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.
Siyuranga O. Koswata received his PhD at Purdue University in May 2008.
Purdue University, West Lafayette, IN