Organic electrochemical transistor
The organic electrochemical transistor (OECT) is a transistor in which the drain current is controlled by the injection of ions from an electrolyte into a semiconductor channel. The injection of ions in the channel is controlled through the application of a voltage to the gate electrode. OECTs are being explored for applications in biosensors, bioelectronics and large-area, low-cost electronics.
OECTs consist of a semiconductor film (the channel), usually made of a conjugated polymer, which is in direct contact with an electrolyte. Source and drain electrodes establish electrical contact to the channel, while a gate electrode establishes electrical contact to the electrolyte. The electrolyte can be liquid, gel, or solid. In the most common biasing configuration, the source is grounded and a voltage (drain voltage) is applied to the drain. This causes a current to flow (drain current), due to electronic charge (usually holes) present in the channel. When a voltage is applied to the gate, ions from the electrolyte are injected in the channel and change the electronic charge density, and hence the drain current. When the gate voltage is removed, the injected ions return to the electrolyte and the drain current goes back to its original value.
OECTs are different from electrolyte-gated field-effect transistors. In the latter type of device, ions do not penetrate into the channel, but rather accumulate near its surface (or near the surface of a dielectric layer, when such a layer is deposited on the channel). This induces accumulation of electronic charge inside the channel, near the surface. In contrast, in OECTs, ions are injected into the channel and change the electronic charge density throughout its entire volume. As a result of this bulk coupling between ionic and electronic charge, OECTs show a very high transconductance. The disadvantage of OECTs is that they are slow, as ionic charge has to get in and out of the channel. Microfabricated OECTs show response times of the order of hundreds of microseconds.
The most common OECTs are based on (PEDOT:PSS), and work in the depletion mode. In this material, the organic semiconductor PEDOT is doped p-type by the sulfonate anions of the PSS (the dopant) and hence exhibits a high (hole) conductivity. Hence, in the absence of a gate voltage, the drain current will be high and the transistor will be in the ON state. When a positive voltage is applied to the gate, cations from the electrolyte are injected into the PEDOT:PSS channel, where they compensate the sulfonate anions. This leads to dedoping of the PEDOT, and the transistor reaches its OFF state. Accumulation mode OECTs, based on intrinsic organic semiconductors, have also been described. Accurate simulation of OECTs is possible using the drift-diffusion model.
OECTs were first developed in the 80’s by the group of Mark Wrighton. They are currently the focus of intense development for applications in bioelectronics, and in large-area, low-cost electronics. Advantages such as straightforward fabrication and miniaturization, compatibility with low-cost printing techniques, compatibility with a wide range of mechanical supports (including fibers, paper, plastic and elastomer), and stability in aqueous environments, led to their use in a variety of applications in biosensors. Moreover, their high transconductance makes OECTs powerful amplifying transducers. OECTs have been used to detect ions, metabolites, DNA, pathogenic organisms, probe cell adhesion, measure the integrity of barrier tissue, detect epileptic activity in rats, and interface with electrically active cells and tissues.
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