The US Department of Energy’s Advanced Research Projects Agency - Energy (ARPA-E) has issued two new Funding Opportunity Announcements (DE-FOA-0000941, DE-FOA-0000942) for high-current wide bandgap (WBG) power semiconductor devices called “Strategies for Wide Bandgap, Inexpensive Transistors for Controlling High Efficiency Systems (SWITCHES).”
Both FOAs seek to fund innovative WBG semiconductor materials, device architectures, and device fabrication processes that promise to enable increased energy density, increased switching frequencies, enhanced temperature control, and reduce power losses in a range of power electronics applications, including high-power electric motor drives and automotive traction drive inverters.
The goal of the SWITCHES program is to enable the development of high voltage (1200V+), high current (100A) single die power semiconductor devices that, upon ultimately reaching scale, would have the potential to reach functional cost parity with silicon power transistors while also offering breakthrough relative circuit performance (low losses, high switching frequencies, and high temperature operation).
These transformational technologies would have promise to reduce the barriers to ubiquitous deployment of low-loss WBG power semiconductor devices in stationary and transportation energy applications.
ARPA-E is allocating up to $25 million for both SWITCHES FOAs, with $15 million in funding being made available specifically to small businesses under ARPA-E’s Small Business Innovation Research (SBIR) and Small Business Technology Transfer (STTR) program and $10 million being made available to all applicants.
Background on WBG materials. Technical advances in power electronics promise enormous energy efficiency gains throughout the economy. Examples of these potential benefits include:
Motor Drives: Electric motors account for over 38% of U.S. electricity consumption. Replacing on/off control or throttling valves with variable frequency drives in industrial pumps and HVAC systems would result in energy savings of up to 65%, according to a study cited by the DOE in the FOA. Their widespread adoption, enabled by low-cost, high-performance power electronics could yield up to a 20% reduction in U.S. electricity consumption.
Automotive: High costs have thus far prevented the widespread electrification of vehicles. While batteries are the dominant factor in powertrain cost, the limitations of current power electronic systems for both battery charging and traction drive inverters also play an important role. Advances in power electronics promise to substantially reduce the weight and additional cost of the electrification of vehicles.
Electric Power Generation: Power converters are required to connect solar photovoltaics and wind turbines to the electric grid. Advances in power electronics have potential to reduce inverter losses by more than 50% while also enabling reductions in cost, weight, and volume. These advances could accelerate the adoption of these new sources of generation, with their commensurate reductions in emissions.
Electric Power Transmission: Congestion of electric transmission networks and the challenges associated with variable power generation (wind and solar) can be mitigated with embedded power electronics-based power flow controllers.
Achieving high power conversion efficiency requires low-loss power semiconductor switches. Today’s incumbent power semiconductor switch technology—silicon (Si)-based MOSFETs, IGBTs and thyristors—has several important limitations:
High Losses: The relatively low silicon bandgap (1.1 eV) and low critical electric field (30 V/µm) require high voltage devices to have substantial thickness. The large thickness translates to devices with high resistance and associated conduction losses.
Low Switching Frequency: Silicon high voltage power MOSFETs require large die areas to keep conduction losses low. Resulting high gate capacitance and gate charge produce large peak currents and losses at high switching frequencies. Silicon IGBTs have smaller die than MOSFETs due to utilization of minority carriers and conductivity modulation, but the relatively long lifetime of minority carriers reduces the useful switching frequency range of IGBTs.
Poor High-Temperature Performance: The relatively low silicon bandgap also contributes to high intrinsic carrier concentrations in silicon-based devices, resulting in high leakage current at elevated temperatures. Temperature variation of the bipolar gain in IGBTs amplifies the leakage and limits the maximum junction temperature of many IGBTs to 125 °C.
WBG semiconductor-based devices, however, are capable of low-loss operation at high voltages (> 1 kV to tens of kV); ;high frequencies (tens of kHz to tens of GHz); and high temperatures (>150 °C). Power converters based on WBG devices can achieve both higher efficiency and higher gravimetric and volumetric power conversion densities.
For example, DOE noted, in a recent demonstration, a 2kW motor driven by high frequency GaN devices resulted in an increase in efficiency of over 2% at full load and 8% at low load relative to the same motor being driven by Si IGBTs.
Much technical progress has been achieved with WBG-based power switches over the past decade, with support from numerous Federal agencies including the Department of Defense and several DOE offices, including the Advanced Manufacturing Office, the Office of Electricity Delivery and Energy Reliability, and the Vehicle Technologies Program.
ARPA-E’s Agile Delivery of Electrical Power Technologies (ADEPT) program, initiated in 2010 (earlier post), also funded several teams to develop new devices and demonstrate their efficacy in system demonstrations.
The ADEPT program focused on transformational power electronics advances in a wide range of high efficiency power conversion systems. Several ADEPT projects focused on WBG devices. For example, several program teams have recently demonstrated enhancement-mode (normally off), 600V+ GaN-on-Si High Electron Mobility Transistors (HEMTs). SiC diodes with 10 kV rating and IGBT’s with blocking voltages exceeding 15kV have also been demonstrated.
Although SiC and GaN have made commercial progress over the past decade, SiC and GaN device technology remains relatively immature relative to Si and currently carries a substantial cost premium, limiting widespread adoption. Because many of the largest opportunities for increased energy efficiency and reduced energy-related emissions exist in extremely cost-conscious industries, cost for an equivalent functional performance remains a major barrier to the widespread adoption of WBG devices, despite opportunities for superior performance (including reductions in system costs).
WBG devices will have to approach functional cost parity with Si power devices to gain widespread adoption, DOE said. In addition to high cost, most WBG discrete devices demonstrated to date have had relatively low current ratings.
SWITCHES. Recent research results indicate that new materials advances, device architectures, and device fabrication processes could substantially accelerate progress towards WBG devices that achieve both higher current ratings and functional cost parity with silicon-based devices, thus gaining ubiquitous deployment. These approaches have, as of yet, received relatively little attention from industry and the research community since they are perceived to be technically unproven and high risk.
The following technical areas are of interest to this FOA:
Wide bandgap power semiconductor devices utilizing novel fabrication processes or device structures not previously supported by ARPA-E, other governmental agencies or previously developed for commercial application; such technologies might include vertical GaN device structures, approaches to device fabrication that are compatible with substrate re-use (i.e. device liftoff), and novel structures compatible with far lower cost fabrication processes and high die current ratings. New approaches to fast, high quality thick film epitaxial growth that enables rapid fabrication of high voltage devices are also of interest.
Investigation of technologies with the potential to enable extremely low cost and highly scalable free standing wide bandgap substrate fabrication. Such technologies might include, but are not limited to, new GaN, ZnO, SnO2, sapphire, or other wide bandgap substrate growth techniques. Approaches that enable larger substrate sizes and substantially reduced defect densities are required. These may include advances in new chemistries for epitaxial growth or substrate refining techniques.