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Picogiga samples composite substrates for GaN

Engineered wafers designed for low-cost GaN radio frequency IC manufacturing are available in 3-inch and 4-inch diameters - and a 6-inch format is in development.

Soitec subsidiary Picogiga has begun sampling the first composite substrate materials designed for manufacturing of high-power GaN-based radio frequency transistors.

The French substrate and epiwafer company says that the "SopSiC" wafers, short for silicon-on-polysilicon-carbide, are able to bridge the gap between the relatively cheap GaN-on-silicon approach, and the high-performance but costly use of crystalline SiC substrates.

GaN-based RF components are just emerging as a commercial reality, with RF Micro Devices recently signing its first supply deal for a military application (see related story). It uses SiC substrates currently.

However, the high-performance technology is still generally regarded as too expensive for widespread market penetration into applications such as broadband wireless infrastructure.

"SopSiC is an excellent example of how Smart Cut engineered substrates can be used to solve challenges for III-V applications," said Picogiga's COO Jean-Luc Ledys. "While GaN on both silicon and SiC is part of our existing epiwafer product line, SopSiC gives our customers a significantly better performing solution than silicon."

"In terms of dollars per Watt, SopSiC is an extremely attractive solution."

Initial characterization studies of the crystal quality, mobility and surface morphology of GaN epilayers grown on SopSiC under a European project have shown encouraging results, says Picogiga. Pilot production yield of HEMT devices is also said to be good.

Picogiga engineers the composite wafers using its Smart Cut process. This approach has proved hugely successful for silicon-on-insulator (SOI) substrates, which are now regularly used by silicon chip giant Advanced Micro Devices (AMD).

The SopSiC wafers comprise a bottom layer of polysilicon carbide, an insulating buried oxide layer, and a high-resistivity top layer of (111) silicon. GaN can be grown on the top layer using either MBE or MOCVD.

The layer of polysilicon carbide conducts away heat that is generated by the high-power HEMTs.

Unhindered by the difficulty of scaling up to larger diameters that has been encountered with single-crystal substrates, initial SopSiC samples are available in 3-inch and 4-inch format, with a 6-inch version currently in development.

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