Efficient power conversion is essential to face the continuously increasing energy consumption of our society. GaN based vertical power field effect transistors provide excellent performance figures for power-conversion switches, due to their capability of handling high voltages and current densities with very low area consumption. This work focuses on a vertical trench gate metal oxide semiconductor field effect transistor (MOSFET) with conceptional advantages in a device fabrication preceded GaN epitaxy and enhancement mode characteristics. The functional layer stack comprises from the bottom an n+/n--drift/p-body/n+-source GaN layer sequence. Special attention is paid to the Mg doping of the p-GaN body layer, which is a complex topic by itself. Hydrogen passivation of magnesium plays an essential role, since only the active (hydrogen-free) Mg concentration determines the threshold voltage of the MOSFET and the blocking capability of the body diode. Fabrication specific challenges of the concept are related to the complex integration, formation of ohmic contacts to the functional layers, the specific implementation and processing scheme of the gate trench module and the lateral edge termination. The maximum electric field, which was achieved in the pn- junction of the body diode of the MOSFET is estimated to be around 2.1 MV/cm. From double-sweep transfer measurements with relatively small hysteresis, steep subthreshold slope and a threshold voltage of 3 - 4 V a reasonably good Al2O3/GaN interface quality is indicated. In the conductive state a channel mobility of around 80 - 100 cm2/Vs is estimated. This value is comparable to device with additional overgrowth of the channel. Further enhancement of the OFF-state and ON-state characteristics is expected for optimization of the device termination and the high-k/GaN interface of the vertical trench gate, respectively. From the obtained results and dependencies key figures of an area efficient and competitive device design with thick drift layer is extrapolated. Finally, an outlook is given and advancement possibilities as well as technological limits are discussed.

Gallium Nitride (GaN) and related alloys have gained considerable momentum in recent years since the improvement in silicon (Si) based power devices is now only incremental. GaN is a promising material for high-power, high-frequency applications due to its wide bandgap, high carrier mobility which result in devices with high breakdown voltage, low on-resistance, and high temperature stability. Despite the superior properties of GaN there is still room for improvement in device design and fabrication to reach theoretical limits of GaN based devices. Reaching the theoretical critical electric field in GaN devices has been challenging due to the presence of threading dislocations, surface impurities introduced during material growth and fabrication process. In order to prevent premature breakdown of the devices, these defects must be mitigated. In this study, avalanche breakdown was observed in p-n diodes fabricated with low power reactive ion etch with a moat etch profile, followed by Mg ion implantation to passivate the plasma damages. Additionally, the devices were fabricated on free standing GaN substrates which has lower dislocation than sapphire or SiC substrates. The electron and hole impact ionization coefficients were extracted separately by analyzing the ultraviolet (UV) assisted reverse bias current voltage measurements of vertical p-n and n-p diodes. GaN and related alloy such as Indium Aluminum Nitride (InAlN) or Aluminum Gallium Nitride (AlGaN) form a high mobility, high density sheet charge at the heterojunction. High electron mobility transistor (HEMT) devices fabricated on these layer stacks are depletion mode (normally-on) devices with a negative threshold voltage. However, normally-on devices are not preferred in power applications due to safety reasons and to reduce the external circuitry. Therefore, the development of an enhancement mode (normally-off) GaN based high electron mobility transistors (HEMT) with positive threshold voltage is important for next generation power devices. Several methods, such as growing a p-GaN on the barrier layer, recessed gate by dry etching, plasma treatment under the gate have been previously studied to develop enhancement-mode HEMT devices. In this study, MOS-HEMT devices were fabricated by selective thermal oxidation of InAlN to reduce InAlN barrier thickness under the gate contact. The thermal oxidation of InAlN occurs at temperatures above 600°C, while GaN oxidation occurs above 1000°C at a slow rate which allows the decrease of the InAlN barrier layer thickness under the gate in a reliable way due to the self-limiting nature of oxidation. A positive shift in the threshold voltage and a reduction in reverse leakage current was demonstrated on MOS-diode structures by thermally oxidizing InAlN layers with In composition of 0.17, 0.178 and 0.255 for increasing oxidation durations at 700°C and 800°C. Enhancement mode device operation was demonstrated on lattice matched InAlN/AlN/GaN/Sapphire MOS-HEMT devices by selective thermal oxidation of InAlN layer under the gate contact. A positive threshold voltage was observed for devices which were subjected to thermal oxidation at 700°C for 10, 30 and 60 minutes. The highest threshold voltage was observed as 1.16 V for the device that was oxidized for 30 minutes at 700°C. The maximum transconductance and the maximum drain saturation current of this device was 4.27 mS/mm and 150 mA/mm, respectively.

GaN is considered the most promising material candidate in next-generation power device applications, owing to its unique material properties, for example, bandgap, high breakdown field, and high electron mobility. Therefore, GaN power device technologies are listed as the top priority to be developed in many countries, including the United States, the European Union, Japan, and China. This book presents a comprehensive overview of GaN power device technologies, for example, material growth, property analysis, device structure design, fabrication process, reliability, failure analysis, and packaging. It provides useful information to both students and researchers in academic and related industries working on GaN power devices. GaN wafer growth technology is from Enkris Semiconductor, currently one of the leading players in commercial GaN wafers. Chapters 3 and 7, on the GaN transistor fabrication process and GaN vertical power devices, are edited by Dr. Zhihong Liu, who has been working on GaN devices for more than ten years. Chapters 2 and 5, on the characteristics of polarization effects and the original demonstration of AlGaN/GaN heterojunction field-effect transistors, are written by researchers from Southwest Jiaotong University. Chapters 6, 8, and 9, on surface passivation, reliability, and package technologies, are edited by a group of researchers from the Southern University of Science and Technology of China.

This book presents the first comprehensive overview of the properties and fabrication methods of GaN-based power transistors, with contributions from the most active research groups in the field. It describes how gallium nitride has emerged as an excellent material for the fabrication of power transistors; thanks to the high energy gap, high breakdown field, and saturation velocity of GaN, these devices can reach breakdown voltages beyond the kV range, and very high switching frequencies, thus being suitable for application in power conversion systems. Based on GaN, switching-mode power converters with efficiency in excess of 99 % have been already demonstrated, thus clearing the way for massive adoption of GaN transistors in the power conversion market. This is expected to have important advantages at both the environmental and economic level, since power conversion losses account for 10 % of global electricity consumption. The first part of the book describes the properties and advantages of gallium nitride compared to conventional semiconductor materials. The second part of the book describes the techniques used for device fabrication, and the methods for GaN-on-Silicon mass production. Specific attention is paid to the three most advanced device structures: lateral transistors, vertical power devices, and nanowire-based HEMTs. Other relevant topics covered by the book are the strategies for normally-off operation, and the problems related to device reliability. The last chapter reviews the switching characteristics of GaN HEMTs based on a systems level approach. This book is a unique reference for people working in the materials, device and power electronics fields; it provides interdisciplinary information on material growth, device fabrication, reliability issues and circuit-level switching investigation.

Gallium nitride (GaN) is a promising material for power electronics due to its outstanding properties, such as high critical electric field and large bandgap. Despite its superior intrinsic properties, fabrication processes and technology for vertical GaN power electronics is still not as mature as in conventional materials. This thesis covers three aspects of vertical power devices on bulk GaN to increase their reliability and performance. The first is the breakdown behavior of GaN under high electric fields. Vertical Schottky diodes with multi-finger anodes are simulated, fabricated and characterized. Evidence of impact ionization and signs of avalanche breakdown are shown. The second aspect is scalable fabrication technologies for vertical power FinFETs. Key processing stesps are refined and demonstrated on large-area devices. The final topic covered is GaN superjunction (SJ) technology in the context vertical power FinFETs. The SJ FinFET concept is first introduced then an underutilized method for p-type doping GaN is explored as an alternative to conventional p-type regrowth and ion implantation. Finally, the proposed GaN SJ FinFET is investigated with simulations. Various standard SJ parameters are optimized and a novel electric field management technique is proposed.

Lateral power devices based on AlGaN/GaN hetero-structures have achieved excellent performance in the medium power range applications. However for higher voltage higher current switches, a vertical structure is preferred since its die area does not depend on the breakdown voltage. This thesis studies vertical GaN power diodes and transistors grown on bulk GaN substrates. The first part of the thesis studies the PiN diode. Low p-GaN ohmic contact resistance is obtained through annealing in oxygen ambient. The breakdown voltage reaches 1200 V with optimized field plate design. The resistance components of the PiN diodes are also analyzed in this part of the thesis. The second half of the thesis presents a novel vertical power FinFET design with only n-GaN epi-layers. One of the key fabrication processes required for this device structure is to achieve a smooth vertical fin sidewall by combining dry/wet etch. The normally-off power FinFET demonstrates excellent performances without the need of p-GaN layer or material regrowth. With the optimization of edge termination structures, 800 V blocking voltage was achieved. A further reduction of on resistance is achieved by increasing the cap layer doping. Switching characteristics are investigated by capacitance measurements. The thesis concludes with the demonstration of spalling off the bulk GaN substrate after device fabrication. Thanks to the substrate spalling technology, the on resistance of the device can be further reduced and the bulk GaN substrate could possibly be reused to save cost.

This unique new resource provides a comparative introduction to vertical Gallium Nitride (GaN) and Silicon Carbide (SiC) power devices using real commercial device data, computer, and physical models. This book uses commercial examples from recent years and presents the design features of various GaN and SiC power components and devices. Vertical verses lateral power semiconductor devices are explored, including those based on wide bandgap materials. The abstract concepts of solid state physics as they relate to solid state devices are explained with particular emphasis on power solid state devices. Details about the effects of photon recycling are presented, including an explanation of the phenomenon of the family tree of photon-recycling. This book offers in-depth coverage of bulk crystal growth of GaN, including hydride vapor-phase epitaxial (HVPE) growth, high-pressure nitrogen solution growth, sodium-flux growth, ammonothermal growth, and sublimation growth of SiC. The fabrication process, including ion implantation, diffusion, oxidation, metallization, and passivation is explained. The book provides details about metal-semiconductor contact, unipolar power diodes, and metal-insulator-semiconductor (MIS) capacitors. Bipolar power diodes, power switching devices, and edge terminations are also covered in this resource.