Technical Insight
Nitronex Grows GaN HEMTs on 4-Inch Silicon Wafers (Nitride News)
Nitronex, a wireless components supplier focusing on GaN-based devices, has announced that it is producing GaN-based HEMTs grown on 4-inch silicon substrates. In contrast, most other such devices have been grown on 2-inch diameter SiC or sapphire wafers. Furthermore, according to Bob Lynch, CEO of Nitronex, the room temperature mobility of the HEMT structures routinely exceeds 1600 cm2/Vs. "For production on a routine basis, this figure represents the highest mobility for GaN devices grown on such a large substrate," says Lynch. "We have also observed much higher mobility values in other samples." The Pendeo Process The high mobility is directly attributable to the quality of nitride material grown by Nitronex, which in turn results from the company s unique growth process combined with its specially designed MOCVD equipment. Precise details are not available at present, but a key feature is the nature of the layers deposited on the silicon substrate. "These layers are instrumental in reducing the strain between the substrate and epilayers, and alleviating the problems associated with lattice mismatches and different thermal expansion coefficients," explains Lynch. The result is a reduction in defect densities of many orders of magnitude compared to material grown by conventional techniques. Nitronex uses the term "Pendeo" to include its entire process from epitaxial growth to device fabrication, and has a number of patents in this specific area, including several licensed from North Carolina State University (NCSU). "Pendeoepitaxy" was a term coined by one of Nitronex s co-founders while at NCSU and refers to a process in which trenches are etched in a GaN layer and growth conditions are adjusted so that subsequent GaN growth occurs from the trench sidewalls. Although Nitronex has licensed the relevant patent from NCSU, Bob Lynch says that Nitronex is not using this exact procedure, and has in fact filed new patents on its proprietary approach. Another key element is the novel MOCVD growth system developed by Nitronex. "We were unable to find a commercial MOCVD growth system that could grow on 4-inch wafers," says Bob Lynch. "Therefore, we built our own. These are multiwafer machines, and we have many of them." Among the unique design features of these machines are the gas distribution systems, along with the methods used to heat the large area substrates. "Uniformity of our epitaxial material is currently 5%, which we feel is excellent for such large wafers," says Lynch. Products As well as growing HEMT epiwafers, Nitronex is building devices; Lynch reports that transistor performance has far exceeded expectations. "We have been working with outside partners all along, and they are now looking at our devices," he says. "Sampling on a wider scale will take place in April or May, and reliability tests are ongoing." The devices are targeted towards a variety of markets in the 1.838 GHz range, including power transistors for cellular base stations. Other potential applications include MMDS (at frequencies around 5 GHz), point-to-multipoint systems, and point-to-point radios. Lynch feels that GaN devices grown on silicon are likely to offer considerable advantages over silicon carbide transistors, particularly in terms of processing. "We have some unique (and patented) backside processes to remove heat from the devices that will negate the advantages of silicon carbide s high thermal conductivity," he says. "SiC is a very difficult material to work with, while all our backside processes are done on silicon." Multi-sourcing and Future Prospects Nitronex has also been able to duplicate its manufacturing process at another company s fab in Europe. "This makes a big difference to our potential customers since it alleviates the risks of working with a single source," says Lynch. The fab in question was chosen since most of Nitronex s early customers are likely to be in Europe, and the facility has a good working knowledge of GaN. Nitronex recently received $9.5 million in its first round of funding, and plans to significantly increase the size of its facility in Raleigh, NC. Not content with the benefits of 4-inch wafers, the company is moving to still larger diameter substrates. "We expect to be able to demonstrate a 6-inch GaN-on-silicon wafer early next year," says Lynch.
