NOVEL STRUCTURES and materials such as multiple gate MOSFETs, carbon nanotube field-effect transistors (CNT-FETs), and molecular based transistors, are expected to be introduced to meet the requirements for scaling. CNT-FETs have been considered in recent years as potential alternatives to CMOS devices due to excellent electronic properties of carbon nanotubes (CNTs). Some of the interesting electronic properties of CNTs are quasi-ballistic carrier transport, suppression of short-channel effects due to one-dimensional electron transport, nearly symmetric structure of the conduction and valence bands, which is advantageous for complementary applications, and high resistance against electro-migration. Since CNTs can be both metallic or semiconducting, an all-CNT electronics can be envisioned.
To explore the physics of CNT-FETs and to find methods to improve the functionality and performance of these devices we performed self-consistent quantum mechanical simulations. The non-equilibrium GREEN's function (NEGF) formalism is used in this work. It provides a very powerful technique for evaluating properties of many-particle systems both in thermodynamic equilibrium and also in non-equilibrium situations.
The numerical implementation of the outlined method is presented. Methods to reduce computational cost and memory requirement are discussed. Employing such techniques allows one to perform simulations in a reasonable amount of time, which is essential for large-scale applications such as device optimizations. For accurate analysis we solved the quantum transport equations with the POISSON equation self-consistently. To solve the system of equations we used an iterative method, the convergence of which is a critical issue. We analyzed the convergence behavior of self-consistent simulations and propose methods to improve the convergence behavior.
The numerical methods are implemented in the multi-purpose quantum-mechanical device simulator VIENNA SCHRÖDINGER-POISSON (VSP) solver, which has extensively been applied to study CNT-FETs. Based on simulation results one can obtain a deeper insight into device operation and its dependence on material and geometrical parameters.
We investigated the ambipolar conduction of CNT-FETs, which deteriorates the device characteristics. Based on the results we propose a double-gate structure to suppress the ambipolar behavior. In this device type carrier injection at the source and drain contacts are controlled separately. The first gate controls carrier injection at the source contact and the second one controls carrier injection at the drain contact, which can be used to suppress parasitic carrier injection.
Reduction of ambipolar conduction of single-gate devices has been studied. We show that the performance of single-gate CNT-FETs can be considerably improved by optimizing the gate-source and gate-drain spacer lengths. The results indicate that the exploited effects are very different from that in conventional MOSFETs.
Finally, the effect of electron-phonon interactions on the device characteristics is discussed in detail. In agreement with experimental data, our results indicate that scattering with high energy phonons reduces the on-current only weakly, but can increase the switching time considerably due to charge pileup in the channel. Scattering with low energy phonons can reduce the on-current more effectively, but has a weaker effect on the switching time. In a CNT at room temperature scattering processes are mostly due to electron-phonon interaction with high energy phonons. Therefore, the on-current of CNT-FETs can be close to the ballistic limit, whereas the switching time is found to be significantly below that limit.
M. Pourfath: Numerical Study of Quantum Transport in Carbon Nanotube-Based Transistors