A competition is underway for the high-performance end of the power semiconductor market presently dominated by silicon MOSFET and IGBT devices. Wide-band gap (WBG) contenders such as silicon carbide (SiC) and gallium nitride (GaN) can deliver impressive performance gains over silicon in almost every respect: Lower losses, which enables better efficiency; higher switching frequencies for leading-edge RF applications; and, superior robustness in temperature and voltage.
Silicon still has an edge in cost, however, and the new technologies are still working out some kinks involving performance over time. So, the evolution, while inevitable, will take time and silicon will always be a large part of the picture. The figure below shows the market research firm IHS Technology’s forecast of the combined SiC and GaN revenues over time.
Figure: SiC and GaN power semiconductors by application (IHS Technology 2018)
Industrial
One of the largest potential MOSFET/IGBT markets – motor drive inverters – will be slow to adopt new technology because silicon is still the best fit when it comes to the design trilogy: cost-of-materials, adequate performance, and design familiarity. In many other applications, silicon and silicon germanium (SiGe) are simply good enough for the task at hand.
Photovoltaic (PV)
Others product groups such as solar inverters, which operate at high voltages, have much more to gain at the system-cost level by adopting SiC/GaN MOSFET devices. Power switches based on the new technologies will also increase switching frequencies, which in turn will reduce inductor and capacitor sizes but also driver and control ICs that are fast and accurate. The 1500-V DC PV (photovoltaic) inverters used by electric utilities to produce power in the range of 30 kW to 100 kW, are projected to gain more than 90% market share of all utility scale inverters in the next two years. The high voltages that can be handled by SiC make it well suited for solar applications.
Communication Infrastructure
In the RF world, wireless (cellular) base stations have already begun to switch over from GaAs to GaN power amplifiers because GaN delivers more power and higher operating voltages. In fact, GaN amplifiers have also been replacing high-frequency electron vacuum tubes in military radar and even in commercial and industrial microwave ovens.
The trend will undoubtedly accelerate as 5G – or some pre-standard version of it – is introduced by wireless operators. Today's 4G LTE mobile standard has an average peak data rate of 100 Mb/s. The goal of 5G is 10 Gb/s – enough for streaming live video. It will also operate in the 5-6 GHz band compared to 4G’s 2.8-GHz.
To make this transition sustainable in terms of power usage, the same number of bits that can be transmitted per watt has to increase significantly. GaAs has a basic power density of about 1.5 W/mm while GaN’s ranges up to 12 W/mm. GaN also has high electron mobility, meaning it can amplify signals well into the upper-gigahertz ranges. Moreover, it can manage this at relatively high breakdown voltage levels of around 80 V.
A bit further down the road
Data Centers
The massive data centers operated by companies such as Google, Amazon and Microsoft to provide the vast computing and storage capacities of the cloud (and Internet) also have massive power requirements. Today they represent a significant percentage (~3%) of the world’s power budget.
According to a Yale University report (on climate change), data-center power use is doubling every four years. If it stays on that track, it will consume as much as 20% of the world’s power by 2025 and will also account for up to 3.5% of global emissions by 2020 – surpassing the carbon emissions of aviation and shipping.
Reducing data-center consumption even slightly would have a big impact on global energy use. Enter the Open Compute Project, and more specifically, a proposal by Google to develop and implement 48-V power distribution architectures for server racks.
Much more efficient than the 12-V systems now in the field, the 48-V architecture realizes a big part of this improvement by using a single power conversion stage that steps down the 48-V distribution voltage to the 1 V or less required by the server. The 48-V architecture also reduces losses in copper transmission components such as bus bars. Market research firm Yole Development has estimated that replacing silicon with SiC or GaN can increase DC-to-DC conversion efficiency from 85% to 95%, which makes this application a perfect fit for SiC/GaN components.
Automotive
The high voltages used for power conversion in electric vehicles make this application a likely candidate for SiC/GaAs power semiconductor components. The technology is available today but given the long product cycles of the automotive industry and the disruption caused by switching from fossil fuels, relatively widespread adoption of electric vehicles in general has to be put about five years into the future.
Except, of course, Tesla, which as integrated a SiC-based inverter in its Model S3. The inverter consists of 24 1-in-1 power modules assembled on a pin-fin heatsink. Each of the 24 modules contains two 650V, 100A SiC MOSFETs with an innovative die-attach solution. The modules are connected directly on the terminals with copper clips and thermally dissipated by copper baseplates. It should be noted that the automotive industry’s interest in SiC/GaN is, in part, a response to aggressive legislation aimed at reducing carbon emissions.
For design engineers, the introduction of SiC- and GaN-based devices might impose a disruptive change in design practice. In short, a higher performance switch driving system is required. More coordination and control will be needed to manage the complex multilevel, multistage power loops required to exploit the performance offered by the next generation of SiC/GaN power converters. These designs will include next-generation of advanced gate driver ICs, sensing ICs, power supply controllers, and embedded processors.
Conclusion
For more than a decade, SiC and GaN devices have held the potential to improve energy conversion efficiency, which in turn would reduce the size, weight, and heat dissipation in power converters and inverters, device chargers, and motor drives. In some instances, SiC or GaN devices could make new applications possible. But WBG materials technologies have been impeded by high fabrication costs, reliability questions and the need for enhanced gate drive designs to match the much faster switching speeds of SiC and GaN.
In 2018, pricing made a breakthrough that will enable wider adoption. In fact, pricing of high voltage GaN devices is approaching that of silicon high-voltage device counterparts. Data garnered from a decade of testing and in-field operation is allaying device reliability concerns and new driver designs are enabling ease of adoption. It won’t be too long before WBG power devices pull into the cruising lane.
