Source : Mouser Electronics
The Pursuit of Higher Performance
I have some Italian heritage. Besides being a self-proclaimed connoisseur of Italian food, primarily what constitutes excellent pizza, I also have a bit of a desire to be driving down the road in a Ferrari SF90 Stradale rather than my Lexus ES300. The supercar’s name is a reference to the 90th anniversary of the foundation of Ferrari’s racing division,. Scuderia Ferrari (SF), which embodies Ferrari’s engineering achievements in the automotive world. The SF90 Stradale is the first Ferrari to feature Plug-in Hybrid Electric Vehicle (PHEV) architecture, which sees the internal combustion engine integrated with three electric motors. Although Ferrari’s gasoline-powered cars are renowned as high-performance engineering works of art, the new hybrid-electric version outperforms (2.5 seconds, 0-100km/h) and pushes the envelope into ultra-high performance.
Like the SF90, the emergence of ultra-wide bandgap (UWBG) semiconductors opens new opportunities in many fields because of their many superior properties. UWBG semiconductors have bandgaps considerably wider than silicon (Si, bandgap 1.1eV) and wide bandgap semiconductors such as gallium nitride (GaN, bandgap 3.4eV) and silicon carbide (SiC, bandgap 3.3eV). Materials such as gallium oxide (Ga2O3), cubic-boron nitride (c-BN), and aluminum gallium nitride (AlGaN) are at the frontier of these semiconductor materials research. This article will introduce these UWBG semiconductor materials, including potential applications in electronic design. (For this article, UWBGs will be defined as semiconductors with bandgaps of ≥ 4eV.)
Before looking at UWBG semiconductors, a review of WBG semiconductors’ status compared to traditional Si is warranted. WBG semiconductors are smaller, faster, and more efficient than counterpart Si-based components. WBG devices also offer greater reliability in more challenging operating conditions. Some of the advantages of WBG semiconductors over Si in power electronics include lower losses for higher efficiency, higher switching frequencies for more compact designs, higher operating temperatures, robustness in harsh environments, and high breakdown voltages. Diverse applications range from industrial functions, such as motor drives and power supplies, to automotive and transportation systems, including hybrid and electric vehicles (HEV/EV), photovoltaic (PV) inverters, railway, and wind turbines. Suppliers producing these products include:
- GaN Systems
- Infineon Technologies
- ON Semiconductor
- ROHM Semiconductor
Aluminum Gallium Nitride (AlGaN)
GaN is a WBG semiconductor. When aluminum (Al) is introduced in with GaN, a UWBG semiconductor can be created, typically in the range of 3.4eV–6.2eV. AlGaN is most often employed to produce light-emitting diodes (LEDs) and laser diodes. AlGaN is used this way because its bandgap makes light in the approximate range of 220nm-450nm. It also finds application as an ultraviolet detector and in high-electron-mobility transistors (HEMT).
Aluminum Nitride (AlN)
Leave out the gallium (Ga) and only work with aluminum (Al) and oxidized nitrogen, also known as nitride; this process produces aluminum nitride (AlN) (Figure 1). Like AlGaN, it is often employed in optoelectronics for items such as ultraviolet (UV) LEDs. AlN has a bandgap of 6.1eV, and has excellent thermal conductivity specifications, and is chemically stable. It can operate at high frequencies and power levels.
Figure 1: Aluminum nitride (formula AlN) is often employed in optoelectronics for items such as ultraviolet (UV) LEDs. (Source: Orange Deer studio/Shutterstock.com)
Cubic-Boron Nitride (c-BN)
Boron and nitrogen (Figure 2) can be assembled to produce boron nitride, one form of which is cubic-boron-nitride (c-BN). C-BN has a UWBG of 6.4eV. One of the things that makes this compound unique is that it has somewhat similar properties to diamond, pure carbon (C), which has a bandgap of 5.5 eV. Diamonds are known for being the hardest material. C-BN is not as hard as diamonds, but it offers higher chemical and thermal stability levels
Figure 2: Vector symbols for Borium and Nitrogenium with periodic table information, and atomic representation on background. (Source: Mouser Electronics)
Gallium Trioxide (Ga2O3)
Table 1: Bandgap characteristics of several UWBG materials. (Source: Author).
|Gallium Oxide (Ga2O3)||4.9|
|Aluminum Nitride (AlN)||6.1|
|Aluminum Gallium Nitride (AlGaN)||6.2|
|Cubic-Boron Nitride (c-BN)||6.4|
WBG semiconductors have enabled more efficient and compact power conversion in various applications. WBG semiconductors offer lower ohmic loss. The motivation to explore UWBG semiconductor materials is motivated by the desire to achieve some undefined order of magnitude improvements in power density analogous to the transition from Si to WBG components. UWBG semiconductors also offer the potential to switch massively large voltages without suffering thermal breakdown or reliability issues. For example, AlGaN might offer ~ 10x lower Ron than GaN. UWBG also offers:
- Higher efficiency at higher frequencies
- Lower ohmic losses
- Lower part count
- Higher reliability
Figures of Merit
With the desire to produce higher converter power densities (Watts/area3), engineers employ figures of merit (FOM). Two essential FOMs used in power density scaling with semiconductor material properties are:
Vertical Unipolar (Baliga) Figure of Merit (vUFOM): (ɛµnEc3)/4
Huang Material Figure of Merit (HM-FOM): Ecµn1/2
The UFOM formula is derived by a mathematical relationship between the Off-state, Gauss’ law, and the On-state. The key item we wish to focus on is EC, the critical electric field. For GaN, EC is between 4 and 5, while for AlN, EC is about 13.
The relative FOM for Si is set at one. Suppose one substitutes the Ec values back into the FOM equations above. In that case, the results will show that AlN and related UWBG semiconductors will produce considerable improvements in FOM, offering scientists and designers hope that UWBG provides excellent promise for high-density power conversion applications (Table 2). Two-terminal devices, including PiN, Schottky Barrier (SBD), Junction Barrier Schottky (JBS), and Merged PiN/Schottky (MPS) diodes, are being evaluated in an attempt to develop viable components.
Table 2: Figures of Merit (FOM) values (Source: Sandia National Labs, Ultrawide Bandgap Power Electronics, SAND2017-13122PE).
UWBG materials also offer benefits over WBG materials at high voltages over mid-frequency ranges (1kHz to 1MHz). This benefit is not as favorable at low and high frequencies because of other effects that impact performance.
UWBG offers benefits over WBG materials in that breakdown voltages increase with larger bandgaps. UWBG semiconductors devices grown using thicker drift regions show higher breakdown voltage. Higher AL compositions in AlGaN devices provide higher breakdown voltages. However, there is a drawback. These AL higher levels also lead to higher electron mobility. Thermal conductivity has a similar issue. Increasing the critical electric field (Ec) can potentially increase the voltage breakdown value (VB). It is theoretically conceivable to develop components with AlN with breakdown voltages 1 x 105.
UWBGs can also offer advantages for radio-frequency (RF) devices. Al-rich AlGaN yields better Johnson Figure of Merit (J-FOM) than GaN because of higher critical electric fields (Ec). This strong scaling of the critical electric field with bandgap provides improved FOMs, which offer significant potential to advance beyond present power electronics boundaries.
Further research is needed and is ongoing. One area that is being investigated further includes basic materials research. Researchers are looking at how to efficiently and effectively grow bulk and epitaxial UWBG semiconductors. Investigations are being performed to ascertain the best way to reduce latent defects while optimizing doping processes and characterizing materials. In particular, p-type doping with increasing Al content produces a challenge.
Additionally, thermal activation of holes is not viable for high-Al alloys. In physics, experimentation is ongoingly related to the best way to efficiently support electronic transport in various electronic field conditions. Optical properties and electrical breakdown offer physicists exciting challenges to attempt to harness and make strides forward. Device architecture, packaging, fabrication, and processing all require work to bring products to commercialization. Proper edge termination is critical to prevent premature breakdown, so various edge termination schemes are being evaluated. Collecting information regarding applications whereby UWBG semiconductors can prove beneficial is ongoing.
UWBG semiconductors represent the next generation of ultra-high-performance for high-power electronics. In learning about their potential advantages and applications in electronic design for the future, one can be sure that innovation will continue to spur new advances that help designers realize and achieve breakthroughs beyond the current limits. As scientists and designers develop process improvements to ultimately capitalize on the superior properties of UWBG semiconductors, I look forward to being able to incorporate them into my next design.
If one day you see a red Ferrari SF90 Stradale zipping past you on the highway, give a wave and tip your hat to the Ferrari engineering team for their future-forward innovation and zealous pursuit of ultra-high-performance.
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