Field-effect transistors made with carbon nanotubes rather than silicon have other advantages as well. Silicon transistors have a doped channel. As they are scaled down, the doping of the channel has to increase proportionately, while its volume decreases. The fluctuations in the number of dopants in this small volume from transistor to transistor produce important differences in their switching properties and degrade the overall performance of the system. Similar degradation of performance occurs because of variations in other parameters (such as the channel length), which increase with decreasing size. In contrast, nanotube transistors can operate even without dopants and are less sensitive to differences in channel length. Rather, they depend on the diameter of the tube and the degree of twist of the honeycomb pattern. And both of these qualities are determined by the chemical synthesis of the nanotubes.
I should point out that carbon nanotubes do not necessarily allow the fabrication of devices with channel lengths of just a few nanometers, because limitations such as those imposed by tunneling from source to drain will afflict nanotube transistors just as they do silicon devices. Moreover, the lengths of nanotube devices are still likely to depend on the ability of lithographic tools to print small structures on wafers. Nevertheless, they will allow us to build, with relative ease, devices with superior performance than that of ultrasmall silicon devices.
The ability to emit and detect light is another advantage of nanotubes. This feature raises the possibility of a future optoelectronics technology based on carbon nanotubes.
When the thickness of the gate insulator of a nanotube transistor gets sufficiently small, the transistor becomes ambipolar--that is, electrons conduct the current when the gate voltage is positive, and holes conduct it when the gate voltage is negative. Such ambipolarity is undesirable in electronic applications, and we have shown that it can be eliminated through proper design of the gate. But it is very valuable in optoelectronic applications.
Because of the ambipolar nature of a nanotube, under appropriate bias conditions, electrons and holes can enter the channel simultaneously from the opposite ends of the device. And when they meet, they can release energy in the form of heat or light. To be more precise, what is really happening is that electrons fall from the conduction band to the valence band, releasing the band-gap energy in the process [see illustration, ].
In 2003 my team at IBM produced the first single-molecule, electrically controlled light source. Unlike ordinary light-emitting diodes, this carbon-nanotube light source does not rely on dopants to create light. Moreover, a diode is a two-terminal device, while the nanotube light source has three terminals. It is, in effect, a new addition to the electronics bestiary: a light-emitting transistor. Using its third terminal, we can control not only the intensity of the emitted light but also the position of the emitting spot along the length of the nanotube [see image, ].
The energy that the electron gives off when it falls from the conduction band into the valence band--the band-gap energy--determines the wavelength of the emitted light. And as we discussed above, it is the diameter of the nanotube that determines the band gap. So we can make nanotube light sources with different wavelengths by using nanotubes with different diameters. We have also been able to perform the reverse process: to generate an electrical current (and voltage) by exposing the carbon-nanotube transistor to light. Thus, we have both a molecular light source and a light detector.
Despite the spectacular properties of carbon nanotubes, we will have to overcome many serious hurdles before we can create an electronic nanotechnology based on them. First, we need an approach for making them that leads to a homogeneous material. Current techniques produce a mixture of semiconducting and metallic nanotubes with different diameters and different amounts of twist in their structures. If we are to make ICs out of nanotubes, we must be able to control completely the nature of the nanotubes we create.
Recently, there has been significant progress in the area of selective synthesis--the process of making nanotubes with specific diameters and twists. Samples containing only a small number of different nanotubes with similar diameters have been made by proper selection of catalysts, starting materials, and reaction conditions. At the same time, researchers are also coming up with chemical and physical techniques for separating the different types of nanotubes after they have been made.
The next important step will be the integration of carbon-nanotube devices into complex CMOS-type circuits, because they will become the building blocks of a new generation of nanotube ICs. This effort will likely employ a mixture of techniques. We may apply lithographic techniques similar to those used for patterning silicon ICs to define the overall structure of the chip. To build the transistors themselves, researchers are working on ways to let the nanotubes assemble themselves in just the right configurations. These self-assembly techniques are still in the early stages of development. Then, too, taking into account carbon nanotubes' unique properties may lead to new types of circuits that are faster and smaller than the standard circuits used in ICs today.
As the dimensions of silicon CMOS transistors continue to shrink well into the next decade, problems resulting from increasing power dissipation, leakage currents, and variations in device parameters will continue to rise. If all goes well, carbon-nanotube electronics will be poised to take over before the problems encountered by the continual downscaling of silicon CMOS dimensions become insurmountable.
To Probe Further
"A general reference on carbon nanotubes is Carbon Nanotubes: Synthesis, Structure, Properties and Applications, eds. M.S. Dresselhaus, G. Dresselhaus, and Ph. Avouris (Springer-Verlag, Berlin), 2001.
"Carbon nanotube electronics," by Ph. Avouris, J. Appenzeller, R. Martel, and S. Wind, Proceedings of the IEEE, Vol. 91, 2003, pp. 1772-84, is a general discussion of the electrical properties of carbon nanotubes.