Modeling and Simulation of CNT based tunnel FET

Abstract

MOSFETs are commonly used in high speed integrated circuits and are yielding smaller, faster and more functions at lower cost. Various problems exist with scaling of MOSFET devices including shorl channel effects such as drain induced barrier lowering, parasitic capacitance, velocity saturation e.t.c. which limit the performances of MOSFETs. Scaling issues of MOSFET devices lead to lower ON to OFF current ratio limited by temperature constrained 60mV/dec subthrcshold swing. A new type of device called 'Tunncl FET" is predicted to overcome these difficulties. TFET can beat 60m V/dec sublhreshold swing of MOSFET. In tunnel FET, carriers are transported by band to band tunneling and its OFF current is very low. This makes it ideal candidate for ultra-low power electronics. Since the tunneling FET relies on the band to band tunneling mechanism, the TFET using conventional material like silicon have very low on state current due to its large indirect band gap. Carbon nanotube has a J)otential to be the appropriate channel material in this new technology due to its unique quasi one dimensional properly such as high electron mobility and high Fermi velocity. This new type of device requires rigorous analysis with various structures and parameters and hence to find out optimum condition for practical device realization, in this thesis we proposed and simulated two double gate CNTTFF.T structures: (i) Oxide only over the channel (OXOC structure) (ii) Oxide extended over source to drain (OXESD structure), taking into account of different device parameters including dielectric strength and thickness of gate oxide materials, channel length, doping concentration and gale underlap. Here the transtr characteristics, on/off current (h)N/h]:J) ratio and subthreshold slope of the device are studied using Non Equilibrium Greens Function (NEGF) formalism in tight binding frameworks. The results are obtained by solving the NEGF and Poisson's equation selfconsistently in NanoTCAD ViDES environment and found that OXOC structure shows significantly better performance than OXESD structure in all cases having highest ON current of 4046 j..tA/tm and suhthreshold swing of 10. 19 mV/decade so far reported for CNT TFET. The results obtained from the simulation for both of the device structures are explained by the energy band diagram and electric field. The presented study is expected to be useful for realizing the switching device capable of operating at high speed and low power applications.