industry power electronics
Utilizing a common approach MicroGaN has a common platform for forming three different types of
device: a 600 V diode and normally off and normally on transistors operating at the same voltage.
The normally on transistor has the simplest construction – it’s just a GaN HEMT with a source, drain and gate. But this class of device is unlikely to be a big seller because in many applications concerns over safety lead to the requirement for a normally off transistor.
Great performance is of little benefit in the commercial arena if manufacturing costs are high. This is certainly not the case, according to Sönmez, who claims that the cost of making these normally off GaN-on-silicon switches is lower than that for manufacturing sophisticated silicon products. “Our processing complexity is very low – lower than sophisticated silicon, such as coolMOS, and far lower than SiC transistors that might have trenches or other technological steps to ensure normally off operation.”
He admits that the costs for epitaxy are higher than those for silicon, but points out that this can be offset by resistance figures up to ten times lower. “You end up with a really competitive cost structure even to silicon.”
Initially, MicroGaN’s products will probably be used as drop-in replacements for those made from SiC and silicon. Employed in that way, circuits don’t have to be re-designed, making it relatively easy to displace SiC free-wheeling and boost diodes that are used in boost stages of power factor correction units, and in H-bridge circuits that are used to control the current flow in solar applications and motor drives.
The architecture for the diode is more complex, combining a GaN HEMT with a silicon Schottky barrier diode. “Silicon’s low-voltage diode properties, which are fantastic at low voltages, are transferred to 600 V,” enthuses Sönmez.
He explains that it’s a similar story for the normally off transistor, which pairs a GaN HEMT with a MOSFET in a cascode configuration. “The advantage then is that you don’t need a free-wheeling diode,” explains Sönmez, who adds that the MOSFET that is used is of very high quality and features a low-voltage barrier diode.
According to him, silicon MOSFETs perform poorly at high voltages due to high levels of stored charge. “But you don’t have this problem for silicon MOSFETs at low voltages – they are incredible.” So, just like the diode, this approach allows the strengths of silicon to be translated to high voltages.
That’s not the only advantage of this approach, though, says Sönmez: “If you want to make an AlGaN/GaN system operate normally off, you have to destroy the nature of the two-dimensional gas at the location of the gate.” By sidestepping that issue, MicroGaN can produce devices that excel in a key figure of merit – the on-resistance, multiplied by the area of the device.
Further inroads into the power electronics market require a re-design of the H-bridge circuit Currently, these circuits require four separate silicon IGBTs and additional free-wheeling diodes. To begin with, MicroGaN’s transistors could replace the IGBTs and eliminate the need for diodes. But further down the road the changes could be far more radical: “I can see a fully integrated power conversion technology that can be made on one chip and will, for the first time, facilitate today’s three-dimensional technology advantages,” says Sönmez.
Over the next few months MicroGaN will be preparing its technology for its initial sampling phase with selected customers that is planned for the first quarter of 2012. Mass production will follow as the Ulm start-up establishes manufacturing partners.
“Our process has been developed from the beginning to be transferable,” explains Sönmez. This allows the company to not just outsource epitaxy, but the entire production process, a move that the company is preparing to make. “This will start using 4-inch, using its maturity at 600 V. But we are tracking 6-inch in parallel, and as soon as the electrical properties fulfil our needs - in terms of yield, homogeneity, breakdown voltage, leakage current, and so on – we will switch.” If the company can make these transitions smoothly, it should to be a major force in the power device market.
© 2011 Angel Business Communications. Permission required.
18
www.compoundsemiconductor.net October 2011
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