Exceptional electronic and mechanical properties together with nanoscale diameters make carbon nanotubes (CNTs) candidates for nanoscale field effect transistors (FETs). The contact between metal and CNT can be of ohmic or Schottky type. Ohmic contact CNTFETs show better performance than Schottky contact devices theoretically and experimentally. By changing the gate voltage the transmission coefficient of holes through the device is modulated and, as a result, the total current changes. In short devices (less than 100 nm) carrier transport through the device is nearly ballistic.
To understand and improve the behavior of CNTFETs, a Schroedinger equation solver in the framework of non-equilibrium Green's function formalism (NEGF) has been integrated into the device simulator Minimos-NT. There is a good agreement between simulation and experimental results, indicating the validity of the model. The quasi-static approximation (QSA) was used to investigate the dynamic response of these devices. Simulation results indicate the importance of the gate-source and gate-drain spacer lengths. We have shown that by appropriately selecting the gate-source and gate-drain spacer lengths we can improve not only the ambipolar behavior and static characteristics but also the dynamic characteristics of the device. Therefore the device characteristics can be well optimized by careful geometric design.
All simulations were based on the assumption of cylindrical symmetry. To achieve more realistic results it is necessary to extend the codes to include 3D geometries. In order to study the static operation of these devices more deeply, we plan to include scattering into our simulations, which can be achieved by using Buetikker probes. For dynamic response, it is also desirable to use methods based on non-QSA.