Modelling of electrically controlled nanofilm gas sensors

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Juan J Velasco Vélez
185 g
211x149x4 mm

Semiconductor gas sensors detect chemical substances and transduce them into changes of electrical conductivity. This response arises from space charge region (SCR) modulation due to the presence of oxidizing and/or reducing species that inject or extract charge carriers or interaction with e.g. preadsorbed oxygen (heterogeneous sensing). These adsorption-desorption processes are described according to the Wolkenstein-Geistlinger theory in the frame of quantum mechanics. Depending on the interaction strength two cases are distinguished: physisorption (ie. of electrostatic nature) and chemisorption (lattice and gas molecule establishes a covalent bond). Two sub-cases are distinguished in chemisorption "weak" (if the bond is neutral) and "strong" (charge transfer between solid and gas). Usually the operation temperature in chemosensors is high. It hinders their use in mobile systems and makes integration into CMOS chips impossible. Thus, the application of an external electric field as adsorption-desorption inductor (electroadsorptive effect, EAE) is proposed.

A gas sensor based on a buried gate with a sensing layer thickness in the range of the Debye length is developed resembling a thin film transistor (TFT). With this configuration the electric field crosses the device, reaches the surface and modulates the surface states. An electric field controls the energy levels, in particular the Fermi level, which governs the reaction paths, allowing for a substantial reduction in the operation temperature. The device is electrically modelled within the framework of classic semiconductor theory together with the charge transfer model (CTM) which describes the charge exchanged between solid and gas.