New chip off the silicon block

A new approach to combining GaN and CMOS devices on silicon takes us a step closer to low-cost high-performance system-on-one chips.

The first monolithically-grown chips that combine gallium nitride and CMOS devices on a silicon substrate have been demonstrated by collaborating researchers at the National Research Council of Canada and Carleton University. The team partnered with the world expert in GaN growth on silicon substrates, Fabrice Semond from CNRS-CHREA in France, to integrate AlGaN/GaN HFETs with silicon MOSFETs on a silicon substrate using a technique that can be easily scaled up for mass production.

“This combination of a high-power and high-frequency technology with mainstream CMOS will give designers much greater flexibility to create more highly integrated chips at a low cost,” said Peter Chyurlia, a researcher at Carleton University. “The cost advantage is a direct result of growth on silicon substrates, which can take advantage of the existing silicon infrastructure, as well as integrated circuitry which in other cases would be on another chip. For example a GaN-based sensor device could be integrated with all the CMOS readout and analysis circuitry right beside it.”

An attractive technology

The limited thermal stability of the GaN layers during the high temperatures needed for MOS processing is the biggest challenge. AlGaN/GaN HFETs have a wide range of possible applications. They are chemically inert which make them ideal for use in chemical and biological sensing and, with their ability to operate at temperatures of around 200oC greater than conventional CMOS, GaN sensors can be placed in harsher environments such as in a car engine where no other semiconductor sensor can function. They can operate at high frequencies and withstand high voltages which make them attractive for use as power transistors in communications applications. GaN-based power amplifiers are starting to appear for mobile-phone base station use as they offer a greater power handling and linearity combination over the previous LDMOS and GaAs based power amplifiers. Currently though the transistors are in limited use owing to the high cost associated with the growth of AlGaN/GaN layers, and GaN technology is only appealing for applications which require both high electrical performance and its ability to withstand harsh environments such as in military and space applications.

A practical achievement

Chyurlia and his colleagues have demonstrated the concept of mass-producible GaN and CMOS integrated devices using a differential heteroepitaxy technique in which the wafer size is not limited as with the alternative hybrid integration or bonding techniques. However, they still need to overcome several challenges for wider use. The limited thermal stability of the GaN layers during the high temperature steps needed for MOS processing meant that enclosed, non-isolated MOS transistors were fabricated for this first demonstration, but they now have a more advanced process underway which will allow for isolated devices and circuits.  And the choice of substrate, silicon <111>, is not the preferred orientation for MOS circuitry due to the high density of interface traps that are formed during oxidation, so they are working towards growing GaN layers on <110> oriented silicon, which is of high interest for modern CMOS technology for high speed devices.   

Switching to the future

To move their integration technology towards mainstream use, the team need to find a suitable application that provides a significant cost advantage over alternatives. “The area of high-power switching circuits as well as sensor integrated CMOS control seems promising, especially with the emergence of electric vehicles in which the portability of this technology could be advantageous,” said Chyurlia. Towards this, one of their projects is to alter the structure of the AlGaN/GaN layers to fabricate normally-off devices, which will reduce loss for switching circuits. They are also investigating higher breakdown field transistors in order to attempt switching thousands of volts at a time without the need for cooling circuitry associated with the current technology. In the future they hope to see more power switching electronics and harsh environment sensors using this technology, and that their work will expand the set of possible applications for GaN devices.

The Letter presenting the results on which this article is based can be viewed on the IET Digital Library. For further reading, please visit www.nrc-cnrc.gc.ca/eng/licensing/ims/gallium.html