Power electronics based on Gallium Nitride (GaN) is expected to significantly reduce the losses in power conversion circuits and increase the power density. This makes GaN devices very exciting candidates for next-generation power electronics, for the applications in electric vehicles, data centers, high-power and high-frequency communications. Currently, both lateral and vertical structures are considered for GaN power devices. In particular, vertical GaN power devices have attracted significant attention recently, due to the potential for achieving high breakdown voltage and current levels without enlarging the chip size. In addition, these vertical devices show superior thermal performance than their lateral counterparts. This PhD thesis addresses several key obstacles in developing vertical GaN power devices. The commercialization of vertical GaN power devices has been hindered by the high cost of bulk GaN. The first project in this PhD thesis demonstrated the feasibility of making vertical devices on a low-cost silicon (Si) substrate for the first time. The demonstrated high performance shows the great potential of low-cost vertical GaN-on-Si devices for 600-V level high-current and high-power applications. This thesis has also studied the origin of the off-state leakage current in vertical GaN pn diodes on Si, sapphire and GaN substrates, by experiments, analytical calculations and TCAD simulations. Variable-range-hopping through threading dislocations was identified as the main off-state leakage mechanism in these devices. The design space of leakage current of vertical GaN devices has been subsequently derived. Thirdly, a novel GaN vertical Schottky rectifier with trench MIS structures and trench field rings was demonstrated. The new structure greatly enhanced the reverse blocking characteristics while maintaining a Schottky-like good forward conduction. This new device shows great potential for using advanced vertical Schottky rectifiers for high-power and high-frequency applications. Finally, we investigated a fundamental and significant challenge for GaN power devices: the lack of reliable and generally useable patterned pn junctions. Two approaches have been proposed to make lateral patterned pn junctions. Two devices, junction barrier Schottky devices and super-junction devices, have been designed and optimized. Preliminary experimental results were also demonstrated for the feasibility of making patterned pn junctions and fabricating novel power devices.

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.

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.

This book demonstrates to readers why Gallium Nitride (GaN) transistors have a superior performance as compared to the already mature Silicon technology. The new GaN-based transistors here described enable both high frequency and high efficiency power conversion, leading to smaller and more efficient power systems. Coverage includes i) GaN substrates and device physics; ii) innovative GaN -transistors structure (lateral and vertical); iii) reliability and robustness of GaN-power transistors; iv) impact of parasitic on GaN based power conversion, v) new power converter architectures and vi) GaN in switched mode power conversion. Provides single-source reference to Gallium Nitride (GaN)-based technologies, from the material level to circuit level, both for power conversions architectures and switched mode power amplifiers; Demonstrates how GaN is a superior technology for switching devices, enabling both high frequency, high efficiency and lower cost power conversion; Enables design of smaller, cheaper and more efficient power supplies.

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)-based devices have entered the power electronics market and shown excellent progress in the medium power conversion applications. For power conversions applications > 10 kW, devices with vertical geometry are preferred over lateral geometry, since the former allows more current for a given chip area, thus provides a more economical solution for high-voltage and high-current applications. Moreover, the vertical geometry is attractive for its dispersion-free performance without passivation, a phenomenon that causes high dynamic on-state resistance (R[subscript on]) in lateral geometry high electron mobility transistors (HEMTs). In this study, GaN-based vertical transistors, which include trench current aperture vertical electron transistors (CAVETs) and in-situ oxide, GaN interlayer based trench field-effect transistors (OGFETs), have been studied both theoretically and experimentally. In order to model the devices for DC and switching performances, a device/circuit hybrid simulation platform was developed based on Silvaco ATLAS. The validation of the model was obtained by calibrating it against commercially available HEMT data. Using this hybrid model, one can start with a two-dimensional (2D) drift-diffusion model of the device and build all the way up to its circuit implementation to evaluate its switching performance. The hybrid model offers an inexpensive and accurate way to project and benchmark the performance and can be extended to any GaN-based power transistors.In the experimental portion of this study, a high voltage OGFET was designed and fabricated. An OGFET shows improved characteristics owing to a 10 nm unintentionally doped (UID) GaN interlayer as the channel. A normally-off (V[subscript th] = 4 V) vertical GaN OGFET with 10 nm UID-GaN channel interlayer and 50 nm in-situ Al2O3 was successfully demonstrated and scaled for higher current operation. By using a novel double-field-plated structure for mitigating peak electric field, a higher off-state breakdown voltage over 1.4 kV was achieved with a significantly low specific on-state resistance (R[subscript on,sp]) of 2.2 m[omega] cm2. The metal-organic chemical vapor deposition (MOCVD) regrown 10 nm GaN channel interlayer enabled a channel resistance lower than 10 [omega] mm with an average channel electron mobility of 185 cm2/Vs. The fabricated large area transistor with a total area of 0.4 mm × 0.5 mm offered a breakdown voltage of 900 V and an Ron of 4.1 [omega]. Results indicate the potential of vertical GaN OGFET for greater than 1 kV range of power electronics applications.In addition to the OGFET, the CAVET with a trench gate structure was studied in this work. By taking advantage of the two-dimensional electron gas (2DEG) in the AlGaN/GaN structure, the trench CAVET can secure an even higher channel electron mobility compared to the OGFET. The first functional trench CAVET with a metal-insulator-semiconductor (MIS) gate structure was fabricated in this work with a breakdown voltage of about 225 V. With the improvement in the fabrication process, an 880 V device with an R[subscript on,sp] of 2.7 m[omega] cm2 was demonstrated. One of the notable features of the fabricated trench CAVET is that it requires a standard MOCVD growth condition for HEMT epilayers. The simplification of the growth process is a significant achievement. Finally, a regrowth-free CAVET was demonstrated and patented. The transformative approach was realized using Si ion implantation based doping compensation in the aperture.