Conventional silicon-based electronics are rapidly approaching a fundamental barrier. Below about five nanometers, quantum effects make their behavior unpredictable. This has led to research into alternative materials such as carbon nanotubes. Now a major collaboration has used a different material: molybdenum disulfide, or MoS2-and used its distinctive properties to create a transistor with a gate size of just one nanometer.
Unfortunately, other parts of the hardware are a lot bigger than that, and we can’t mass-produce them just yet. But the work confirms that MoS2Thanks to the properties of silicon, we can shrink electronics below the limits of silicon.
The idea behind the work is that a property of silicon that we normally think of as beneficial becomes a problem once things get small enough. That property is the mobility of electrons in silicon. On the plus side, the electrons move with less resistance when we want them to, but they also move more easily when we want them to do not wants them to. This unwanted motion causes an increase in current leaking across the transistors when they should be turned off. Once the silicon functions become small enough (the 5 nm limit mentioned above), the leakage becomes so large that it is impossible to tell whether a transistor is on or off.
Moss2 offers a possible solution to this. In this material, electrons move as if they were heavier than in silicon. This slows them down, which limits device performance, but also makes it much harder for them to randomly leak across a transistor, even as the transistor continues to shrink. Plus, Moss2 naturally forms sheets that are just one atom thick, making it relatively easy to create incredibly small devices.
However, to get a functioning transistor you need more than just a semiconductor like MoS2: you need a port to check whether it is conducting or not. To get something of the right size, the team making the device switched to carbon nanotubes. The wires for the power source and drain were nickel.
Unfortunately, two of these materials – MoS2 and carbon nanotubes – are extremely difficult to make where you want or to put in place when you’re done. So the production procedure for the small transistor was a bit complicated. The carbon nanotubes were first made by chemical vapor deposition on a silicon substrate. Instead of moving them, the team simply identified where they were and then used standard lithography techniques to wire the nanotubes together.
All this was then embedded in a layer of zirconia, after which flakes of MoS2 were placed above the carbon nanotubes. The authors tested different sizes of flakes, but most of the work was done with two atomically thin layers stacked on top of each other. Next, the nickel source and drain wiring was coupled to the MoS2completing the circuit.
All the wiring was a good deal larger than the small size of the two main ingredients, so this isn’t a complete 1nm feature. But the resulting circuit was fully functional; the presence of current in the carbon nanotube allowed it to be turned on and off in a controlled manner. And the amount of current that passed through the MoS2 in the on position was 106 times greater than the off-state current.
While it worked, even the researchers involved warned people not to get too excited just yet. “This work demonstrated the shortest transistor ever,” said Ali Javey, the paper’s senior author. “However, it’s a proof of concept. We haven’t packaged these transistors on a chip yet, and we haven’t done this billions of times.” The concept, of course, is that materials other than silicon can help us push beyond the limits of current transistors.
Science2016. DOI: 10.1126/science.aah4698 (About DOIs).