GaN Electronics for Next Generation Cars
By Srabanti Chowdhury
Technology is reshaping our lives. It is changing the way we think, communicate, commute and operate. The invention of wheels marks the journey that continues today into electric vehicles, Cars powered with all luxuries of a home was possible in science fiction in the 60’s is now a reality. Thanks to electronics that enables our imaginations take shape
The evolution of cars carries the history of our society through the changing times depicting the culture, challenges and priorities. From gas-guzzlers to the electric vehicles has been a road marked by inventions of various proportions, often revolutionary, setting significant leaps into the future.
One such breakthrough was caused by semiconductor technology that enabled new and efficient way of designing systems. Silicon (Si) technology has been the most effective and productive semiconductor technology that delivered solution and created new technologies never-thought-of before. From computation to power conversion, Silicon has led to unprecedented developments and challenging our imagination even further. However, like many other successful technologies, progress in Si based technology has slowed down and the any advancement is now incremental. The next generation power electronics demand new and sustainable technologies beyond Si. Gallium Nitride (GaN) is one of the two new semiconductor technologies (the other semiconductor being Silicon Carbide (SiC)) that is pushing the roadmap beyond Silicon.
GaN and its unique properties
The saturation in Si technology is due to its material limit. A semiconductor material is characterized by its physical properties like band gap. These properties define the performance space of the device (like transistors and diodes) establishing roadmaps for the device and other related technologies. When a device performance hits it’s material’s limit, improvements of significant size is deterred not only at the device level but also all the subsequent levels of technology that was enabled by the material. Fundamentally the material plays a very big role in determining the future of any technology. Power electronics is no exception and starts with the material.
GaN is an artificially synthesized compound semiconductor that presents material properties extremely favorable for power electronics application. A member of the III-V family Gallium Nitride has a bandgap of 3.4eV, which is almost 3 times than that of Si with a theoretical electric field of 3MV/cm (10 times more than Si). GaN can be alloyed with other Gr III materials like Aluminum and Indium to create semiconductors with a wide variety of bandgaps. The other very special property of GaN that has made it a very dominant player in high frequency electronics is the presence of polarization charge. Both piezoelectric polarization (a result of lattice strain) and spontaneous polarization (a property of the polar crystal) adds up to values as high as 2E13/cm2. Alloying GaN with 25% Aluminum and layering it on a GaN surface leads to formation of a 2 dimensional electron gas (2DEG) – which forms the channel of a High electron mobility transistor (HEMT). The electrons confined in the 2D well is of very high density (1E13/cm2) can travel with velocities up to 2E7cm/ under an electric field, which forms the basis of high frequency transistors when operated as amplifiers
GaN-based devices for power switching applications
GaN looks extremely attractive for power switching applications. A switch, which is in heart of every power converters, is made of a solid-state transistor. The switch used in today’s converters is based on Si and devices (such as MOSFETs and IGBTs) are dominantly used in these applications. An ideal switch should offer “zero” resistance in it’s On-state and infinite resistance in the Off-state– very much like the light bulb switch in our homes. When the switch is turned on, the circuit is completed and a low resistance path is offered for the current to flow and light up the bulb. When the switch is off, the circuit is broken offering infinite resistance to the current. High maximum current, low On-resistance (Ron), low off state leakage current at high breakdown voltage are some of the essential properties to qualify as a good switching transistor. One other very important parameter of a good switching transistor is the dynamic Ron. This is the resistance offered by the transistor in the On state, during each switching cycle and is therefore the resistance that is responsible for the conduction loss (I2Rondy) during one switching cycle. The second form of loss incurred in a switch is the switching loss, which proportional to the frequency of switching the transistor. Faster switching enables compact form factor at the circuit level and hence at the system level due to reduction in the size of the passive components in the converter circuit. However, faster switching increases the total switching loss in the transistor reducing the converter’s efficiency. This is a serious limiting factor in Si-based transistors and restricts newer functionalities. For the medium level power conversion (1-15KW) GaN-based transistors offer a high mobility, high charge – and hence low conductivity – channel that enables switching frequencies at least 30 times higher than the current Si standard. GaN-based devices can go up to frequencies of 1 MHz with efficiencies never possible using Si. In some of today’s converter designs with GaN, the switching frequency is around 100-300KHz and limited only by the application design. With a 3MV/cm breakdown electric field and high electron mobility channel GaN based devices are very well suited to serve the various power electronics requirements in hybrid and electric cars without elaborate cooling needs, The less stringent cooling requirements and the less bulky passive components make the converter design extremely compact, light weight, cost-effective and aesthetic.
The two flavors of GaN
GaN can serve both high power (engine drive) and medium power (auxiliary electronics) applications in electric or hybrid vehicles. Vertical GaN transistors [1,2], where the blocking voltage is held vertically in the bulk material, is an emerging technology that has the potential of replacing the Si-based IGBTs in the inverters and generators, providing solutions at lower cost due to reduced cooling requirements. The more matured lateral GaN-on-Si [2,3] technology is suited to serve medium voltage auxiliary electronics such as the air conditioning unit. Whereas vertical GaN will rely on the quality of the bulk GaN material targeting higher bulk mobility, lateral GaN-technology will continue to expand its market taking the advantage of the high mobility-high charge density 2DEG channel that lowers the output charging energy of the switch enabling faster operation.
In conclusion, GaN presents a unique palette of viable solutions for both high power and medium power applications that can power the next generation cars efficiently and economically. It is important for our generation of engineers to understand the urgency for such clean and hi-tech solutions to keep the wheels running.
“Current status and scope of gallium nitride-based vertical transistors for high-power electronics application”
Srabanti Chowdhury, Brian L Swenson, Man Hoi Wong and Umesh K Mishra, Semiconductor Science and Technology Volume 28 Number 7 07401(8pages) [Invited Review]
“Lateral and Vertical transistors using the AlGaN/GaN heterostructure”
Srabanti Chowdhury and Umesh K Mishra, IEEE Transaction on Electron Devices, Volume 60, Issue 10, (7 pages) [Invited Review]
“High-Speed GaN Switches for Motor Drives”, Power Electronics Europe,.J.Honea and J.Kang 2012. 3: 38-41.
Srabanti Chowdhury is an Assistant Professor at Arizona State University. She received her Ph.D. in December, 2010 from University of California, Santa Barbara. As part of her Ph.D. thesis she fabricated the first Gallium Nitride vertical transistor that demonstrated high voltage handling igniting interest in vertical devices in GaN. The effort was funded by Toyota Motor Corporation, Japan for developing energy efficient hybrid car inverter switches. She joined the Transphorm team, a leader in the Gallium Nitride technology, in May 2010 to develop reliable and manufacturable Gallium nitride-based high voltage device (>900V) for efficient power conversions. During her stay in the company she successfully designed and fabricated 900V-1200V Gallium Nitride High Electron Mobility Transistors (HEMTs) on both Silicon Carbide and Si substrates which are highly desired for economic viability for the technology. The devices with breakdown voltages greater than 1kV was successfully incorporated into modules for motor drive applications resulting in the first demonstration of high frequency GaN-on-Si based motor drive in the world. She has also been involved in developing novel techniques to make the devices fail-proof and reliable – highly needed for commercialization. Her ideas and designs have resulted in over 8 patent applications in the last 2 years.