Study of Carbon NanoTube Field Effect Transistors for NEMS

Abstract

The recent developments of Carbon NanoTube Field Effect Transistor (CNTFET) technology indicate the perspective of the Nanoelectromechanical systems (NEMS). Carbon nanotubes (CNT) are ideal candidates for NEMS due to their chemical and physical structures, low masses and exceptional stiffness. Study of NEMS devices in the light of quantum mechanics requires understanding the interplay between the physical, geometrical and electrical parameters of the system (Dang et al., 2006). An analytical representation of (CNT) based field effect transistor is developed for high frequency NEMS applications to examine the characteristics observed from the fabricated devices. The analytical models enable us to gain deep insights of the device performance and behavior. The developed analytical model of CNTFETs represents its viability into transistor applications for NEMS switches, RF circuits, memory cells, field emission displays, biomedical instruments etc (Polash & Huq, 2008). The metal-nanotube contacts in the CNTFETs are treated as Schottky barriers and analyzed by means of a ballistic model (Natori et al., 2005). The famous Landauer formula is used to calculate the conductance of the tube by relating the energy dependant transmission probability within the tight binding approximation of the CNTFET (Datta, 2000). Transmission function of the CNT is expressed in terms of the Green’s functions of the conductors and the coupling of the conductor leads. The Green’s function is incorporated with the transfer Hamiltonian approach to calculate the tunneling currents. The nonequilibrium Green’s function transport equation is solved iteratively along with a 2D Poisson equation to improve the numerical convergence. The charge density is calculated by integrating the 1D universal density-of-states along with the source-drain Fermi-Dirac distribution function over energy within the energy gap of the CNT. The calculations show that the proposed device can perform stable operation at high current levels (670 A/m). Upper limits of device characteristics are considered for the model. Degradation in measured data is observed due to the limitations in device fabrication technology and imperfect contact placement on the CNT. Commercialization of CNTs NEMS/sensors is a great challenge due to the price and size of such measuring equipments (Fujita et al., 2007) . When small bias current is used, resistance measurement of CNTs sensor becomes difficult and high accuracy current source and analogue to digital converter (ADC) are required to maintain a reasonable signal to noise ratio (SNR). Controlled growth of CNT is yet to be 21