Makingnew silicon

Making the new silicon

9:01 AM, 18th August 2015
Making the new silicon
Shown here is a prototype laptop power adapter made by Cambridge Electronics using GaN transistors. At 1.5 cubic inches in volume, this is the smallest laptop power adapter ever made.

CAMBRIDGE, US: An exotic material called gallium nitride (GaN) is poised to become the next semiconductor for power electronics, enabling much higher efficiency than silicon.

In 2013, the Department of Energy (DOE) dedicated approximately half of a $140 million research institute for power electronics to GaN research, citing its potential to reduce worldwide energy consumption. Now MIT spinout Cambridge Electronics Inc (CEI) has announced a line of GaN transistors and power electronic circuits that promise to cut energy usage in data centres, electric cars and consumer devices by 10 to 20 percent worldwide by 2025.

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 power-electronics 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’s GaN transistors have at least one-tenth the resistance of such silicon-based transistors, according to the company. This allows for much higher energy-efficiency, and orders-of-magnitude faster switching frequency - meaning power-electronics systems with these components can be made much smaller. CEI is using its transistors to enable power electronics that will make data centres 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.

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 CEI co-founder Tomas Palacios, alumnus Bin Lu, Omair Saadat PhD ’14 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.

But GaN transistors are typically “normally on” -meaning, by default, they’ll 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 and DOE grants- developed GaN transistors that were “normally off” by modifying the structure of the material.

To make traditional GaN transistors, scientists grow a thin layer of GaN on top of 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.

But GaN and other nonsilicon semiconductors are also manufactured in special processes, which are expensive. To drop costs, the MIT researchers-at the Institute and, later, with the company- developed new fabrication technologies, or “process recipes,” Lu said.

“Basically, we are fabricating our advanced GaN transistors and circuits in conventional silicon foundries, at the cost of silicon. The cost is the same, but the performance of the new devices is 100 times better,” Lu said.

Major applications

Among the other feasible applications for the transistors, Palacios said, is better power electronics for data centres run by Google, Amazon, Facebook, and other companies, to power the cloud.

Currently, these data centres eat up about 2 percent of electricity in the United States. But GaN-based power electronics, Palacios said, could save a very significant fraction of that.

Another major future application, Palacios added, will be replacing the silicon-based power electronics in electric cars. These are in the chargers that charge the battery, and the inverters that convert the battery power to drive the electric motors. The silicon transistors used today 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.

© Massachusetts Institute of Technology News

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