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. Rather than move them around, the team simply identified where they were and then used standard lithography techniques to connect the nanotubes to wiring.Īll this was then embedded in a layer of zirconium oxide, after which flakes of MoS 2 were placed above the carbon nanotubes. The carbon nanotubes were made first, through chemical vapor deposition onto a silicon substrate. So the production procedure for the tiny transistor was a bit involved. Unfortunately, two of these materials-MoS 2 and carbon nanotubes-are extremely difficult to make where you want or to move into place when you're done. The wires for the current source and drain were made of nickel. To get something of the appropriate size, the team making the device turned to carbon nanotubes. To get a functioning transistor, however, you need more than just a semiconductor like MoS 2: you need a gate to control whether it conducts or not. Plus, MoS 2 naturally forms sheets that are just a single atom thick, making it relatively easy to make incredibly small devices. 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. In this material, electrons move as if they were heavier than they are in silicon. MoS 2 offers a potential solution to this. 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. This unwanted movement causes an increase in current leaking across transistors when they're supposed to be off. 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. That property is the mobility of electrons within silicon. The idea behind the work is that a property of silicon we normally view as beneficial becomes an issue once things get small enough. But the work validates that MoS 2's properties can allow us to shrink electronics down below silicon's limits. Unfortunately, other parts of the hardware are quite a bit larger than that, and we have no way of producing these in bulk yet. Now, a large collaboration has taken a different material-molybdenum disulfide, or MoS 2-and used its distinctive properties to craft a transistor that has a gate size of just one nanometer. That's led to research into alternative materials such as carbon nanotubes. Below about five nanometers, quantum effects make their behavior unpredictable. (Early satellites such as Echo had been passive radio reflectors, containing no amplifying transistor circuits to be affected by radiation.Conventional silicon-based electronics are rapidly approaching a fundamental barrier. The first active communications satellite placed in orbit by the United States, Telstar (1962), failed when its transistors were exposed to unexpected levels of ionizing radiation. Ionizing radiation, furthermore, may cause transistors to fail by permanently altering the crystal structure. Usually these effects are unwanted, because one does not want the properties of a circuit to vary with temperature. Heat, light, or ionizing radiation may all increase the semiconductor's electrical conductivity by liberating these electrons to support current. They are not bound too tightly to break loose if given a small amount of extra energy, but cannot wander easily through the crystal. Covalent bonding also prevents these outermost electrons from moving through the crystal (i.e., flowing as an electric current) as easily as do the conduction electrons in metals. This sharing holds the atoms together by the process termed covalent bonding. That is, each atom is held in its place in the crystal's orderly structure because each atom shares its four outermost electrons with the outermost electrons of four nearby atoms. Each atom in a silicon or germanium crystal lattice has four atoms as close neighbors.
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