Organic Field-Effect Based Devices for Pressure Detection
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Organic Field-Effect Based Devices for Pressure Detection Ileana Manunza1,2 and Annalisa Bonfiglio1,2 1 CNR-INFM S3 NanoStructures and BioSystems at Surfaces, Via Campi 213/a, Modena, 41100, Italy 2 Dept. of Electrical and Electronic Engineering (DIEE), CNISM-University of Cagliari, Piazza d'Armi, Cagliari, 09123, Italy ABSTRACT We describe pressure sensors realized starting from completely flexible organic thin film transistors (OTFTs). A flexible and transparent plastic foil (Mylar) is employed both as substrate and gate dielectric; gold source and drain electrodes are patterned on the upper side of the foil while the gate electrode lies on the opposite side; a vacuum-sublimed pentacene film acts as active layer. The pressure dependence of the output current has been investigated by applying to the gate side of the device a mechanical stimulus by means of a pressurized air flow. Experimental results show a reversible current dependence on pressure; further data analysis suggests that current variations are due to pressure-induced variations of mobility and threshold voltage. INTRODUCTION For a broad range of sensing applications there is an increasing demand for small, portable and low cost sensors. Transistor-based sensors are active devices: this implies that they can be electrically characterized in a more complex way than passive monoparametric devices; they are multiparametric sensors and offer the possibility of using a combination of variables in order to characterize their response to the parameter to be sensed. Moreover, active sensors combine in the same device both switching and sensing functions, and this allows to easily obtain a sensing matrix of limited size and optimized reliability. Although the mobility of organic semiconductors is very lower (even three orders of magnitude) than that of poly- and single-crystalline silicon, the slower speed is tolerable for most applications of large-area sensors [1]. Only a few examples of organic base devices for mechanical stimuli detection have been reported so far [1,2,3,4]. Here we propose a flexible OTFT structure for pressure detection. Our approach takes advantage of the favourable structure properties that allow to combine the sensor with any kind of substrate or to use it as a free-standing device. EXPERIMENT Bottom gate OTFT sensors [5], whose structure is shown in Fig. 1(a), were fabricated using a 1.6 µm thick Mylar sheet (Du Pont, dielectric constant of 3.3, dielectric rigidity of 105 V/cm ) as gate insulator and also as mechanical support of the whole structure [6]. Bottom contact gold electrodes (thickness 100 nm) with W/L=100 (W=5 mm and L=50 µm are the channel width and length, respectively) were patterned on the upper side of the flexible
dielectric, using a standard photolithographic technique while the gold gate electrode (thickness 100 nm) was patterned on the opposite side of the Mylar foil. To study the potential role of structural effects (in particular of contact/semiconductor interface) on the pressure sensitivity, we h
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