What happens inside an atomically thin transistor?

Have you ever wondered what happens inside an atomically thin semiconductor? Well, wonder no more: a team of physicists at the University of Texas at Austin has, and they’ve written about it in the Proceedings of the National Academy of Sciences.

They’ve also discovered that an essential function for computing may be possible within a space so small that it’s effectively one-dimensional.

In the PNAS paper, the researchers describe seeing the inner workings of a new type of transistor that is two-dimensional.

 

In this visualisation of what happens inside a 2D transistor made of a promising new material called MoS₂, electric currents appear initially at the outer edges and then inside of the device. Thread-like flaws can be seen in the interior part of the transistor.

Transistors act as the building blocks for computer chips, sending the electrons on and off switches required for computer processing. Future tech innovations will require finding a way to fit more transistors on computer chips, so experts have begun exploring new semiconducting materials including one called molybdenum disulfide (MoS₂). Unlike today’s silicon-based devices, transistors made from the new material allow for on-off signalling on a single flat plane.

Keji Lai, an assistant professor of physics, and a team found that with this new material, the conductive signalling happens much differently than with silicon, in a way that could promote future energy savings in devices. Think of silicon transistors as light bulbs: The whole device is either turned on or off at once. With 2D transistors, by contrast, Lai and the team found that electric currents move in a more phased way, beginning first at the edges before appearing in the interior. Lai said this suggests the same current could be sent with less power and in an even tinier space, using a one-dimensional edge instead of the two-dimensional plane.

“In physics, edge states often carry a lot of interesting phenomenon, and here, they are the first to turn on. In the future, if we can engineer this material very carefully, then these edges can carry the full current,” Lai said. “We don’t really need the entire thing, because the interior is useless. Just having the edges running to get a current working would substantially reduce the power loss.”

Researchers have been working to get a view into what happens inside a 2D transistor for years to better understand both the potential and the limitations of the new materials. Getting 2D transistors ready for commercial devices, such as paper-thin computers and cell phones, is expected to take several more years. Lai said scientists need more information about what interferes with performance in devices made from the new materials.

“These transistors are perfectly two-dimensional. That means they don’t have some of the defects that occur in a silicon device. On the other hand, that doesn’t mean the new material is perfect,” said Lai.

Lai and his team used a microscope that he invented and that points microwaves at the 2D device. Using a tip only 100 nanometres wide, the microwave microscope allowed the scientists to see conductivity changes inside the transistor. Besides seeing the currents’ motion, the scientists found thread-like defects in the middle of the transistors. Lai said this suggests the new material will need to be made cleaner to function optimally.

“If we could make the material clean enough, the edges will be carrying even more current, and the interior won’t have as many defects,” Lai said.