<p>This paper presents a bio-electrolyte gated organic thin film transistor (BEG OTFT) structure as a pH sensor. The organic thin film transistor is designed and simulated with and without gate oxide (insulator layer) in biomedical applications. The proposed structure uses pentacene as an organic layer in the thin film transistor. The BEG OTFT has been examined and analyzed as a pH sensor for various fluids of human body. The active layer of pentacene in the OTFT is a channel material, which conducts the current, when there is a change in the pH value of electrolyte. A gate oxide of high permittivity (Al₂O₃) has been used in BEG OTFT with a gate oxide structure. The gate oxide in the pH sensor plays an important role, as it provides significant and proficient electrostatic control over the channel. The voltage sensitivity measured for the proposed sensor has been evaluated at 160mV/pH, thrice the value of the Nernst limit (&gt; 59mV/pH).</p>