Previously, combining graphene with other materials has degraded the speed of the resulting electronics. The IBM group prevented this by making sure other materials didn’t contact the graphene in a harmful way. They made arrays of graphene transistors on the surface of silicon carbide wafers coated with graphene. They then etched away the extra graphene surrounding the transistors, leaving a clear surface that was easier for metal inductors to stick to. Ensuring separation between the graphene transistors and the metal inductors also prevented degradation of the transistor’s electrical properties.
The resulting circuits operate at 10 gigahertz—much faster than previous graphene circuits. Lin concedes that they are less reliable than the state of the art silicon frequency mixers but says they expect to close that gap soon.
The IBM researchers plan to make them on the scale of tens rather than hundreds of nanometers. “They can easily be ten times smaller, which would help us surpass the record,” says Lin. “We haven’t seen the limits of graphene devices in terms of speed—we think they can get into the terahertz range.”
The next step is to improve the reliability of the circuits, says Xiangfeng Duan, professor of chemistry at the University of California, Los Angeles. “The signal comes out weaker at the other end,” he notes. “Improving the transistors will help get better circuit performance.”
The IBM group is working on this problem, and is developing more complex graphene integrated circuits. Lin says the method used for the frequency-mixer circuits will work for other types of circuits. “This is the first step towards a new level of potential,” he says. “Perhaps we won’t see the real impact of graphene for another five to ten years.”