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15 April 2015

HiPoSwitch project completed with development of fast, efficient normally-off GaN-on-Si power transistors

Lasting from September 2011 to end-August 2014 with a budget of €5.57m (including €3.58m of funding from the European Union), the recently completed three-year project HiPoSwitch ('High Power Switch') has developed prototype fast, high-efficiency power switches using gallium nitride (GaN) operating in enhancement-mode. Such transistors are essential for producing energy-efficient, compact and light-weight power converters that make electrical energy more usable. The market potential is reckoned to be enormous, since these converters are found in nearly every electronic device.

Power converters using GaN transistors have less than half the losses of existing technologies and make conversion efficiencies of over 98% practical. A great deal of primary energy consumption can be saved with their widespread use. "More than 3000 terawatt-hours of power are generated in Europe annually," says Joachim Würfl, head of both HiPoSwitch and the GaN Electronics business area at Berlin-based project coordinator Ferdinand-Braun-Institut, Leibniz-Institut fur Hoechstfrequenztechnik (FBH). "If you only converted a quarter of the electricity produced annually in Europe to a different level and increased the efficiency level by two percentage points, you can turn off at least two coalfired plants," he adds.

From high-performance materials to mass-production techniques

GaN possesses ideal physical properties for a semiconductor. "GaN components are therefore very efficient and very fast power switches. And this is because of their low on-state resistance with negligible losses," Würfl says. Higher switching frequencies also mean that passive elements of the power converter (i.e. the inductive coils and capacitors) can be considerably smaller in size (an improvement on the systems side).

GaN has already been used in microwave transistors for many years, and applied in thin layers mostly on silicon carbide (SiC) substrates. Over the last few years this technology has been further developed by FBH for 600V-rated power transistor switches. "This works well, but it is too expensive for mass markets," Würfl says. "As an alternative, the processes developed for SiC can be transferred to considerably more cost-effective but technologically more challenging silicon substrates." 

The advances made in the HiPoSwitch project fit with FBH's project partners. The eight European partners provided complementary competences spanning the value chain, from R&D (FBH; the Slovak Academy of Sciences, Institute of Electrical Engineering; Vienna University of Technology in Austria; and Italy's University of Padua) to industrial application (German deposition equipment maker Aixtron SE, Artesyn Austria GmbH & Co KG, Belgian epiwafer supplier EpiGaN, and Infineon Technologies Austria AG).

FBH was so successful in optimizing the processing of GaN switching transistors on SiC and silicon that nearly ideal components became feasible, it is reckoned. Among others things, comprehensive investigations of drift and degradation effects carried out by University of Padua and Vienna University of Technology provided the foundation for this. The finished transistor chips were finally assembled into low-induction ThinPAK housings by Infineon in Malaysia. The single transistor measures only 4.5mm x 2.5mm and is optimized for switching 600V. It has an on-resistance of 75mΩ and handles a maximum of 120A. "We are the only ones in Europe who can manufacture these kinds of normally-off transistors at present," says Würfl.   

EpiGaN - together with Aixtron - transitioned the epitaxy to silicon, so that manufacturing costs for the substrates are reduced by more than a factor of ten. At the same time, the wafer diameter is increased to 6" or even 8" (a necessary step towards cost-effective industrial production). Chip maker Infineon matched up the newly developed GaN technology with a silicon process line for industrial production of power semiconductors at its Austrian location in Villach.

Part of the project had a decidedly exploratory character, says Würfl, due to the new techniques and processes for implementing GaN power transistors. Together with colleagues at Vienna University of Technology and the Slovak Academy of Sciences, promising ideas for producing semiconductors were successfully tested, says FBH.

Positioned at the end of the value-added chain as a systems-level partner, Austria's Artesyn has developed a 3kW rectifier for telecoms applications including cellular base-stations. This unit converts line voltage to DC with an efficiency of 98%. A specialized switching topology was developed and implemented that is matched to the properties of the GaN switching transistors. Due to their broad usage, the market for such energy-saving power converters is enormous, it is reckoned, while their smaller size and weight also makes them attractive for aerospace applications.

See related items:

EU-funded HiPoSwitch project launched, targeting more efficient power electronics

Tags: GaN power transistors EpiGaN

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