Modeling of Carbon Nanotube Field Effect Transistors

Abstract

In this thesis, models are presented for the design and analysis of carbon nanotube field-effect transistors (CNFETs). Such transistors are being seriously considered for applications in the emerging field of nanotechnology. Because of the small size of these devices, and the near- one-dimensional nature of charge transport within them, CNFET modeling demands a rig- orous quantum-mechanical basis. This is achieved in this thesis by using the effective-mass Schr¨ odinger Equation (SE) to compute the electron and hole charges in the nanotube, and by using the Landauer Equation to compute the drain current. A Schr¨ odinger-Poisson (SP) solver is developed to arrive at a self-consistent potential distribution within the device. Normaliza- tion of the wavefunction in SE is achieved by equating the probability density current with the current predicted by the Landauer Equation. The scattering matrix solution is employed to compute the wavefunction, and an adaptive integration scheme to obtain the charge. Overall convergence is sought via the Picard or Gummel iterative schemes. An AC small-signal circuit model, employing the DC results from the SP solver, is also constructed to obtain estimates of the high-frequency capabilities of the transistors. The DC results predict the unusual ambipolar behaviour of CNFETs reported in the literature, and explore the possibilities of using work-function engineering to tailor I-V characteristics for different device applications. The model qualitatively agrees with some experimental results in the literature, and gives confidence that the performance of coaxial devices, when they become available, will be well predicted by the models. In the AC regime, it was found that under somewhat ideal operating conditions the operating limit of these devices might just reach into the 1-10THz regime. In addition to the development of rigorous modeling procedures for CNFETs, a preliminary compact model is developed, in which some of the essence of the detailed model is distilled into a set of simpler equations, which may prove useful in guiding device design towards CNFETs for applications in nanoelectronics. ii