Nernst–Planck–Poisson analysis of electrolyte-gated organic field-effect transistors

Abstract

Electrolyte-gated organic field-effect transistors (EGOFETs) represent a class of organic thin-film transistors suited for sensing and biosensing in aqueous media, often at physiological conditions. The EGOFET device includes electrodes and an organic semiconductor channel in direct contact with an electrolyte. Upon operation, electric double layers are formed along the gate-electrolyte and the channel-electrolyte interfaces, but ions do not penetrate the channel. This mode of operation allows the EGOFET devices to run at low voltages and at a speed corresponding to the rate of forming electric double layers. Currently, there is a lack of a detailed quantitative model of the EGOFETs that can predict device performance based on geometry and material parameters. In the present paper, for the first time, an EGOFET model is proposed utilizing the Nernst-Planck-Poisson equations to describe, on equal footing, both the polymer and the electrolyte regions of the device configuration. The generated calculations exhibit semi-qualitative agreement with experimentally measured output and transfer curves.

Field-Effect Transistor with a Plasmonic Fiber Optic Gate Electrode as a Multivariable Biosensor Device

“…A significant dependency of the recorded transfer characteristics on the FO core diameter was observed. Only a few publications on EG-FETs address the influence of the geometry and the electrode surface area on the electrical signal. ,− The ratio of gate area to channel area has been identified as a crucial parameter for the observed effect . The FO gate surface area was calculated by approximating a cylindrical shape with a length derived by analysis using ImageJ .…”

Inkjet‐Printed, Coplanar Electrolyte‐Gated Organic Field‐Effect Transistors on Flexible Substrates: Fabrication, Modeling, and Applications in Biodetection

“…This difference has been seen before in EGOFET devices and was related to the experimental way used to measure the currents in the output and transfer curves. [36] In the following, we focus on the concentration and potential profiles at the electrolyte-organic semiconductor interface, where the electric double layer is formed, outlining the difference between the standard operation of EGOFETs observed at higher negative voltages |V GS | > 0.4 V and the offcurrent regime at lower voltages |V GS | < 0.4 V. The operation of EGOFETs is relatively well understood: [7,36,49] the application of a negative voltage at the gate electrode changes the distribution of the ions within the electrolyte, such that positive ions come close to the gate, whereas negative ions accumulate close to the electrolyte/OSC boundary. The accumulation of the negative ions at the interface induces accumulation of the holes on the other side of the boundary, in the organic semiconductor region.…”

Sub-3 V, MHz-Class Electrolyte-Gated Transistors and Inverters

“…Ion mobilities are orders of magnitude smaller than electron or hole mobilities in semiconductors, which means the ionic resistance of the intervening electrolyte between the gate electrode and the semiconductor channel is relatively large. This ionic resistance can be viewed as a parasitic resistance R p in series with the gate, and it is dependent on intrinsic electrolyte conductivity, electrolyte thickness, and cross-sectional area. − As we demonstrate below, for an EGT-based circuit, it can unfortunately be the case that the limiting ON-state resistance R total ON controlling signal propagation delay is not determined by the semiconductor channel resistance R channel ON or the source–drain contact resistance R contact ON , but rather is dominated by R p due to slow ion motion in the electrolyte (i.e., R total ON = R channel ON + R contact ON + R p , where R p dominates). − We, and others, have measured ionic conductivity for gel electrolyte films based on ionic liquids and polymers and found it to be as large as 4 mS/cm depending on the precise gel composition. − Thus, for typical EGTs with channel areas of ∼1000 μm 2 , and gel film thicknesses of ∼1 μm, R p is on the order of 1 kΩ, which is a large value. Less resistive, ultrathin gate–electrolyte films are a clear goal for EGTs .…”