## 2 Functioning Principles and General Concepts for Transduction  
### Functioning Principles  
The OECT is a three-terminal device (source, drain and gate) in which source and drain electrodes are connected by an organic channel made up of a conductive polymer and an electrolyte which separates this polymer from a third contact, the gate electrode. The development of OECTs was initiated in 1984 by [PERSON] et al. who elaborated an electrochemical transistor based on an electropolymerized polypyrrole film [24]. This early OECT worked inside an aqueous electrolyte solution. Its working principle was based on changes in the polypyrrole conductivity induced by the application of a gate voltage able to switch the polymer between its oxidized and reduced states. Since this pioneering work, several organic conducting polymers (OSCs) have been investigated, such as polyaniline [25], polycarbazole [26] or polythiophene [27]. However, the most commonly employed material for OECTs is poly(3_A_-ethylenedioxythiophene) doped with poly(styrene sulfonate) (so-called PEDOT:PSS) [28; 29], a positively-doped (\(p\)-doped) OSC. The working principle of PEDOT:PSS OECTs (i.e., the modulation of the source-drain current) relies on the electrochemical doping/dedoping effect of the OSC channel that occurs when an appropriate gate voltage is applied. In such devices, the source electrode is grounded, so that voltages applied to the drain and gate electrodes are against the source. At zero bias voltage on the gate electrode, the transistor is in its \"on\" state; high current flows through the channel between source and drain under external polarization of the drain contact. When the gate electrode is put at a positive potential (against the source), the channel is consequently reduced. Cations in the electrolyte solution enter into the PEDOT:PSS and compensate the negatively charged sulfonate ions on the PSS, thus dedoping the OSC; this is the so-called depletion mode of operation, illustrated in Figure 1. Ionic interactions reduce the density of positive charge carriers (hole polarons), which leads to the decrease of the OSC conductivity (i.e., smaller currents cross the channel). In that sense, OECTs can convert ionic currents into electric currents. [PERSON] and [PERSON] proposed a model that reproduces the transient response (the rate at which the transistor responds to external changes such as a potential change at the gate) and the steady-state by considering OECTs as a combination of an ionic and an electronic circuit [30]. The electronic circuit models the flow of holes inside the channel and its variations during the dedoping process, while the ionic circuit has been modeled as a capacitor and a series resistor. In the model, the resistance represents the ionic strength of the electrolyte and the capacitor the number of ions that can be stored in the channel bulk. The operating principle of the OECT then relies on the interplay between the ionic and electronic currents. One should thus consider the parameters that influence transport properties (i.e., nature of the gate material, resistance of the electrolyte, and the geometry of the gate and channel) and tune them according to the targeted application. The most important feature of OECTs is their amplification property. A recent review by [PERSON] et al. [31] describes very well the relation between the amperometric gate current, apparent gate voltage and drain current. The figure of merit corresponding to amplification is called the gain. High gain is indeed crucial for transistors in the field of biosensing as it can provide high sensitivity, low limit of detection (LOD) and good signal-to-noise ratio. The efficiency of the amplification is measured by the transconductance \(g_{m}\), which is defined as the slope of the transfer characteristic of the transistor: \(g_{m}=\Delta I_{d}/\Delta V_{g}\), where \(I_{\rm D}\) represents the drain current and \(V_{\rm GS}\), the gate voltage. Therefore, a better gain is related to higher values of \(g_{m}\).