Current transport modeling of carbon nanotube field effect transistors for analysis and design of integrated circuits

The purpose of this study was to develop a complete current transport model for carbon nanotube field effect transistors (CNT-FETs) applicable in the analysis and design of integrated circuits. The model was derived by investigating the electronic structure of carbon nanotubes and using basic laws of electrostatics describing a field effect transistor. We first derived analytical expressions for the carrier concentration in carbon nanotubes for different chiral vectors (n,m) by studying and characterizing their electronic structure. Results showed a strong relation to the diameter and wrapping angle of carbon nanotubes. The charge distribution in a CNT-FET is characterized from the charge neutrality and potential balance conditions. Mathematical techniques are used to derive analytically approximated equations describing the carbon nanotube potential in terms of the terminal voltages. These equations are validated by comparing them with the respective numerical solutions; furthermore, the expressions for the carbon nanotube potential are used to derive current transport equations for normal and subthreshold operations. Threshold and saturation voltages expressions are each derived in the process and the I-V characteristics for CNT-FETs are calculated using different combinations of chiral vectors. Results showed a strong dependence of the I-V characteristics on the wrapping angle and diameter of carbon nanotubes, as expected from the carrier concentration modeling. Results were also compared with available experimental data showing close agreement within the limitations and approximations used in the analysis. In addition, the current model equations were used to generate the voltage transfer characteristics for basic logic circuits based on complementary CNT-FETs. The voltage transfer characteristics exhibit characteristics similar to the voltage transfer characteristics of standard CMOS logic devices, with a sharp transition near the logic threshold voltage depending on the input conditions. A small-signal radio frequency (rf) model was also developed and it is shown to have cut-off frequencies in the upper GHz range with a strong dependence on the chiral vector and corresponding transconductance (gm). Finally, due to the rapid growth of carbon nanotubes as bio- and chemical sensing devices, we have also presented, using our current model equations, possible methods to interpret and analyze CNT-FETs when utilized as biosensors