Evaluation of the Vapor and Chemical Sensing Mechanism for Pentacene Field Effect Transistors
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Evaluation of the Vapor and Chemical Sensing Mechanism for Pentacene Field Effect Transistors Davianne A. Duarte, Deepak Sharma, Brian Cobb, and Ananth Dodabalapur Microelectronics Research Center, The University of Texas at Austin, Austin, TX 78758
ABSTRACT Properties such as semicondutor film grain size, morphology, and channel length are known to effect the sensing response in pentacene based organic thin film transistors (OTFTs). The sensing behavior for low and high mobility pentacene devices are reported here exhibiting different temperature dependent behaviors. The lower mobility OTFT exhibits an expected thermally activated response during alcohol testing with an increasing mobility with temperature along with a decreasing mobility at each temperature for increasing concentration. The higher mobility device exhibits a decrease in mobility with increasing temperature along with a decrease in mobility with increasing concentration at each temperature. In both sets of devices, the polar analyte produced reductions in drain current and shifts in threshold voltage. INTRODUCTION Thin film field-effect transistors (TFTs) provide a methodology for chemical and biological sensing by exhibiting a change in the tranpsort properties such as shifts in mobility, threshold voltage and conductivity. TFTs based on organic semiconductors (OTFTs) have been studied as vapor sensors due their ability to exhibit a high sensitivity to various vapors. This feature along with the potential for low cost fabrication have made OTFTs a promising alternative to existing inorganic and other technologies. Previous work has demonstrated the effects of carrier concentration, gate insulator, grain size (surface morphology), and channel length on the sensing response to analytes such as alcohols, which exhibit a dipole moment [1-4]. At low carrier concentrations, the added charge has the effect of producing an increase in current for the sensing response. At higher carrier concentrations the occurrence of trapping overwhelms the effect of the positive charge and you see and reduction in current. Torsi et al. and Wang et al. have demonstrated the correlation between vapor response and grain size [1,4]. The sensing response increases with decreased grain size which yields more grain boundaries or interaction sites. Device mobility is also related to grain size, higher mobility devices typically have larger grains and tighter molecular packing which, as we will show below, leads to a decreased sensing response. Low mobility OTFT devices were previously used in studies regarding transport phenomena in relation to vapor sensing including temperature dependent data taken in our group. In this work we studied the transport characteristics of low and high mobility devices utilizing temperature dependent measurements to gain insight into the trapping behavior and charge transport in the presence of polar analyte vapor.
EXPERIMENT A 150 nm thick thermal oxide was grown on a heavily doped n-type Si substrate as the gate dielectric. Pentac
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