<p>Organic electrochemical transistors (OECTs) are emerging as promising platforms for low-voltage bioelectronics applications. However, the use of sharp switching behavior for sensing has remained a challenge due to effects of structural disorders and operational stability. In this study, we report OECTs with subthreshold swings close to or even below the thermodynamic limit of 60 mV dec⁻¹. The sharp switching owe to the semicrystalline nature of the employed thin films showing reversible dopant ion intercalation responding to the gate bias. Furthermore, the introduction of a Langmuir-Blodgett amphiphilic membrane at the semiconductor-electrolyte interface effectively suppressed performance drift. The drift in threshold voltage was as small as 0.18 mV per hour, enabling the use of subthreshold regions for sensing applications. By leveraging the pH-responsivity of the functional monolayer, pH sensing was demonstrated. This work demonstrates benchmark switching performance and stability of OECTs, and the potential of supramolecular interface engineering for sensing platforms.</p>