technology industry
better power supplies for computers and more efficient power conversion in solar converters and hybrid electrical vehicles.
The power electronics market that could underpin this revolution in efficiency savings is growing at more than 8 percent per year, and is worth billions and billions of Euros. Silicon devices dominate this sector. However, the room left for improvement in these devices is vanishing fast, because the best ones are now operating close to limits imposed by their intrinsic characteristics. It is widely accepted that silicon IGBTs deliver a relatively poor performance at high frequencies and MOSFETs struggle to handle high powers.
Fortunately, there is a superior class of materials that can come to the rescue: Wide bandgap semiconductors. This includes GaN-on-silicon devices, which excel at combining affordability with great performance.
One of the biggest attributes of these nitride electronic devices is their high breakdown voltage, which stems from a field strength that is an order of magnitude higher than that of silicon. Thanks to the high mobility and large carrier concentration associated with the two- dimensional electron gas (2DEG) of the AlGaN/GaN heterostructure, nitride devices built for switching applications can also combine a low on-resistance with a high switching speed. What’s more, the wide band- gap properties associated with GaN-based devices enable them to operate at high temperatures.
Development of nitride power devices has been underway for more than a decade and progress in this direction has enabled today’s switching devices to outperform their silicon rivals. In the performance stakes, SiC is a tougher opponent, but GaN more than holds its own.
Making the nitrides affordable Great performance is no guarantee of market success: Products must also be affordable. GaN-on-silicon excels in this regard, because it is the most cost-efficient wide bandgap technology. It is already possible to deposit advanced heterostructures on silicon substrates up to 150 mm in diameter, and in the near future this growth process will be extended to 200 mm silicon. There is also an opportunity to develop process compatibility with standard CMOS technology, which would open the door to further cost reduction by enabling these wafers to be put through lines at under-utilized 200 mm silicon fabs that are dotted around the globe.
Given it’s potential, it is of no surprise that GaN power electronics technology is attracting more and more interest. But no one is yet to deliver the first big commercial breakthrough – a reliable device operating at 600 V. One of the biggest challenges is to drive the
Figure 1. In-situ SiN passivation, a unique concept for controlling surface states filling,
enhancing device stability during operation as well as reliability tests
first significant penetration for a compound semiconductor technology of a domain where silicon dominates, and many potential customers have been scarred by the experience of SiC. Although the performance of SiC diodes is attractive for power converter manufacturers, these chips are too pricey. In addition, until recently these diodes couldn’t be paired with a SiC transistor, which is detrimental to the uptake of this first-on-the-market wide bandgap solution.
Another reason behind the lack of a commercially attractive, reliable 600 V device is that it is tough to manufacture GaN-on-silicon epitaxial structures, which are the starting point for making power electronics.
This is precisely the challenge that we have picked up at EpiGaN, a spin-off of imec, a large international independent nanoelectronics centre located in Leuven, Belgium. Our start-up, which we co-founded in May 2010, is built on our expertise developed at imec, where we were involved in GaN research from 2001. Key successes during that time included the world’s first low-sheet-resistivity 6-inch HEMT structures in 2006, and the first GaN-on-silicon 200 mm epiwafers, a feat achieved in partnership with the MOCVD toolmaker Aixtron.
Our approach differs from that of several players, which employ SiC as the substrate for their nitride devices. We have always focused our efforts on GaN-on-silicon devices, due to their cost advantages. Initially, we developed material for RF devices (we also offer epiwafers for RF applications). However, given the strengths of GaN-on-silicon for power electronics, it was an obvious move for us to switch our focus and target this lucrative market.
During our time at imec advances in device performance governed our development of materials growth. This approach worked effectively and efficiently, thanks to very fast feedback of device performance.
March 2012
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