In the race to create ever-tinier electronic devices, nanowires are looking like a better bet. This month a team of Harvard researchers disclosed that they had created several functional nanoscale semiconductor devices, including the world's smallest bipolar transistor.

Using silicon nanowires, semiconductive filaments only 20 nanometers wide, researchers in the lab of Harvard professor Charles Lieber also built a nanoscale diode and an inverter, the first devices ever assembled from both positive- and negative-type nanoscale semiconductors.

These devices represent "a step toward a 'bottom-up' paradigm for electronics manufacturing," Lieber and graduate student Yi Cui wrote in the Feb. 2 issue of Science. In bottom-up manufacturing, electronic building blocks will be synthesized in volume and then assembled to create complete devices. "The use of nanoscale structures as building blocks for self-assembled structures could potentially eliminate conventional and costly fabrication lines," Lieber and Cui wrote.

Researchers in Lieber's lab have demonstrated one application for these devices: tiny light-emitting diodes made of crossed nanowires. Another relatively near-term application may be nanoscale sensing, a field in which others have already made great strides (see "Nanotech Goes to Work").

Twice as Nice

Other research teams, including Lieber's, have previously built nanoscale electronic devices that use only one type of semiconductor. Last year, Lieber's team made a nanoscale field-effect transistor from silicon nanowires and conductive metal (other researchers have made field-effect transistors from carbon nanotubes). Bipolar transistors make better switches and are fundamental to modern microprocessors, but unlike field-effect transistors, they require both positive- and negative-type semiconductors.

To create nanowires with both types of semiconductors, the Harvard researchers "doped" (chemically modified) the silicon filaments with either boron or phosphorus. According to computer architect Phil Kuekes, who works on related research at Hewlett Packard Labs in Palo Alto, California, this was a major step. "The problem is that you can't dope them by conventional techniques," Kuekes told technologyreview.com. "There was a lot of concern that doping would make the wires too brittle."

Next, Lieber and Cui used fluid to lay down the nanowires in a crisscross pattern (see "Nanowires Cross the Line"). Using an electron microscope, they looked for patterns suitable to their purpose and then connected the nanowires to much larger electrical contacts, creating functional semiconductor devices.

Early Days

However, the Harvard authors and their colleagues agree that mass production of nanoscale electronics is still many years away.

"This is something that's ultimately going to work, but there are very large barriers to introducing such technology in the near term," Lieber told technologyreview.com.

Chad Mirkin, director of the Institute of Nanotechnology and Center for Nanofabrication and Molecular Self-Assembly at Northwestern University, agreed with Lieber. According to Mirkin, the largest obstacle to useful nanodevices lies in individually addressing and controlling each transistor.

"Once you make these structures, how do you interface them with the macroscopic world?" Mirkin asked. "Until we address this question, we have a long way to go."