Silicon Carbide Nanomechanical Switches
NEMS switches can operate at extremely high temperatures, endure millions of cycles, and could lead to low-power logic
The end of Moore’s Law feels closer than ever. Chip-industry leaders are going to strange lengths—fin-shaped transistors and exotic semiconductors—to keep delivering better integrated circuits every year, and some alternatives are starting to look attractive. A number of research teams are working on substituting the transistor switches that form an IC’s logic and memory circuits with nanoscale electromechanical switches, or NEMS. One obstacle to real-world implementation has been that these devices wear out quickly and then break, but new findings presented at the International Electron Devices Meeting in Washington, D.C., yesterday show that these switches can actually function for days or weeks in air, putting them closer to a commercially viable life span.
Engineers at Case Western Reserve University, in Cleveland, created silicon carbide mechanical switches that could open and close tens of millions of times during multiple days of testing. Previous research conducted at Case Western and in other labs focused on achieving power and temperature goals and involved only switches that functioned properly for a handful of cycles. What’s more, the nanoswitches in these trials performed comparably in ambient air (about 25 °C) and in high-temperature conditions (500 °C).
“The mechanical relay was replaced after the Second World War by the newly invented solid-state transistor,” says Philip Feng, a NEMS engineer at Case Western who oversaw the study. “But now it’s actually worth revisiting relay technology and using our new micro-/nanofabrication technology to make really small mechanical switches with very close to ideal switching characteristics.”
One advantage to nanomechanical switches is that they don’t consume power when they’re in their off state, as common MOSFETs do. (Although the complementary logic scheme used in processors today, called CMOS, ideally should not consume power except when switching, current does leak through the transistors in practice, wasting power.) But by using nanomechanical switches, logic gates can draw less power and be more efficient. “It switches very abruptly. When it’s on, it’s on. When it’s off, it’s totally off,” Feng says. The engineers used their switches in integrated logic gates to show their potential for digital computing at a broad range of voltages.
Tsu-Jae King Liu, a NEMS expert at the University of California, Berkeley, says that the potential advantage of silicon carbide nanoswitches over silicon transistors is the former’s ability to operate with zero leakage over a wide range of temperatures. Liu says that the Case Western results represent a significant advancement because “they show that these switches can operate in room air for many days with a lot of endurance.”
Liu notes some remaining challenges, though. The switches would ultimately need to be much smaller to be competitive with today’s transistors. She also points out that the devices have relatively high on-state resistance ( greater than 1 kilo-ohm) and require much larger operating voltage (15 volts) than transistors. Feng agrees, noting that scaling the operating voltage would be crucial as the technology moved closer to real-world applications. “These are baby steps,” he says. “I admit that all the devices are rudimentary, but their promise is high.”