Electron Phonon Interaction in Carbon Nanotube Devices
With the end of silicon technology scaling in sight, there has been a lot of interest in alternate novel channel materials and device geometry. Carbon nanotubes, the ultimate one-dimensional (1D) wire, is one such possibility. Since the report of the first CNT transistors, lots has been learned about CNT device physics, their scaling properties, and their advantage and disadvantages as a technology. Most of the analysis, however, treats the nanotube as a ballistic device or use some simplified form of phonon scattering. It is an experimental fact that phonon scattering is particularly strong in CNTs, yet every attempt to explain the experimental I-V using the theoretically calculated electron-phonon scattering rates has failed. In this work, we show that experimental I-V can be reproduced if the non-equilibrium phonon population is considered. In the first part of this work, a simple yet reasonably accurate method to calculate electron phonon scattering rates over the full band is developed, along with a direct solution scheme of the coupled electron-phonon Boltzmann transport equations (BTE). This coupled transport model is then applied to the study of metallic single walled carbon nanotubes, both supported on a solid substrate and suspended over a trench. It is shown that for on-substrate tubes, the current saturation at high bias is the result of the significant build up of optical phonons, which is captured by the phonon BTE. For suspended tubes, there is the additional self-heating effect which is responsible for the negative differential resistance (NDR). After showing the importance of hot-phonon and self-heating effects in metallic tubes and the success of the calculated scattering rates, this model is then applied to the assessment of CNT-MOSFETs. It is found that the self-heating does not play an important role in a single tube CNT-MOSFETs, but that hot-phonons have a significant effect, both on DC and AC characteristics.
Sayed Hasan received his PhD from Purdue in May 2007.
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Purdue University, West Lafayette, IN