Pushing the limits of carbon nanotube transistors

Carbon nanotubes are arrangements of carbon atoms in the form of thin, hollow tubes. Their physical and electrical properties have attracted much interest in recent years---carbon nanotubes are among the strongest materials currently known and they can perform both as active and passive components in nanoscale circuits. Their electrical properties are the focus of this thesis. The work presented here shows that carbon nanotube transistors can operate in the gate quantum limit and can be used as high-frequency mixers.; In the first experiment, an aqueous electrolyte solution was used as the gate electrode of a carbon nanotube transistor. This approach accomplishes the task of efficiently increasing the electrostatic capacitance between the nanotube and the gate via a thin, high-kappa dielectric (water). The total capacitance is shown to reach the quantum limit where charging is dictated by the energy level spacing in the nanotube. Additionally, the gate coupling is nearly ideal, as found from the subthreshold swing of ∼80 mV. The coupling results in record transconductances of ∼7 muS/nm, a result important to applications of carbon nanotubes as chemical and biological sensors.; The second experiment measures the high-frequency properties of carbon nanotube transistors by means of frequency mixing. A dc current results from the response of the device to an ac voltage. The amplitude of the current is measured as a function of the frequency of the input, revealing the existence of a cutoff between 1 and 10 GHz. The origin of the cutoff is not completely understood yet, since it is in order-of-magnitude agreement with the cutoff obtained from the contacts while its value does not seem to vary substantially from device to device, suggesting an external limiting factor. Despite the cutoff, the mixing response was measured up to 50 GHz. This is the highest frequency at which the electrical properties of carbon nanotubes have been measured to date.