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 oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

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

Recent success with the fabrication of high-performance GaN-on-Si high-voltage HFETs has made this technology a contender for power electronic applications. This paper discusses the properties of GaN that make it an attractive alternative to established silicon and emerging SiC power devices. Progress in development of vertical power devices from bulk GaN is reviewed followed by analysis of the prospects for GaN-on-Si HFET structures. Challenges and innovative solutions to creating enhancement-mode power switches are reviewed.

https://iopscience.iop.org/article/10.1088/0268-1242/28/7/074011/pdf

Gallium nitride devices for power electronic applications

We report 30-nm-gate-length InAlN/AlN/GaN/SiC high-electron-mobility transistors (HEMTs) with a record current gain cutoff frequency (fT) of 370 GHz. The HEMT without back barrier exhibits an extrinsic transconductance (gm.ext) of 650 mS/mm and an on/off current ratio of 106 owing to the incorporation of dielectric-free passivation and regrown ohmic contacts with a contact resistance of 0.16 Ω·mm. Delay analysis suggests that the high fT is a result of low gate-drain parasitics associated with the rectangular gate. Although it appears possible to reach 500-GHz fT by further reducing the gate length, it is imperative to investigate alternative structures that offer higher mobility/velocity while keeping the best possible electrostatic control in ultrascaled geometry.

InAlN/AlN/GaN HEMTs With Regrown Ohmic Contacts and $f_{T}$ of 370 GHz

This letter reports lattice-matched In0.17Al0.83N/GaN high-electron-mobility transistors on a SiC substrate with a record current gain cutoff frequency (fT) of 300 GHz. To suppress the short-channel effects (SCEs), an In0.15Ga0.85N back barrier is applied in an InAlN/GaN heterostructure for the first time. The GaN channel thickness is also scaled to 26 nm, which allows a good immunity to SCEs for gate lengths down to 70 nm even with a relatively thick top barrier (9.4-10.4 nm). In a 30-nm-gate-length device with an on-resistance (Ron) of 1.2 Ω · mm and an extrinsic transconductance (gm.ext) of 530 mS/mm, a peak fa of 300 GHz is achieved. An electron velocity of 1.37-1.45 × 107 cm/s is extracted by two different delay analysis methods.

300-GHz InAlN/GaN HEMTs With InGaN Back Barrier

This letter reports a GaN vertical fin power field-effect-transistor structure with submicron fin-shaped channels on bulk GaN substrates. In this vertical transistor design only n-GaN layers are needed, while no material regrowth or p-GaN layer is required. A combined dry/wet etch was used to get smooth fin vertical sidewalls. The fabricated transistor demonstrated a threshold voltage of 1 V and specific on resistance of 0.36 ${\mathrm {m}}\Omega {\mathrm {cm}}^{2}$ . By proper electric field engineering, 800 V blocking voltage was achieved at a gate bias of 0 V.

High-Performance GaN Vertical Fin Power Transistors on Bulk GaN Substrates

Textbook-like device characteristics are demonstrated in vertical GaN p-n diodes grown on bulk GaN substrates. These devices show simultaneously an avalanche breakdown voltage (BV) of >1.4 kV under reverse bias, an ideality factor plateau of ∼2.0 in a forward bias window followed by a near unity ideality factor of 1.1, which are consistently achieved over a temperature range of 300–400 K. At room temperature (RT), the diode with a mesa diameter of 107 μm showed a differential on-resistance Ron of 0.12 mΩcm2, thus resulting in a record figure-of-merit BV2/Ron of ∼16.5 GW/cm2, which is the highest ever demonstrated in any semiconductors. Analytical models are used to fit experimental I-Vs; based on the recombination current with an ideality factor of ∼2.0, a Shockley-Read-Hall lifetime of 12 ns is extracted at RT with an estimated recombination center concentration of 3 × 1015 cm−3.

/pdf/near-unity-ideality-factor-and-shockley-read-hall-lifetime-3da11aajmp.pdf

Near unity ideality factor and Shockley-Read-Hall lifetime in GaN-on-GaN p-n diodes with avalanche breakdown

We report lattice-matched In0.17Al0.83N/GaN high-electron mobility transistors on a SiC substrate with a record current gain cutoff frequency (fT). The key to this performance is the use of an oxygen plasma treatment to form a thin oxide layer on the InAlN barrier and to reduce the gate leakage current by more than two orders of magnitude. In addition, the RF transconductance (gm) collapse is reduced in the O2-treated devices, which results in a significant improvement in the fT . In a transistor with a gate length of 30 nm, an fT of 245 GHz is achieved, the highest value ever reported in GaN-based field-effect transistors.

Xiang Gao

Papers

InAlN/AlN/GaN HEMTs With Regrown Ohmic Contacts and $f_{T}$ of 370 GHz

300-GHz InAlN/GaN HEMTs With InGaN Back Barrier

High-Performance GaN Vertical Fin Power Transistors on Bulk GaN Substrates

Near unity ideality factor and Shockley-Read-Hall lifetime in GaN-on-GaN p-n diodes with avalanche breakdown

245-GHz InAlN/GaN HEMTs With Oxygen Plasma Treatment