Mechanism of Gas Sensing in Carbon Nanotube Field Effect Transistors
Gas sensors based on carbon nanotubes in the field effect transistor configuration have exhibited impressive sensitivities compared to the existing technologies. However, the lack of an understanding of the gas sensing mechanism in these carbon nanotube field effect transistors (CNTFETs) has impeded setting-up a calibration standard and customization of these nano-sensors for specified gas sensing application. Calibration requires identifying fundamental transistor parameters and establishing how they vary in the presence of a gas and influence the overall sensing behavior. This work focuses on modeling the sensing behavior of a CNTFET in the presence of oxidizing (NO2) and reducing (NH3) gases and determining how each of the transistor parameters, namely: the Schottky barrier height, Schottky barrier width and doping level of the nanotube are affected by the presence of these gases.Earlier experiments have shown that the carbon nanotube-metal interface is responsible for the observed change in the CNTFET response. The interface consists of the metal contact and the depletion region in the carbon nanotube. A change in the metal work function will change the Schottky barrier height, whereas doping of the depletion region will affect the Schottky barrier width and the doping level of the carbon nanotube. A theoretical model containing these parameters was systematically fitted to the experimental transfer characteristics for different concentrations of NO2 and NH3. A direct correlation between the measured changes in the CNTFET saturated conductance and the Schottky barrier height was found. These changes are directly related to the changes in the metal work function of the electrodes that I determined experimentally, independently, with a Kelvin probe system. The overall change in the CNTFET characteristics were explained and quantified by also including changes due to doping from molecules adsorbed at the carbon nanotube-metal interface through the parameters Schottky barrier width and the doping level.
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