Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

/pdf/the-2018-gan-power-electronics-roadmap-2iunv619fh.pdf

The 2018 GaN power electronics roadmap

J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

/pdf/ultrawide-bandgap-semiconductors-research-opportunities-and-49ev1zu2ym.pdf

Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

There is a great interest in wide-bandgap semiconductor devices and most recently in monolithic GaN structures for power electronics applications. In this paper, vertical p-n diodes fabricated on pseudobulk low defect density ( $10^{4}$ – $10^{6}$ cm $^{-2}$ ) GaN substrates are discussed. Homoepitaxial low-pressure metal organic chemical vapor deposition growth of GaN on its native substrate and being able to control and balance the n-type Si doping with background C impurity has allowed the realization of vertical device architectures with drift layer thicknesses of 6 to 40 $\mu $ m and net carrier electron concentrations of $4\times 10^{15}$ to $2.5\times 10^{16}$ cm $^{-3}$ . This parameter range is suitable for applications requiring breakdown voltages (BVs) of 600 V–4 kV with a proper edge termination strategy. Measured devices demonstrate near power device figure of merit, that is, differential specific on-resistance ( $R_{{{\textrm {sp}}}}$ ) of 2 m $\Omega $ cm $^{2}$ for a BV of 2.6 kV and 2.95 m $\Omega $ cm $^{2}$ for a 3.7-kV device, respectively. The improvement in the substrate quality over the last few years has resulted in the fabrication of diodes with areas as large as 16 mm $^{2}$ , with BVs exceeding 700 V and pulsed (100 $\mu $ s) currents of 400 A. The structures fabricated are utilized to study in detail the temperature dependency of $I$ – $V$ characteristics, impact ionization and avalanche characteristics, and extract (estimate) modeling parameters such as electron mobility in the GaN $c$ -direction (vertical) and hole minority carrier lifetimes. Some insight into device reliability is also provided.

Vertical Power p-n Diodes Based on Bulk GaN

In this letter, vertical GaN transistors fabricated on bulk GaN substrates are discussed. A threshold voltage of 0.5 V and saturation current >2.3 A are demonstrated. The measured devices show breakdown voltages of 1.5 kV and specific ON-resistance of 2.2 mΩ-cm
 2
, which translates to a figure-of-merit of V
 BR
 2
/R
 ON
 ~1 × 10
 9
 V
 2
 Ω
 -1
 · cm
 -2
.

1.5-kV and 2.2-m $\Omega $ -cm $^{2}$ Vertical GaN Transistors on Bulk-GaN Substrates

Wide-bandgap semiconductors are expected to be applied to solid-state lighting and power devices, supporting a future energy-saving society. While GaN-based white LEDs have rapidly become widespread in the lighting industry, SiC- and GaN-based power devices have not yet achieved their popular use, like GaN-based white LEDs for lighting, despite having reached the practical phase. What are the issues to be addressed for such power devices? In addition, other wide-bandgap semiconductors such as diamond and oxides are attracting focusing interest due to their promising functions especially for power-device applications. There, however, should be many unknown phenomena and problems in their defect, surface, and interface properties, which must be addressed to fully exploit their functions. In this review, issues of wide-bandgap semiconductors to be addressed in their basic properties are examined toward their ?full bloom?.

/pdf/wide-bandgap-semiconductor-materials-for-their-full-bloom-7wjoaf5zpd.pdf

Wide-bandgap semiconductor materials: For their full bloom†

In this paper, we report on 1.2-kV-class vertical GaN-based trench metal–oxide–semiconductor field-effect transistors (MOSFETs) on a free-standing GaN substrate with a low specific on-resistance. A redesigned epitaxial layer structure following our previous work with a regular hexagonal trench gate layout enables us to reduce the specific on-resistance to as low as 1.8 mΩcm2 while obtaining a sufficient blocking voltage for 1.2-kV-class operation. Normally-off operation with a threshold voltage of 3.5 V is also demonstrated. To the best of our knowledge, this is the first report on vertical GaN-based MOSFETs with a specific on-resistance of less than 2 mΩcm2.

http://iopscience.iop.org/article/10.7567/APEX.8.054101/pdf

1.8 mΩ·cm2 vertical GaN-based trench metal–oxide–semiconductor field-effect transistors on a free-standing GaN substrate for 1.2-kV-class operation

This paper reports the characteristics of vertical GaN-based trench metal–oxide–semiconductor field-effect transistors on a free-standing GaN substrate with a blocking voltage of 16 kV The high blocking voltage was obtained by using field plate edge termination around the isolation mesa of the transistor To our knowledge, the blocking voltage is the highest ever reported for vertical GaN-based transistors on free-standing GaN substrates Normally off operation with a threshold voltage of 7 V is also demonstrated

https://iopscience.iop.org/article/10.7567/APEX.7.021002/pdf

Vertical GaN-based trench metal oxide semiconductor field-effect transistors on a free-standing GaN substrate with blocking voltage of 1.6 kV

This paper reports on vertical GaN Schottky barrier diodes (SBDs) fabricated on a free-standing GaN substrate with different sizes of Schottky electrode. The fabricated SBDs with 3 ? 3 mm2 Schottky electrodes exhibited both a forward current of 50 A and a blocking voltage of 790 V. To our knowledge, the characteristics of operation with a simultaneous high forward current and high blocking voltage are reported for the first time for vertical GaN SBDs on free-standing GaN substrates. The dependence of these characteristics on the Schottky electrode size is also reported in detail.

https://iopscience.iop.org/article/10.7567/APEX.8.071001/pdf

50 A vertical GaN Schottky barrier diode on a free-standing GaN substrate with blocking voltage of 790 V

A high activation ratio of Mg ion implantation by conventional rapid thermal annealing (RTA) was demonstrated. To obtain the high activation ratio of Mg ion implantation, the dependence of hole concentration on Mg dose was investigated. A maximum hole concentration and a high activation ratio of 2.3% were obtained at a Mg dose of 2.3 × 1014 cm−2 between 9.2 × 1013 and 2.3 × 1015 cm−2. The ratio is, to the best of our knowledge, the highest ever obtained by conventional RTA.

https://iopscience.iop.org/article/10.7567/APEX.10.091002/pdf

High carrier activation of Mg ion-implanted GaN by conventional rapid thermal annealing

In this paper, we demonstrate 1.2 kV-class vertical GaN trench MOSFETs fabricated on a bulk GaN substrate with high current and switching operations. The fabricated multi-cell MOSFET achieved a blocking voltage of 1.3 kV by using a field-plate edge termination around an isolation mesa. The MOSFET chip with the size of 1.5 mm × 1.5 mm operates at current rating of as high as 23.2 A with fast switching characteristics. To our knowledge, this is the first report for vertical GaN-based transistors with a high current operation exceeding 10 A and dynamic characteristics.

Tohru Oka

Papers

1.8 mΩ·cm2 vertical GaN-based trench metal–oxide–semiconductor field-effect transistors on a free-standing GaN substrate for 1.2-kV-class operation

Vertical GaN-based trench metal oxide semiconductor field-effect transistors on a free-standing GaN substrate with blocking voltage of 1.6 kV

50 A vertical GaN Schottky barrier diode on a free-standing GaN substrate with blocking voltage of 790 V

High carrier activation of Mg ion-implanted GaN by conventional rapid thermal annealing

Over 10 a operation with switching characteristics of 1.2 kV-class vertical GaN trench MOSFETs on a bulk GaN substrate