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.

Silicon-based power devices are reaching their fundamental performance limit. The use of wide-bandgap semiconductors with superior material properties over silicon offers the potential for power electronic systems with much higher power densities and higher conversion efficiency. GaN, with a high critical electric field and carrier mobility, is considered one of the most promising candidates for future high-power, high frequency and high temperature applications. High voltage transistors and diodes based on both lateral and vertical structures are of great interest for future power electronics. Particularly, vertical GaN power devices have recently attracted increasing attention due to their many unique properties. This paper reviews recent progress and key remaining challenges towards the development of high-performance vertical GaN transistors and diodes with emphasis on the materials and processing issues related to each device architecture.

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This work presents the first experimental study on capacitances, charges and power-switching figure-of-merits (FOMs) for a large-area vertical GaN power transistor. A 1.2 kV, 5 A GaN vertical power FinFET was demonstrated in a chip area of 0.45 mm 2 , with a specific on-resistance of 2.1 mΩ•cm 2 and a threshold voltage of 1.3 V. Device junction capacitances were characterized and their main components were identified. This was used to calculate the switching charges and practical switching frequencies. Device FOMs were then derived that take into account the trade-offs between conduction and switching power losses. Our GaN vertical FinFETs exhibit high frequency (~MHz) switching capabilities and superior switching FOMs when compared to commercial 0.9-1.2 kV Si and SiC power transistors. This work shows the great potential of GaN vertical FinFETs for next-generation medium-voltage power electronics.