Cambridge Electronics Inc (CEI) – which was spun off from Massachusetts Institute of Technology (MIT) in 2012 – has announced a range of gallium nitride (GaN) transistors and power electronic circuits targeted at cutting energy usage in data centers, electric cars, and consumer devices by 10-20%.
Power electronics is a ubiquitous technology used to convert electricity to higher or lower voltages and different currents — such as in a laptop's power adapter, or in electric substations that convert voltages and distribute electricity to consumers. Many of these systems rely on silicon transistors that switch on and off to regulate voltage but, due to speed and resistance constraints, waste energy as heat.
CEI says that its GaN transistors have at least one-tenth the resistance of such silicon-based transistors, allowing much higher energy efficiency and orders-of-magnitude faster switching frequency, so that power electronics systems made with these components can be much smaller. CEI is using its transistors to enable power electronics that will make data centers less energy-intensive, electric cars cheaper and more powerful, and laptop power adapters one-third the size — or even small enough to fit inside the computer itself.
CEI's co-founders and and co-inventors of the technology include Tomás Palacios (an MIT associate professor of electrical engineering and computer science); technical advisory board chair Anantha Chandrakasan (the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering); VP for device development Dr Bin Lu; director of operations Dr Ling Xia; director of epitaxy Dr Mohamed Azize; and director of product reliability Dr Omair Saadat.
Making GaN feasible
While GaN transistors have several benefits over silicon, safety drawbacks and expensive manufacturing methods have largely kept them off the market. But Palacios, Lu, Saadat and other MIT researchers managed to overcome these issues through design innovations made in the late 2000s.
Power transistors are designed to flow high currents when on, and to block high voltages when off. Should the circuit break or fail, the transistors must default to the 'off' position to cut the current to avoid short circuits and other issues — an important feature of silicon power transistors. However, GaN transistors are typically 'normally on' (so, by default, they will always allow a flow of current), which has historically been difficult to correct. Using resources in MIT's Microsystems Technology Laboratory, the researchers — supported by Department of Defense (DoD) and Department of Energy (DoE) grants — developed GaN transistors that were 'normally off' by modifying the structure of the material.
To make traditional GaN transistors, a thin layer of GaN is grown on a substrate. The MIT researchers layered different materials with disparate compositions in their GaN transistors. Finding the precise mix allowed a new kind of GaN transistors that go to the off position by default. "We always talk about GaN as gallium and nitrogen, but you can modify the basic GaN material, add impurities and other elements, to change its properties," Palacios says.
But GaN and other non-silicon semiconductors are also manufactured in expensive processes. To drop costs, the researchers — at MIT and, later, with the company — developed new fabrication technologies, Lu says. This involved, among other things, replacing gold metals (used in manufacturing GaN devices) with metals that were compatible with silicon fabrication, and developing ways to deposit GaN on large wafers used by silicon foundries.
"We are fabricating our advanced GaN transistors and circuits in conventional silicon foundries, at the cost of silicon," Lu says. "The cost is the same, but the performance of the new devices is 100 times better."
Major applications
CEI is currently using its transistors to develop laptop power adaptors that are about 1.5 cubic inches in volume (the smallest ever made, it is claimed).
Palacios says that other possible applications for the transistors include better power electronics for data centers run by Google, Amazon, Facebook, and other companies, to power the cloud. Currently, these data centers consume about 2% of electricity in the USA. But GaN-based power electronics could save a very significant fraction of that, Palacios says.
Another major application is replacing the silicon-based power electronics in the battery chargers and in the inverters that convert the battery power to drive the electric motors within electric vehicles. The silicon transistors used currently have a constrained power capability that limits how much power the car can handle. This is one of the main reasons why there are few large electric vehicles.
GaN-based power electronics could boost power output for electric cars, while making them more energy-efficient and lighter — and, therefore, cheaper and capable of driving further. "Electric vehicles are popular, but still a niche product. GaN power electronics will be key to make them mainstream," Palacios says.
Innovative ideas
In launching CEI, the founders turned to MIT's entrepreneurial programs. "MIT's innovation and entrepreneurial ecosystem has been key to get things moving and to the point where we are now," Palacios says.
Palacios first earned a grant from the Deshpande Center for Technological Innovation to launch CEI. Afterward, he took his idea for GaN-based power electronics to Innovation Teams (i-Teams), which brings together MIT students from across disciplines to evaluate the commercial feasibility of new technologies. That program showed him the huge market pull for GaN power electronics, and helped CEI settle on its first products.
