Carbon nanotube based field-effect transistors: Comparison between atomistic quantum transport and numerical device simulation

We study carbon nanotube based field-effect transistors (CNTFETs) by means of two different approaches: numerical device simulation (NDS) based on the effective mass Schrödinger equation and atomistic quantum transport simulation based on the non-equilibrium Green's function formalism (NEGF). The required parameters for the NDS model are extracted from density functional theory data. An all-carbon CNTFET with n-doped source- and drain-electrodes in a gate-all-around geometry is investigated. The NDS predicts a band-to-band tunnel current once the valence band edge is shifted to the Fermi energy. This increases the off-current and leads to slightly ambipolar behavior. Using the NEGF on the other hand, localized states inside the channel can be observed because a potential well is created by the gate. As a result, the band-to-band tunnel current is suppressed and improved transistor properties are predicted by NEGF calculations. By varying the channel length, we demonstrate the potential of the studied CNTFET for future applications, which shows an on/off current ratio above 106 and a subthreshold swing below 80 mV/dec down to channel lengths of about 8 nm.