Moore’s Law looked like it was in trouble a few years ago. The prediction by Intel’s cofounder that the number of transistors on a square inch of integrated circuit would double every year started to tire, so the timing got revised to every year and a half. This is now the accepted definition of Gordon’s rule, and most electronics pundits expect it to hold up for around another 20 years.
But, then what? Well, if nanotechnology has its way, Moore’s Law could get a massive life extension.
Enter carbon nanotubes, which will be used to create high-density, nonvolatile memory chips. Carbon nanotubes are tiny (10 atoms wide), stronger than diamond, and perform the functions of wires and transistors with better speed, power, density, and cost.
Essentially, these wires of pure carbon feature nanometre diameters and lengths of many microns. A single-walled carbon nanotube (SWNT) is simply a single-atomic-layer-thick sheet of graphite (called graphene) rolled into a cylinder. Multiwalled carbon nanotubes (MWNTs) consist of several concentric nanotube shells.
It’s the electronic properties of the graphene sheet that creates the electronic properties of carbon nanotubes. Graphene is a zero-gap semiconductor. There’s a bandgap for most directions in the graphene sheet, and electrons aren’t free to flow along those directions unless they’re given extra energy. However, in certain directions graphene is metallic and electrons flow along those directions.
When graphene is rolled to make a nanotube, though, a special direction is selected, which is along the axis of the nanotube. Sometimes this is a metallic direction, and sometimes it’s semiconducting, which means that certain nanotubes are metals and others are semiconductors. Since both metals and semiconductors can be made from the same all-carbon system, nanotubes are ideal candidates for molecular electronics technologies.
Dutch breakthrough
Dutch researchers at the Delft University of Technology created a transistor made from a nanotube that toggles on and off with the flow of a single electron. It’s said to be the first such single-electron transistor (SET) to operate at room temperature. Scientists are convinced it represents a leap forward in the quest to create ever-smaller electronic components.
Ordinary transistors rely on the motion of millions of electrons that, when compressed together, generate a lot of heat. As a result, these transistors can only be shrunk down to a certain size. A single electron switch makes it possible to avoid this size barrier.
These devices consist of a metallic island separated from two electrodes by a surrounding area (referred to as a “sea”) of material. Voltage applied to a gate on the island makes it possible for electrons to tunnel through the surrounding sea and hop on or off the island. But the scientists also discovered that heat could supply the electrons with enough energy to island hop.
Delft’s researchers skirted the heat issue by making their SET so tiny—one nanometre wide and 20 nanometres long—that heat fluctuations proved irrelevant. They used the tip of an atomic-force microscope to put sharp bends in a single carbon nanotube. The function of these bends was to provide a way to regulate the number of electrons passing through the SET.
Single-molecule circuit
In another research effort, IBM scientists created a working computer circuit within a carbon nanotube. The nanotube was used as a voltage inverter, or NOT gate, which is one of the three fundamental types of logic gates that all computers rely on.
Carbon nanotubes are now seen by IBM researchers as a serious candidate to replace silicon when current chip features can’t shrink any further, a barrier that’s expected to be reached in approximately 15 years. Again, this lends to the hope that not only will Moore’s Law continue, but may also receive an upgrade thanks to nanotechnology.
IBM research work has already resulted in the creation of carbon- nanotube, p-type transistors. In this development, instead of electrons, current is carried by "holes" that lack electrons. To make a NOT gate—which basically flips an incoming item of binary code from a zero to a one or vice versa—requires ntype transistors in which electrons pass the current. The breakthrough came when IBM recently found that they could convert p-type nanotube transistors into n-type transistors simply by heating them in a vacuum.
Not only were the scientists able to convert an entire nanotube, but they also discovered they could convert just part of a nanotube. As a result, they were able to build the carbon nanotube NOT gate.
To make the device, the nanotube was placed over gold electrodes to generate two ptype transistors in series. An insulated layer of polymethyl methacrylate and lithography techniques were used to create a window, which exposes part of the nanotube. Potassium passing through the window converts the one p-type transistor to an ntype transistor.
Of particular note, researchers found that the current in the carbon- nanotube NOT gate came out stronger than it went in, a highly desired feature for circuit design.