Study of thin film devices and organic biosensors: parasitic phenomena, modelling and characterization

Organic and Inorganic Thin-Film Transistors (O-TFTs and I-TFTs, respectively) have been widely studied during the last years, due to appealing properties such as low-cost fabrication processes, flexibility, lightweight and (semi-) transparency. Therefore, to help the study and development of such technologies, we presented and discussed a new simple and easy to use technique for parameter extraction in thin film transistors. We experimentally validate our procedure by performing a complete characterization of both organic and inorganic transistors featuring dihexyl-quaterthiophene and indium-gallium-zinc-oxide (IGZO) as semiconducting materials, respectively. However, the reliability of IGZO TFTs are not fully understood and for this reason, we studied the impact of stair-case gate bias stress on them and we estimated the breakdown voltage for different channel aspect ratios. Our results show that the breakdown voltage exhibits a remarkable dependence on the channel width, while exposing no, or marginal, dependence on the channel length. In order to ensure the accuracy of the above-mentioned results, the used model require that working principle of the analysed devices must be well known at priori. Unfortunately, in organic and amorphous electronic this hardly ever the case. In particular, we focused on the non-linear parasitic effects in the region between the Source/Drain electrodes and the transistor channel. We can represent this region as metal-insulator-metal (MIM) structure. Hence, we propose a model that can describe the parasitic voltage drop at the contacts of the OTFT and at the same time we explained the properties of the MIM devices. Furthermore, we proposed an enhanced model that consider also the effects of the surface roughness on the metal semiconductor interface, and, by means of simulations, we highlighted the macroscopic effect of the surface roughness. Among the thin film transistor technology, researchers have spent many efforts in the healthcare area, working with different polymers and small molecules where organic semiconductors are at the interface with an ionic solution. In addition, the improvement of water gated devices, such as organic electrochemical transistors and electrolyte-gated organic field effect transistors (EGOFETs), is paving the way to the development of new biosensors. Hence, we presented a general equivalent circuit model for the metal/organic semiconductor /liquid/metal system. To underline the importance of our model, we reported two cases of study of electrochemical impedance spectroscopy of devices featuring NaCl and MilliQ water as gate medium, showing that both cases can be considered as a particular case of the general model presented in this thesis. Among the different organic materials, TIPS-Pentacene was recently employed to make EGOFETs, which are promising devices for biosensing applications. For this reason, we fabricated EGOFETs using TIPS-pentacene as active material. Despite the organic semiconductor being deposited in air by drop casting, our EGOFETs showed performance comparable with state-of-the-art technologies. In addition, we successfully investigated, the biocompatibility of the material, promoting the use of TIPS-pentacene-based EGOFETs for biosensing applications. Such devices can be used also as Reference-Less EGOGET (RL-EGOFETs) that are a new candidate for in vivo stimulation and recording of cells activity. Therefore, we characterized the fabricated EGOFETs in Reference-Less configuration, shedding light on the self-polarization mechanism, demonstrating that EGOFETs can feature a field-effect behavior even without the presence of the gate electrode. In summary, the works and the results of this thesis allowed a deeper and accurate study of thin film devices. Hence, we believe that the results here represented could help the in improvement of state of art devices and in the development of new devices.