INDUSTRY POWER ELECTRONICS
WITH ELECTRICITY NOW accounting for a staggering 43 percent of primary energy consumption, according to the International Energy Agency, the benefits of efficient energy conversion – from both an environmental and an economic perspective – are bigger than ever. And this pay-off is only going to grow as more electrical systems are manufactured for deployment in electric vehicles, computer power supplies, solar cell inverters and power converters for LED lighting.
One area where the use of electrical systems will grow fastest is in electrical vehicles. Speaking on behalf of Ford at CS International 2014, Power Semiconductor Research Engineer Ming Su revealed that 25 percent of the company’s vehicles are expected to be electrified by 2020. Making the electrical conversion in these vehicles as efficient as possible will be high on the priority list of engineers based in the US, because this will help automobiles to satisfy a government mandate for 2025 that demands a fuel efficiency of at least 54.5 miles per gallon.
Thanks to its low cost, widespread availability and familiarity, silicon has been the semiconductor of choice for many years in electrical systems, where it is used to perform various roles, including voltage conversion. However, judged purely in terms of performance, this incumbent is inferior to wide bandgap semiconductors, such as SiC and GaN. Switch from silicon to GaN and it is possible to construct devices that combine a low on-resistance with a low parasitic capacitance, culminating in low power losses when the device is on and when it is switching between states.
These great attributes, which stem from the use of a high-mobility two- dimensional electron gas at the interface between the GaN and AlGaN layers of a HEMT, make this class of transistor a promising device for incorporation into switched-mode power supplies serving many of the applications outlined above. According to the GaN power semiconductor manufacturer EPC of El Segundo, CA, 600 V devices account for a quarter of the overall power transistor
market, with devices rated at 200 V or below pulling in three-quarters. Since GaN HEMTs are capable of covering all these voltages, there is good reason to believe that this wide bandgap semiconductor can be adopted in a broad range of power conversion applications.
Foundations for GaN Ideally, manufacture of GaN HEMTs would involve growth on a native substrate. However, GaN substrates are prohibitively expensive and limited in size and availability, so different foundations must be used. The most common alternative, silicon, enables the fabrication of GaN HEMTs that are competitively priced compared to the incumbent. However, the penalty to pay for growth on silicon is that the production of these devices is far from easy. Engineers don’t just have to contend with bow of the epiwafers, caused by differences in lattice constants and thermal expansion coefficients – there are also challenges related to unwanted chemical reactions at the silicon surface.
Figure 1. It is possible to control the bow using REO layers. The plot second from the right is a silicon wafer prior to any depositions. Bow measurements are taken using an FRT MicroProf 3D optical profilometer
Copyright Compound Semiconductor October 2014
www.compoundsemiconductor.net 29
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