Gallium Nitride-enabled High Frequency and High Efficiency Power Conversion

One of the major factors in determining the quality of GaN technology is the epitaxial step. This chapter reviews two different approaches: the use of bulk GaN substrates and GaN-on-Si epitaxy.

This chapter describes the basic GaN-based HEMT device, including the polarization effects and surface states responsible for the formation of the 2DEG in AlGaN/GaN heterostructures. GaN HEMT device structure innovations for increasing the channel mobility, reducing the current collapse phenomena, achieving high voltage breakdown, and enabling normally off operation are presented.

The chapter titled “Vertical GaN Transistors for Power Electronics” takes the reader through the research and development cycle of GaN vertical-device technology, detailing out the three-terminal devices developed over the last decade. Power converters rely on solid state devices featuring diodes and transistors as their basic building blocks. GaN technology is an ever-expanding topic for R&D, proving its potential to solve several challenges in power conversion that cannot be addressed by Si. Medium-voltage (650–900 V) devices using the HEMT configuration have been able to reduce form factor at the system level by driving circuits at higher frequencies (100KHz–1 MHz) and eliminating heat sinks or reducing cooling requirements. Such potentials sparked the interest in GaN device research to address power conversion needs. However, in power conversion the demand of high current (50A and higher) from a single chip for a rated voltage (1KV and higher) is a standard requirement. Particularly when the market is favorable toward electrification of cars and other means of transportations, GaN must expand its scope to provide high power solutions with higher power density compared to Si and even SiC. Vertical devices have been the choice of power device engineers for economic use of the material and maximum use of its physical properties (which allow highest possible blocking field, field mobility, etc.). In this chapter, we discuss vertical transistors first in its normally on form (CAVETs) and then in its normally off design (MOSFET). The advantages and disadvantages are discussed for each type besides describing their operation principles. We have tried to make this chapter scholastic and informative by use of modeling and experimental data for each device we describe. The chapter will help the reader to realize the most recent status of GaN vertical transistors and appreciate its potential in power conversion.

No new product is possible without reliability: this is especially true for new and emerging technology, such as gallium nitride-based devices. For GaN power transistors, breakdown mechanisms play a significant role. The reduction of the robustness and of the long-term reliability still represents a serious issue that must be taken into consideration. The first part of the chapter deals with the above mentioned aspects and mainly focuses on the permanent degradation induced in GaN-based devices by off-state time-dependent mechanisms.

In recent years, as GaN power transistors come into widespread use as switches for power converter applications, it is all the more crucial and inevitable to guarantee their reliability. In this chapter, we discuss how to evaluate the robustness of GaN power transistors. In the switching of GaN power transistors, they can be subject to the so-called current collapse that is a specific phenomenon for GaN in which the ON state resistance is increased once the device is exposed to a high voltage. Since the current collapse induces instability of the device in the form of the increase in the temperature, non-uniform internal electric field distribution and so on, it may lead to the reliability issue. Therefore the robustness of GaN transistors should be examined under switching operation besides the conventional reliability tests standardized for Si power transistors. Since current collapse is crucially dependent on the drain current – voltage locus curve during the switching event, the switching reliability of GaN transistor depends on the switching locus. Accordingly a concept of Switching Safe Operating Area (SSOA) is proposed to define the switching conditions wherein the device can be switched safely. As an example, we define the SSOA for our Hybrid-Drain-embedded Gate Injection Transistor (HD-GIT) that is now commercially available. Furthermore we propose the long-time SSOA (lSSOA) in which we guarantee the robustness of HD-GIT under long-time switching operations (e.g., 10 years). The proposed method for confirming the robustness of GaN power transistors can be utilized to estimate the devices’ lifetime when they are employed in a given switching application.

The concept of circuit “parasitics” has, for the most part, been an attempt to vilify the unwanted or unexpected device and system-level characteristics whenever they were found to be limiting the system operation or performance in some manner. The current approach has always been to mitigate the effects of these unwanted parasitics through design improvements, be it on a device, package, or system level. As these parasitics are, for the most part, determined by their geometry, the methods for their improvements are also spatial in nature. In this chapter, we will discuss the impact of a wide range of so-called parasitics with respect to GaN-based power conversion. We will conclude with brief speculative discussion on the future impact of these parasitics, as GaN technology continues to improve over the coming years, including some thoughts on relevant areas of research for future system improvements.

To start, the relevant GaN power conversion parasitics can, roughly, be broken up into three distinct categories, namely:

With low power loss and high switching capability, GaN device technology can benefit power electronics converters in a number of ways. GaN devices can substitute Si devices in a converter for improved efficiency and power density. In addition, GaN devices can simplify the converter topology, leading to lower cost and higher reliability. GaN devices can also introduce system-level benefits as a result of their fast speed and therefore better dynamics. They can also enable new applications. This chapter presents how GaN technology can benefit AC/DC converters, covering both the single-phase and three-phase cases. Associated challenges in using GaN devices and potential solutions are discussed.

Radio-frequency (rf) power is important to a wide range of applications, including radio transmitters, plasma generation, medical imagers (e.g., MRI), power converters, and wireless power transfer (WPT) among myriad other applications. Advances in power semiconductor devices, magnetics, and circuit design are opening the door to much more efficient generation and delivery of power at radio frequencies. This chapter presents an overview of switched-mode power amplifiers – or radio-frequency inverters – encompassing their design, control, and construction. We focus on the high-frequency (HF, 3–30 MHz) and very-high-frequency (VHF, 30–300 MHz) ranges. We explore key aspects of rf power conversion, including power circuit architecture and design, selection and efficient drive of power devices at rf, and control methods for modulating power and managing load variations. We also address circuit construction, including the design and application of passive components at radio frequencies. Magnetics for power applications at HF and VHF pose a special challenge when compactness and high efficiency are desired. We explore the design of air-core and magnetic-core magnetics for this frequency range, including winding design, core material evaluation and selection, and application of magnetic cores.