Leakage and performance of zero-Schottky-barrier carbon nanotube transistors

Undoped carbon nanotube field-effect transistors (CNTFETs) with zero-Schottky-barrier contacts have the current-voltage response of field-effect transistors (FETs), but the physics of their operation is that of voltage-controlled tunnel barriers. The carbon nanotube (CNT) body itself provides the tunnel barrier. The leakage current in CNTFETs is a combination of both interband and intraband tunneling and this current can be significantly reduced by changing the CNT diameter as well as the CNT length and source/drain asymmetry. Source and drain extensions significantly reduce the leakage current and increase the ON/OFF current ratio. Asymmetry with the gate closer to the source further reduces leakage, improves the ON/OFF current ratio, decreases the switching time, and increases the cutoff frequency despite the higher gate capacitance. An ON/OFF current ratio of > 10(4) can be obtained from a 50-nm-long, 1.5-nm-diameter CNT with a 2 nm gate. The switching time is very small in the 0.1 ps range and the cutoff frequency is very high in the 4 THz range. Coulomb blockade is expected to block the interband resonant tunneling (ambipolar) leakage current so that the CNTFETs become effectively unipolar devices. Poisson's equation is solved self-consistently with the nonequilibrium Green's-function equations using a pi-bond model for the CNT. (c) 2005 American Institute of Physics.