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Ph.D. Defense Thursday, November 9, 2006 Bourns Hall A171 11:00am Title: Quantum Electron Transport in Nanoscale Devices Abstract: Due to aggressive down scaling, the silicon technology is expected to reach the ultimate size limit in a decade. An alternative material, the carbon nanotube, is at the top of the substituent list. The carbon nanotube is a direct band gap material with symmetric conduction and valence bands. One dimensional transport in the carbon nanotube is near ballistic at the nanometer scale. Moreover, the use of a high-K gate dielectric as an insulator does not degrade the mobility. Theses attractive features make the carbon nanotube a unique choice for future nano electronic applications. In this work, a full band quantum mechanical simulation model using pz orbital of carbon atom is developed to study the transport physics of carbon nanotube transistors, dielectric scaling issues, optimal device design, and the role of doping. Carbon nanotube field-effect transistors have the current-voltage response of a field-effect transistor, but the physics of their operation is that of a voltage controlled tunnel barrier. This tunnel barrier is provided by the nanotube itself. The leakage current of the transistor is found to be an important factor for the transistor performance. This leakage current is a combination of inter-band and intra-band tunneling, and it can be significantly reduced by changing the nanotube diameter, the nanotube length, and the use of asymmetric source/drain underlap with the gate closer to the source. Our study on dielectric scaling shows that the subthreshold behavior of the carbon nanotube transistor does not change when the dielectric is changed from high-K ZrO2 to low-K SiO2. The principal change is found to be the reduction of the parasitic gate capacitance, which is the main obstacle for faster operation of the transistors. A simple carbon nanotube on insulator top gate device structure is proposed that can further reduce the parasitic capacitance and hence improve the device performance. For doped nanotube transistors, we notice an optimal doping concentration that shows high performance. Inclusion of kinetic channel inductance becomes essential in the assessment of cut-off frequency for doped carbon nanotube transistors. |
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