Tight-binding versus effective-mass modeling of carbon nanotube FETs

Two different approaches have been compared for the simulation of carbon nanotube field effect transistors (CNT-FETs). They are both based on the self consistent solution of the Poisson and Schroedinger equation with open boundary conditions within the non-equilibrium Green’s function (NEGF) formalism. In the first approach, we assume an ideal cylindrical surface for the CNT and a constant effective mass (CEM) Hamiltonian. The second one is based on a tight-binding Hamiltonian with an atomistic (pz orbitals) mode space basis in a three-dimensional domain. An extensive analysis has been carried out by comparing the simulation results of a (13,0) CNT-FET from deep-subthreshold to ON regime. The main limitation of the CEM approach has been found in the calculation of transmission through the band-gap states, i.e., in the determination of the direct-tunneling currents.