Hybrid Chips of Gallium Nitride and Silicon

Compound semiconductors, made of two or more elements—gallium nitride (GaN) or indium phosphide, for example—have attractive properties that silicon lacks. They can make faster transistors than silicon, handle more power, and emit or collect light more easily. For its part, silicon is inexpensive and plentiful, and decades of development have led to processes for making complex, nanoscopic circuits in silicon. It would be useful to mix devices made out of different materials on the same chip, but differing physical characteristics make it difficult to grow compound semiconductors on top of silicon.

Rather than try to grow another semiconductor on silicon, Tomás Palacios, an assistant professor of electrical engineering and computer science at MIT, opted to make a separate wafer of GaN and bond it to the silicon. He then uses standard photolithography to create circuits in both the silicon and the GaN. "It basically for the first time allows circuit and system designers to choose the best semiconductor material for each device they want in the chip," Palacios says. "They don't have to compromise anymore."

The basic problem with growing any nitride on the kind of silicon used in ICs is that the crystal lattices in each material are oriented differently, which causes defects to spread through the nitride that would render it useless. Palacios and his team grew a wafer of aluminum gallium nitride/gallium nitride on a different type of silicon, one cut along a crystal lattice that renders it useless for ICs but good for growing GaN. They coated the top of the nitride with a thin layer of an oxide called hydrogen silsesquioxane, then pressed a wafer of electronics-grade silicon wafer down on top of that and heated the wafer sandwich to 400 °C for an hour, gluing them together. Next, they etched away the unusable silicon at the bottom, then used standard processes to inscribe circuits on the good silicon on top, being careful to leave some of the wafer blank. After that, they removed silicon from the blank areas of the wafer, exposing the nitride, and inscribed circuits in that. The result was a fast-switching GaN device called a high electron mobility transistor, right next to an ordinary field-effect transistor made of silicon.