Design and Simulation Analysis of Electrolyte-Gated Aluminum Oxide Organic Thin-Film Transistor Biosensor for High Sensitivity

Field-effect transistors (FETs) have been extensively studied for pH and biosensing. Inorganic FETs like Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) and Ion-Sensitive Field-Effect Transistor (ISFET) offer high sensitivity, and Complementary Metal-Oxide-Semiconductor (CMOS) provides compatibility. However, they suffer from poor long-term stability in ionic environments, leading to increased interest in organic FETs (OFETs), which are flexible, biocompatible, and suitable for low-cost, scalable production.

This paper explores an electrolyte-gated organic thin-film transistor (EG-OTFT) with a pentacene structure and an aluminum oxide gate dielectric for an efficient pH sensor. The device includes a micro-scale rectangular cavity of 8 μm × 10 μm for introducing the bioanalyte. A 30 nm-thick pentacene semiconductor layer is deposited over a 5.7 nm-thick aluminum oxide dielectric.

Aluminum oxide is selected for its high hydration sensitivity, which enhances pH responsiveness and improves the device’s chemical stability. Direct contact between the organic semiconductor and the electrolyte improves charge injection, lowers threshold voltage, and increases switching speed, key to low-power and real-time sensing. The aluminum oxide layer also suppresses leakage currents and extends charge carrier lifetime, boosting overall sensor performance and durability.

Device behavior is evaluated using the SILVACO TCAD Atlas tool, employing models such as drift-diffusion, Poole–Frenkel, hopping mobility, and Langevin recombination. The electrolyte is simulated as a semiconductor via the Poisson–Boltzmann equation, modeling ions (Na⁺, Cl⁻) as charge carriers and setting water’s permittivity to 80.

A key contribution is the dual-aspect simulation, i.e., modeling interfacial potential changes from pH-dependent surface reactions, and analyzing coupled ionic and electronic charge transport. The oxide–electrolyte interface is central to pH response, where protonation and deprotonation reactions shift the surface potential, modelled logarithmically like the Nernst equation. A detailed capacitive model incorporates both Stern and Gouy–Chapman layers to represent the electric double layer (EDL).

The device exhibits effective pH sensing through its electrical and ionic responses. As pH increases from 1 to 10, the transfer characteristics shift rightward by approximately 1.3 V, with the threshold voltage moving from −0.5 V at pH 1 to +0.8 V at pH 10. This shift results from surface deprotonation of the aluminum oxide dielectric, which reduces positive surface charge and decreases hole accumulation in the pentacene channel under alkaline conditions.

The sensor achieves a high voltage sensitivity of 255.9 mV/pH, significantly exceeding the Nernst limit of 59 mV/pH. Drain current sensitivity also rises with pH, reaching its maximum at pH 14. Additionally, the ON/OFF current ratio improves from 10⁵ at pH 1 to 10⁸ at pH 14, indicating excellent switching performance. The threshold voltage decreases with increasing pH, while the sensitivity peaks around pH 7, demonstrating strong responsiveness in biologically relevant acidic environments.

The research demonstrates the feasibility of achieving high sensitivity

through electrolyte gating and also introduces a scalable, biocompatible, and energy-efficient sensing platform. The development of the pent-EG-OTFT pH sensor highlights a significant shift in the field of chemical and biosensors. Its high sensitivity, low cost, flexibility, and low power requirements make it a promising platform for healthcare diagnostics, environmental monitoring, and next-generation wearable sensing applications.