Current Status and Future Prospects of GaN Power HEMTs
Power semiconductor devices are key components of power electronics technology, used primarily as switches or rectifiers in circuits and systems. Currently, more than 70% of all electrical energy consumed is processed by power electronics. The main function of power electronics is to control and transfer the flow of electrical energy from one form to another and in a form that is suitable to the user. Semiconductor devices are widespread and can be found in almost every electrical and electronic piece of equipment or product. Their power range depends on the application and can be from milliwatts to megawatts. Power devices have a significant impact on the economy as they determine system cost and efficiency.
More than half of century was necessary for researches from industry and university to develop different types and power levels Si power semiconductor devices that have high reliability and low price. But the need for high-voltage, high power density devices operating at high frequencies and junction temperatures higher than 150 °C is growing, especially for advanced power electronics. Silicon-based devices are not able to meet these requirements without connecting, in series and in parallel, to a large number of devices using costly active or passive snubbers and expensive cooling systems. For this reason, the limitations of Si material properties for power devices have been debated during the last two decade; and wide band gap semiconductors, made of SiC and GaN have attracted considerable attention.
Significant research into GaN as material for semiconductor devices has been carried out during the last 20 years. Its wide band gap energy enables the devices to operate at elevated temperatures (600 °C) while retaining low leakage current. The higher breakdown strength for a given blocking voltage of GaN results in smaller channel lengths as compared to Si devices. As a result, the storage of the minority carriers or the input and output capacitance and, therefore, the associated switching losses at a given switching frequency, are reduced. This leads to an increase of the switching frequency to 0.5-1 MHz with acceptable switching losses, which significantly reduces the size and cost of the passive components in power electronic systems.
Regardless of the advantages that GaN materials possess, they have not been adopted for manufacturing of the entire family of high-power devices. Difficulty with crystal growth, the presence of crystal defects, such as micropipes and dislocations, and the absence of abundant wafer suppliers have all contributed to a lack of progress in the fabrication of GaN power devices by the commercial sector. Several industrial research groups and university labs have been working on improving the material properties and development of GaN power devices., In February 2006, the first commercially available GaN high electron mobility transistor (HEMT) products were launched by Eudyna Devices (now Sumitomo Electric Device Innovations USA, Inc.) (12). Since then new GaN devices and new companies have been introduced to the market.
Most of us remember the Little Box Challenge by Google and the IEEE Power Electronics Society. The challenge was to build a power inverter that was about 10 times smaller than the state-of-the-art at the time. From the 18 finalists, the prize of $1 million was awarded to Red Electrical Devils from CE+T Power, who presented their 2 kVA inverter that had a power density of 143 W/inch3. Gallium nitride power transistors from GaN Systems were critical parts of the winning design. The benefits of GaN devices have been presented in different seminars, conference and forums, but the price and reliability of the power devices are a stopping factor for them to be imbedded in current designs. Trends are still unknown.
The structure of Si and SiC high voltage devices that are on the market is vertical, while the GaN devices are HEMT made on Si substrate. Some of the company are specialized of producing enhancement mode devices while other in depletion mode devises.
The enhancement mode devices (for example GaN systems Inc.) are normally-OFF device and are preferable because they prevent current from flowing when no voltage is applied to the gate, and therefore, it can be easily utilized in power applications. But GaN enhancement mode device, the gate switching voltages are -3 and 6 V which made the drive circuit more complicated.
The depletion mode device are normally ON devices and therefore not preferable by the power designers. A cascode circuit, includes a high-voltage, normally-ON GaN HEMT with a low-voltage MOSFET that can operate as a normally-OFF high-voltage device (example Transphorm) . The high voltage GaN HEMT is a normally-ON device and has a negative threshold voltage, and the low-voltage MOSFET is capable of blocking 30 V with a standard power MOSFET threshold voltage. The cascode circuit has the gate-driving characteristics bility of the high-voltage GaN HEMT. Since the MOSFET is a low-voltage device, its ON-resistance, RDS_ON, is significantly smaller than in the GaN HEMT, and the associated losses will be insignificant. Substituting the Si low voltage devices with a GaN devices make the losses smaller (Navitas Semiconductor). TI and VisiC have a direct drive device. It is similar to a cascade configuration: a demolition mode HEMT with a low voltage Si MOSFET. The low voltage MOSFET is used to enable the circuit while the drive is done by the HEM. VisiC has also reported 1200 V/ 50 A device.
Session Chairs:
Tanya Kirilova Gachovska received her M.Eng., and Ph.D. Degrees, all in Electrical Engineering, from the University of Ruse, Bulgaria, in 1995 and 2003. She earned her second Ph.D. Degree in Electrical Engineering (Power Electronics), at the University of Nebraska-Lincoln (UNL), Lincoln, USA in 2012. Her Ph.D. thesis was “Modeling of Power Semiconductor Devices”. She worked as an Assistant Professor at the University of Ruse from 1999 to 2003. She conducted research from 2004 to 2006 and taught for a semester in 2006 at McGill University in Montréal. She worked as a Postdoctoral Research Scientist in the area of Pulsed Electric Fields at UNL from 2012 to 2013. During her Ph.D. studies at UNL, she taught various courses and labs, and continued a collaboration in Pulsed Electric Fields research with McGill University, University of Ruse, University of Djiali Liabes, Sidi Bel Abbes, Algeria and École Nationale Supérieure Agronomique, El Harrach, Algeria. She joined Solantro Semiconductor, Corp., Ottawa in 2013. Dr. Gachovska authored or coauthored more than 30 technical papers and conference presentations, two books, and two book chapters and holds a world patent in Pulsed Electric Fields. In 2019 Dr. Gachovska become a professional engineer of Ontario. She is a vice chair of IEEE-IAS Power Electronics Devices and Components Committee and Co-chair of PELS Ottawa. She is also a senior IEEE member and a P.Eng.
Ruxi Wang received the B.S. and M.S. degrees in electrical engineering from Xi’an Jiaotong University, Xi’an, China, and the Ph.D. degree in the Center for Power Electronics Systems (CPES), Virginia Tech, Blacksburg, USA, in 2004 and 2007, and 2012, respectively. In 2012, he joined the Global Research Center of General Electric Company, Niskayuna, USA as a Senior Electrical Engineer. Since 2019, he joined General Electric Aviation as a Senior Staff Engineer. His research interests include high-power-density converter design in transportation application, Healthcare electronics, electromagnetic interference technology, more electrical aircraft system, advanced components and packaging technology. He has published over 50 papers in refereed journals and international conference proceedings. He has above 30 U.S. awarded and pending patents. Dr. Wang received the William M. Portnoy Award for the Best Paper published in the IEEE Energy Conversion Congress & Expo in 2012.
Dr. Wang is an associate editor for the IEEE Transaction on Industrial Applications and also served as Chair for Power Electronics Devices and Components Committee in IEEE Industry Applications Society.
Speakers:
Julian Styles, GaN Systems LLC.
Philip Zuk, Transphorm
Stephen Oliver, Navitas Semiconductor
Luc Van de Perre, VisIC
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