Conventional silicon-based electronics are rapidly approaching a fundamental barrier. Below about five nanometers, quantum effects make their behavior unpredictable. That's led to research into alternative materials such as carbon nanotubes. Now, a large collaboration has taken a different material—molybdenum disulfide, or MoS2—and used its distinctive properties to craft a transistor that has a gate size of just one nanometer.
Unfortunately, other parts of the hardware are quite a bit larger than that, and we have no way of producing these in bulk yet. But the work validates that MoS2's properties can allow us to shrink electronics down below silicon's limits.
The idea behind the work is that a property of silicon we normally view as beneficial becomes an issue once things get small enough. That property is the mobility of electrons within silicon. On the positive side, the electrons move with less resistance when we want them to, but they also move more readily when we don't want them to. This unwanted movement causes an increase in current leaking across transistors when they're supposed to be off. Once silicon features get small enough (that 5nm limit mentioned above), leakage becomes large enough that it's impossible to tell whether a transistor is on or off.
MoS2 offers a potential solution to this. In this material, electrons move as if they were heavier than they are in silicon. This slows them down, which limits device performance, but it also makes it much harder for them to randomly leak across a transistor even as the transistor size gets ever smaller. Plus, MoS2 naturally forms sheets that are just a single atom thick, making it relatively easy to make incredibly small devices.
To get a functioning transistor, however, you need more than just a semiconductor like MoS2: you need a gate to control whether it conducts or not. To get something of the appropriate size, the team making the device turned to carbon nanotubes. The wires for the current source and drain were made of nickel.
Unfortunately, two of these materials—MoS2 and carbon nanotubes—are extremely difficult to make where you want or to move into place when you're done. So the production procedure for the tiny transistor was a bit involved. The carbon nanotubes were made first, through chemical vapor deposition onto a silicon substrate. Rather than move them around, the team simply identified where they were and then used standard lithography techniques to connect the nanotubes to wiring.
All this was then embedded in a layer of zirconium oxide, after which flakes of MoS2 were placed above the carbon nanotubes. The authors tested a variety of flake sizes, but most of the work was done with one that was two atomically thin layers stacked on top of each other. The nickel source and drain wiring was then linked to the MoS2, completing the circuitry.
All the wiring was quite a bit larger than the tiny dimensions of the two main ingredients, so this is not a complete 1nm feature. But the resulting circuit was fully functional; the presence of current in the carbon nanotube allowed it to be switched on and off in a controlled manner. And the amount of current that transited across the MoS2 in the on state was 106 times larger than the current in the off state.
While it worked, even the researchers involved caution 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 have not yet packed these transistors onto a chip, and we haven't done this billions of times over." The concept, of course, is that materials other than silicon might help get us past the limits of today's transistors.
