Organic field-effect transistors (OFETs) promise for printed intelligence embedded into, and coated onto many different items that previously have been considered impossible to make electronically active. It is crucial that this technology is driven at low voltage and power. Also, we need to obtain solid understanding of the charge transport in organic semiconductors. Different materials [1] and architectures have been utilized as the probe to reveal the nature of charge transport along the transistor channel and to achieve low-voltage switching. In transistors operating according to field-effect or electrochemical principles, respectively, vacuum, air, [2] oxides, [3] high-permittivity dielectrics, [4] organic mono-layers, [5] and electrolytes [6] have successfully served as the medium to electronically separate the gate from the transistor channel, of which the latter three allow low-voltage operation. In particular electrolytes have attracted much attention lately since they generate very high electric fields at the organic transistor channel/electrolyte interface already at very low voltages, i.e., below 1 V. One issue with those devices is that electrochemical switching and field-effect modulation of the organic channel often coexist, [7] which result in transistors that are typically slow and that exhibit a great degree of hysteresis. Here, we report OFETs gated via pure water that operates entirely in the field-effect mode of operation. Our findings shed new light on low-voltage operating OFETs, their charge transport characteristics under exposure to water [8] and opens for sensor applications using water-gated OFETs as transducers in aqueous media.[9] Because of the simplicity and readiness of its production, it could also reveal a very helpful tool for rapid testing of new organic semiconductor compounds.Electrolyte (insulator)/semiconductor interfaces have attracted much attention during the last decades, in part driven by an interest to achieve high-performing sensors operating in water, to reach low-voltage operation for OFETs and to study the fundamentals of charge transport in semiconducting solids. In ion-sensitive field-effect transistors (ISFETs), the electrical potential at the electrolyte/insulator interface is translated into a modulation of the transistor output characteristics.[10] Actually, the modulation originates from that the threshold voltage (V T ) is sensitive to the ion concentration. The high electric field that is possible to establish at electrolyte/solid interfaces becomes a powerful tool for probing various features of the transport and accumulation of charges inside solids. Electrostatic field-operated transistors and switches, including for instance silicon, carbon nanotubes, [11] rubrene, [12] or manganites [13] as the active material, have been extensively studied in the past. In all those cases pure field-effect operation, without any parasitic electrochemical reactions of the bulk of the solid, is achieved simply because the materials included in the devices are known ...

Organic electronics have, over the past two decades, developed into an exciting area of research and technology to replace classic inorganic semiconductors. Organic photovoltaics, light-emitting diodes, and thin-film transistors are already well developed and are currently being commercialized for a variety of applications. More recently, organic transistors have found new applications in the field of biosensors. The progress made in this direction is the topic of this review. Various configurations are presented, with their detection principle, and illustrated by examples from the literature.