Inelastic transport in carbon nanotube electronic and optoelectronic devices

Discovered in the early 1990's, carbon nanotubes (CNTs) are found to have exceptional physical characteristics compared to conventional semiconductor materials, with much potential for devices surpassing the performance of present-day electronics. Semiconducting CNTs have large carrier mobilities and a direct electronic bandgap, resulting in enhanced band-to-band tunneling (BTBT) as well as optical properties that could lead to novel electronic and optoelectronic applications. Therefore, detailed modeling and simulation of electronic transport in CNTs is required for a comprehensive understanding of the operation of CNT based devices. We have used the nonequilibrium Green's function (NEGF) formalism for dissipative quantum transport simulation of CNT field-effect transistors. Previous experiments have shown that BTBT in CNT-MOSFETs can lead to subthreshold swings below the 60mV/decade conventional limit, which makes these devices promising candidates for low-power applications. Our simulations indeed confirm this observation, and further show that this regime of operation is dominated by phonon-assisted tunneling which degrades desirable device behavior. A detailed investigation of a CNT based p-i-n tunneling transistor (TFET) geometry that has much favorable device characteristics is also presented. We observe less than 60mV/decade subthreshold swing for this geometry that leads to smaller off-state leakage and standby power dissipation compared to the conventional MOSFET operation. Under on-state performance, the drive current and the switching speed of p-i-n TFETs are dominated by the tunneling barrier properties. Interestingly, the switching energy of the p-i-n TFET is observed to be fundamentally smaller than that for the MOSFET at the quantum capacitance limit of operation. Finally, a study on the modeling and simulation of inelastic transport in a CNT based optoelectronic device using the semiclassical Boltzmann transport equation is presented. The optical emission in these devices is attributed to an excitonic process. Localized exciton generation under high-field conditions is explored, and detailed device optimization schemes are discussed. These devices have the potential for ultra-bright light emission, among many other optoelectronic applications.