEMTs for power electronics applications. By designing novel device structures and optimizing the process flow, we have developed p-GaN gate HEMTs superior to the best-in-class products in Threshold voltage, ON-resistance, and gate breakdown. We have also investigated the failure mechanisms and reliability issues related to p-GaN gate HEMTs.
The background, motivation, objectives, and organization of the thesis are highlighted in Chapter 1. Chapter 2 discusses power device fundamentals, material properties of GaN, GaN high electron mobility transistor, and reliability issues associated with GaN. Chapter 3 details the fabrication of the baseline device and DC and HTGB reliability characterization of 650V p-GaN E-Mode devices. Chapter 4, we proposed and validated a novel successive multi-stage compliance ramp voltage sweep (MSC-RVS) methodology to thoroughly examine the breakdown mechanisms of the p-GaN gate lateral HEMT. Three breakdown patterns, soft (Schottky), progressive (Schottky + Passivation), and hard (Schottky + PiN / Passivation), have been observed and confirmed by Electrical Failure analysis (EFA) and further validated through the TCAD Electric Field simulation. Chapter 5 introduces a novel analysis method for devices under constant voltage stress. The temperature dependences of the breakdown voltages reveal the presence of an additional fourth failure, which was observed in the isolation region and confirmed through EFA.
Chapters 6 and 7 discuss two device designs and fabrication. A novel p-GaN (gate)/AlGaN/GaN lateral E-Mode HEMT structure with additional layers in the gate stack is fabricated to to improve the gate breakdown and reliability by reducing the electric-field through widening the depletion profile. This improved the gate breakdown voltage of the p-GaN gate HEMTs. Secondly, a novel p-GaN(gate)/AlGaN/GaN lateral HEMT with an additional layer in the AlGaN/GaN interface is fabricated. This structure can enhance the mobility, electron density, and drain current. This improves the RON of the baseline device.
In Chapter 8, TCAD modeling was employed to understand the fundamental physics that dictates the operation of reference devices, enabling better optimization of the experiment's design. TCAD-simulated stack design and process flow demonstrate superior performance devices in threshold voltage, gate breakdown voltage, and ON resistance. Finally, Chapter 9 concludes the dissertation and suggests directions for future research.