There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

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

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.

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

This paper reports on vertical GaN-based trench MOSFETs operating at 100 A for the first time. A current distribution layer (CDL) in a drift layer is employed for the high current operation. An effective insert position of the CDL is designed and, thereby, the current density of the MOSFET with the CDL is increased about 1.17 times higher than that of the MOSFET without the CDL. Large MOSFET chips with a drain current of up to 100 A are fabricated and their switching characteristics are demonstrated.

100 A Vertical GaN Trench MOSFETs with a Current Distribution Layer

An object is to avoid an increase in contact resistance of an ohmic electrode by etching in a semiconductor device. There is provided a method of manufacturing a semiconductor device. The method of manufacturing comprises forming a semiconductor layer; forming an ohmic electrode by stacking a plurality of metal layers, on the semiconductor layer; forming another metal layer that is mainly made of another metal different from a material of an outermost layer among the plurality of metal layers, on the ohmic electrode; removing the another metal layer from top of the ohmic electrode by etching; and processing the ohmic electrode by heat treatment, subsequent to the etching.

Tsutomu Ina

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

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

100 A Vertical GaN Trench MOSFETs with a Current Distribution Layer

Semiconductor device, method of manufacturing the same and power converter