Electronic Design

GaN-Based HEMTs Fabricated On 4-in. Silicon Wafers

To harness the high-frequency benefits of gallium nitride (GaN), Nitronex Corp. of Raleigh, N.C., has developed proprietary processing technology for fabricating GaN-based high-electron mobility transistors (HEMTs) on larger low-cost silicon wafers. This startup plans to fabricate these microwave transistors on inexpensive 4-in. silicon wafers.

Until now, industry efforts were limited to 2-in. wafers using silicon-on-insulator (SOI) and sapphire (Al2O3) substrates, which are comparatively expensive materials. GaN device makers believe a conventional silicon substrate would be a third alternative.

To fabricate the transistors, Nitronex employs a metal organic chemical-vapor-deposition (MOCVD) growth process. A proprietary transition layer is first grown on the silicon substrate, followed by the GaN and aluminum-gallium-nitride (AlGaN) layers. In essence, the AlGaN provides the heterostructure interface needed to obtain the two-dimensional electron gas with very high mobility. To minimize contact resistivity, this structure uses unique metal alloys for source and drain metallization contacts, while the gate employs a dissimilar metal (see the figure).

This patent-pending epitaxial growth and deposition technology is known as Pendeo. According to T. Warren Weeks, Nitronex's director of materials engineering, it reduces GaN crystal defects by more than 100,000 times relative to conventional growth techniques.

"Overcoming thermal and lattice mismatches between the materials, reducing stress, and dramatically lowering defect densities were the primary objectives of this Pendeo-epitaxy technology," he says.

"GaN cannot be grown by itself as a semiconductor crystal in an economically viable manner because of basic physical limitations," Weeks explains. "We addressed those issues to provide a commercially viable solution for growing GaN on large-area substrates."

Although detailed specifications of the HEMT transistors being developed were not disclosed, Nitronex claims that dc and RF device measurements have exceeded expectations. The company is expecting 10 to 12 dB of gain from these microwave transistors, with high efficiency and power density. Maximum power density reported for GaN transistors is approximately 10 W/mm. Plus, internal tests show that room-temperature two-dimensional electron gas mobilities exceed 1600 cm2/Vs.

"This is one of the highest electron mobilities reported for GaN transistors, demonstrating the high quality of the material used," Weeks says.

Targeting next-generation wireless communications applications, Ni-tronex is readying GaN-derived HEMTs in the 2- to 4-GHz frequency range. These transistors will require dual power supplies, one at 28 V or higher, and the other ranging from −5 to −10 V. The company anticipates initial prototypes and complete results by the end of next quarter. For that, it has built a pilot production line at its Raleigh facility. Nitronex also has partnered with a major packaging house to deliver these microwave devices in high-frequency, high-performance packages.

Efforts are under way for scaling the growth of GaN by Pendeo-epitaxy on substrate wafers with diameters beyond 4 in. This is critical to the economical manufacturing of GaN-based semiconductor devices.

For additional information, visit www.nitronex.com.

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