## 2 Organic Electrochemical Transistors (OECTs)  
Organic Electrochemical Transistors (OECTs) are considered hybrid transistors due to their unique ability to combine electronic operation with electrochemical response to control the flow of electrical current. These devices represent a unique fusion of electronics and chemistry, offering distinctive features that make them suitable for a wide range of applications, from flexible electronics to bioelectronics and tissue engineering [10; 11; 12; 13; 14]. The basic structure of an OECT includes three main components: the organic active layer (channel), a source electrode, a drain electrode, and a gate electrode connected to an electrolyte (Figure 1) [15]. The channel, typically made of a conductive polymer, is situated between the source and drain electrodes. When a voltage is applied between these two electrodes, current flows through the channel. The gate electrode is in contact with an electrolyte that covers the active layer of the channel. When a voltage is applied to the gate electrode, ions from the electrolyte diffuse into the channel, altering its oxidation state and thus its conductivity. The operation of an OECT is based on the modulation of the channel's conductivity through the injection and extraction of ions, which are controlled by the potential applied to the gate electrode. When a positive voltage is applied to the gate, positive ions from the electrolyte enter the channel, inducing oxidation of the organic material and increasing its conductivity. Conversely, a negative voltage at the gate causes ions to be withdrawn from the channel, leading to deoxidation of the material and a reduction in conductivity. This operating mechanism, which relies on the interaction between the electrolyte ions and the conducting organic material, allows the OECT to regulate the current flowing between the source and drain electrodes. The control of the channel's conductivity through electrochemical modulation distinguishes OECTs from other types of transistors and defines their fundamental operating principle. It is important to note that the above explanation applies specifically to p-type semiconductors. In n-type semiconductors, the polarities of the processes described would be reversed: a positive voltage at the gate would lead to the extraction of electrons (reduction) and a decrease in conductivity, while a negative voltage would induce the injection of electrons (oxidation), increasing the conductivity of the channel. The parameters that evaluate the efficiency of an OECT include several key factors that directly influence the device's performance. Firstly, electron mobility (\(\mu\)) is crucial as it determines how efficiently charge carriers (electrons or holes) can move through the channel material under an applied electric field. High electron mobility allows for faster switching speeds and improved overall performance of the transistor, making it a critical parameter for the optimization of OECTs. It is given by:  
\[\mu=I_{D}L/WC_{OX}V_{GS}\]  
where \(I_{D}\) is the drain current, \(L\) is the length of the channel, \(W\) is the width of the channel, \(C_{ox}\) is the gate oxide capacitance per unit area and \(V_{GS}\) is the gate-source voltage. Secondly, the volumetric capacitance (\(C^{*}\)) of the channel material is a vital parameter that reflects the material's ability to store charge per unit volume. This capacitance is linked to the material's swelling capability, which facilitates the penetration of ions from the electrolyte into the channel layer. A higher volumetric capacitance implies that more charge can be injected or extracted with a smaller change in gate voltage, enhancing the modulation of the channel's conductivity and thereby improving the device's sensitivity and responsiveness. It is defined by:  
\[C^{*}=Q/V\]  
where \(Q\) is the total charge stored and \(V\) is the volume of the channel. Lastly, ion mobility (\(\mu_{ion}\)) within the channel material is another important factor.