Power GaN Devices

After introducing the fundamental properties of GaN and related materials, introductory device design is described in view of applying them for switching power device. In order to increase the efficiency in power conversion systems, important device parameters, such as on-resistance (R _on), blocking or breakdown voltage (B _V), and switching frequency have to be considered in relevant to the trade-offs among them. Additionally, potential advantages of vertical device structure are discussed that may increase the current handling capability, where conductivity modulation effect may have a role to realize a future GaN power devices.

The choice of substrate and the material requirements for GaN-based power transistors for switching applications strongly depend on the device architecture. While to date most efforts have been directed toward the fabrication of lateral devices, vertical device layouts have recently gained interest, catalyzed by the progress in the development of larger size bulk GaN substrates. The vertical devices have the advantage that the high fields are held within the bulk of the material rather than on the surface. Large-area GaN substrates, however, are still very expensive, making a lateral device layout on a foreign substrate such as silicon, which is available in wafer sizes up to 12″, currently more attractive.

To achieve high-volume manufacturing of GaN technology, GaN-on-Si wafers need to be processed in highly efficient and productive CMOS fabs. To achieve this objective, the GaN-on-Si epitaxy and processing technology need to be adapted to the strict standard of CMOS fabs. In this chapter, the challenges related to the growth and (Au-free) processing of 200-mm GaN-on-Si wafers in a CMOS fab are analyzed.

GaN and related materials have exploded onto the semiconductor landscape because of the broad range of applications that they address. The most visible is that it has enabled the solid-state lighting revolution with GaN-based LEDs now replacing incandescent and fluorescent light bulbs because of both their low cost and return on investment due to electricity savings afforded by their enhanced efficiency. GaN has also enabled full color displays from cell phones to stadiums, automotive and airplane mood cabin lighting to head lamps based on LEDs and most recently lasers. These are just a few of the applications that have created a >$10B and rapidly expanding market. In the wake of the photonics applications, GaN electronic applications are now emerging in their own right to serve a multi-billion market from cell phone infrastructure to RADAR and communications to power conversion applications. The dominant device that is used for electronic applications is the AlGaN/GaN HEMT. This chapter will address the materials aspects (briefly) and the various device designs that are of value in different applications.

The evolution of LEDs, lasers and HEMTs using gallium nitride as the semiconductor has taught us about its many unique capabilities including high breakdown electric field, polarization-induced high charge density in the channel and reliable operation at high temperature, to name a few. These superior performances due to GaN’s exceptional material properties make GaN ideal for power switching besides RF applications. Thus, power electronics got recently added to GaN’s portfolio showing great progress. Lateral HEMTs, already available as products for RF application, were the obvious first choice for designing power electronic switches. The last ten years have witnessed an incredibly fast advancement in the lateral HEMT technology building a market space for GaN in medium (up to 15 kW) power electronic applications. Although the maximum industrially feasible and economically viable limits of power conversion using the lateral GaN technology are yet to be determined, vertical devices start to look attractive for power conversion ranging above 15–20 kW. The availability of bulk GaN substrates has stimulated the development of vertical GaN technology. Vertical GaN devices, analogous to Si-DMOSFETs in some ways, can uniquely be designed with a high-mobility AlGaN/GaN channel combined to a thick-drift region in bulk GaN to offer very low on-resistance and high breakdown voltage—the two key parameters of benchmarking a power switch. While the channel of a vertical device can be designed either horizontally or vertically along the sidewalls, the peak electric fields in these devices are always buried in the bulk material, far from the surface. This allows the device to be reasonably dispersion-free without involving field plates, unlike used in lateral HEMTs. Attaining high electron mobility in bulk GaN that forms the drift region will be of key importance to outperform the competing technologies based on Si and SiC. This chapter will focus on the design space, challenges, current performance, cost and roadmap of vertical GaN devices for next-generation power conversion.

The outstanding electronic properties of GaN semiconductors, such as large breakdown voltage, high critical electric field, high electron mobility and saturation velocity, high-temperature operation, make them an ideal material for power switches, converters, and RF power amplifiers.

The most direct method for measuring the influence of defects on HEMTs is to examine how the I _ds changes under a given bias condition due to thermally or optically stimulated transitions of electrons and holes into or out of deep levels. The primary effect of changing the occupancy of a deep-level defect on HEMT operation is the formation of a local space-charge that acts like a floating gate. Filling a defect with excess electrons creates a local negative potential that acts to partially pinch-off the channel and reduce I _ds. Conversely, electron emission from a defect state makes the local potential more positive and increases I _ds. Thus, defect states act to self-bias the HEMT and lead to instability in device operation as the occupancy of deep levels changes under dynamic operating conditions, such as switching or self-heating.

We describe an approach to modelling of power GaN HEMTs, aimed at full-system optimization through concurrent simulation of device, package, and application. We believe this “virtual prototyping” approach is an effective means to link fundamental understanding of the device properties to circuit- and system-level performance. Results are specifically presented from detailed simulations and comparison with experiments for both normally-on insulated-gate GaN HEMTs and normally-off pGaN devices in real switching applications.

This chapter describes the properties of the performance-limiting defects in GaN-based transistors. The first part of the chapter summarizes the properties of the most common defects in GaN, by describing a database of defects that has been prepared by our group based on a collection of more than 80 papers on the topic. The second part of the chapter describes the results of our most recent experiments on the impact of common native defects (vacancies, surface states, etc.) and impurities (such as Fe and C) on the dynamic performance of GaN HEMTs. Information on the correlation between epitaxial structure, process quality, and dynamic performance is given in the text.

High-voltage GaN is redefining power conversion, providing cost-competitive and easy-to-embed solutions that reduce costly energy loss by more than 50 %, shrink size and simplify the design and manufacturing of power supplies (for servers, telecom, industrial, gaming, and adapters), PV inverters, motor drives and EV/HEV inverters/converters. Transphorm has established the next power conversion platform—demonstrating breakthrough performance by introducing the world’s first 600 and 650 V GaN products with its EZ-GaN^™ platform, using a cascode high-voltage GaN HEMT configuration. Following successful completion of JEDEC qualification as well as establishment of a high-voltage lifetime of >10 M hours, cascode high-voltage GaN HEMT on silicon switches is now a reality. The ability to produce large-area (6 in.) GaN-on-Si wafers and manufacture them in existing high-volume Si foundries has also made high-voltage GaN commercially attractive.

This chapter describes an enhancement-mode (E-mode) GaN transistor named as Gate Injection Transistor (GIT) and various technologies to improve the performances. The operation principle of the GIT as well as its state-of-the-art DC and switching performances is described after summarizing reported E-mode GaN power transistors. Status of the reliability including Current collapse is summarized, followed by the results of the application of the GIT to practical power switching circuits targeting at high efficiencies. Future technologies to improve the performances and to extract the potential are also described.

GaN-based heterojunction devices, in the form of high electron mobility transistors (HEMTs) and metal–insulator–semiconductor HEMTs (MIS-HEMTs), are capable of delivering superior performance in high-frequency power amplifiers and high-voltage power switches. Conventional Ga-face C-plane GaN-based heterostructures offer record-high 2DEG (two-dimensional electron gas) density without any intentional doping because of the strong intrinsic charge polarization. High-density 2DEG channel inevitably results in depletion-mode (D-mode) GaN transistors with negative threshold voltage (V _th). However, enhancement-mode (E-mode) HEMTs are highly desirable for their inherent fail-safe operation and simple circuit configurations. The key to a feasible normally off HEMT technology is a post-epitaxy threshold voltage control technique that allows localized conversion from D-mode to E-mode or vice versa, since a high-performance E-mode HEMT requires a gate-controlled E-mode channel and D-mode regions (for low access resistance). Gate recess, p-type cap (AlGaN or GaN), and fluorine plasma ion implantation are the three commonly used approaches to fabricating normally off HEMTs. In this chapter, a comprehensive discussion on the underlying physical mechanisms of the fluorine implantation is presented, including atomistic simulation and experimental studies. Further development of the F-implant technique and its integration with other advanced techniques such as gate recess and AlN passivation is described. Finally, the robustness of the F-implant technique is further illustrated with the demonstration of various mixed-signal circuits fully integrated with GaN power devices.

Drift effects in semiconductors are changing electrical device properties in dependence on their electrical, thermal and other treatments. In a similar manner as degradation effects they are adversely influencing device performance. However, in contrast to degradation, drift effects are fully recoverable. This means that device performance can be brought back to its initial property by certain treatments such as device biasing at specific conditions, light exposure by heating up the un-biased device or by combining these procedures. The knowledge on device drift effects is imperative for predicting device performance in real system environment. This chapter provides an overview on drift effects in GaN power switching devices, describes the physical background and discusses proven technological concepts to minimize device drift.

GaN devices are promising candidates for the next generation power devices for energy efficient applications. Although astounding performance is already proven by many research papers, the widespread adoption of GaN power devices in the market is still hampered by (1) yield and reproducibility; (2) cost; (3) reliability.

The use of gallium nitride devices is gathering momentum, with a number of recent market introductions for a wide range of applications such as point-of-load (POL) converters, off-line switching power supplies, battery chargers, and motor drives. GaN devices have a much lower gate charge and lower output capacitance than silicon MOSFETs and, therefore, are capable of operating at a switching frequency 10 times greater. This can significantly impact the power density of power converters, their form factor, and even current design and manufacturing practices. To realize the benefits of GaN devices resulting from significantly higher operating frequencies, a number of issues have to be addressed, such as converter topology, magnetics, control, packaging, and thermal management. This chapter studies the switching characteristics of high-voltage GaN devices including some specific issues related to the cascode GaN. An evaluation is presented of the cascode GaN based on a buck converter in hard-switching and soft-switching modes, which shows the necessity of soft switching for cascode GaN devices at high frequencies. High dv/dt- and di/dt-related gate drive issues associated with the higher switching speed of GaN devices are addressed, and many important design considerations are presented. Additionally, this chapter illustrates the utilization of GaN in a wide range of emerging applications.