Picogiga: GaN RF composite substrates ready for samples

January 29, 2007 – Following closely on the heels of work done by a European R&D consortia, Picogiga International Picogiga International, a division of Soitec, says it has developed pre-production samples of its SopSiC (silicon-on-polycrystalline silicon carbide) substrate for gallium nitride (GaN)-based power devices, which it says handles heat better than silicon “at a fraction of the cost” of bulk silicon carbide.

The new substrate “bridges the void” between low-performance, low-cost GaN-on-silicon and high-performance, high-cost single crystal silicon carbide for GaN HEMT devices, the company claims. The work follows a couple of months after a European R&D program, HYPHEN, said it achieved “excellent” initial material characterization results for GaN on compound engineered substrates, in its three-year efforts to develop and evaluate new silicon and silicon carbide-based substrates for GaN-based RF devices.

The SopSiC structure, engineered using Soitec’s Smart Cut layer transfer and bonding technology also used with silicon-on-insulator wafers, incorporates a bottom layer of polysiliconcarbide, an insulating buried oxide layer, and a high resistivity (1-1-1) silicon top layer. The top layer serves as the seed layer for GaN epitaxial growth through molecular-beam epitaxy (MBE) or metallic organic CVD (MOCVD) technology, while the bottom polysiliconcarbide layer is designed to evacuate the heat generated by high-power HEMT devices.

Picogiga says samples are now available for 3-in. and 4-in. diameter wafers, with a 6-in. (150mm) wafer currently in development — adding that larger wafer sizes standard for silicon are achievable, since the fabrication process isn’t limited by bulk SiC sizes.

“While GaN on both silicon and silicon carbide is part of our existing epiwafer product line for high-power applications, SopSiC gives our customers a significantly better performing solution than silicon — and a considerably less expensive solution than SiC,” said Jean-Luc Ledys, COO of Picogiga, in a statement.

In 2005 the HYPHEN project, which includes Picogiga as well as other European academic and R&D consortia participants, compared GaN on bulk silicon and GaN on bulk SiC with GaN grown on two composite engineered substrates, SopSiC and SiC on polycrystalline SiC (SiCopSiC). Examined with MOCVD and MBE techniques, the GaN-composite substrate materials showed critical performance factors (e.g. crystal quality, mobility, and surface morphology) that equaled or exceeded current materials.

The results also suggested the composite substrates were better attuned to pilot production yields and repeatability. Preliminary results showed epitaxy of GaN HEMT on SopSiC composite substrates were more reliable than on conventional silicon substrates, in addition to being cheaper and better suited to high volumes for frequency scale of <10GHz.


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