International Rectifier plans this year to introduce a power-device technology employing gallium nitride on silicon. The company grows an epitaxial layer of gallium nitride on a conventional silicon wafer, and the gallium nitride forms the active devices in that layer.

Gallium nitride and silicon have significantly different coefficients of thermal expansion, as well as mismatched crystal lattices. One of the key problems that researchers have encountered in using silicon as an underlying substrate is getting the gallium-nitride layer to stay attached under operating stresses. International Rectifier has developed interposer layers that solve this problem. The company also asserts that, once it has overcome the adhesion obstacle, the economics of silicon—as opposed to other substrate materials that researchers have used, such as sapphire or silicon carbide—are compelling and open the way to volume manufacture of power transistors.

Unlike other experimenters in gallium nitride in the context of power devices, another thread of research deals with the material as the basis for RF-power transistors. International Rectifier begins with low-voltage transistors—for applications such as dc/dc converters—and progresses to higher voltages. “Other [companies and researchers] have worked with gallium nitride for high-power, traction-type applications,” said Tim McDonald, vice president of emerging technologies, speaking at the Electronica 2008 trade fair in November in Munich, Germany. However, the company believes that silicon carbide will likely take over that segment and that gallium nitride has a lot to offer for the high-volume, lower-voltage-product segment. The company intends this year to market products based on 30V transistors, following those products with 120V devices in 2010, and 600V parts in 2011.

Gallium-nitride technology offers substantially lower transistor on-resistance than the on-resistance that MOSFETs currently achieve. Today’s FETs, McDonald says, are nearing some fundamental physical limits; with gallium nitride, a further reduction to approximately only 20% or even less of today’s best values is feasible. Gallium-nitride transistors have low stored charge and consequently low turn-off overshoot and transients. With the reduced transients, EMI (electromagnetic interference) also diminishes proportionally. The basic device in the company’s technology is an HEMT (high-electron-mobility transistor), a lateral device with conduction paths across the surface.