Real-time cell culture media monitoring can be conducted by Organ-on-Chip (OoC) ion-sensitive floating-gate field-effect transistor based sensors (ISFGFET). A method of modelling the sensor is described and implemented in the Advanced Design System (ADS) design and simulation software. The model is validated using measurement data of the sensor when exposed to an aqueous solution and in a 'dry' scenario. The model is utilized for an overall sensitivity analysis and the effect of the sensor dimensions on the sensitivity to charge variation that are immobilized on the sensing area surface. In addition to a SPICE compatible floating gate, level 3 MOSFET parameters are extracted, along with a sensing area model that links changes in the floating gate voltage to changes in the pH of the solution. Subsequently, a bias point is determined based on measurement data and limiting factors. Finally, the sensitivity of the ISFGFET is analysed. Our results characterize the effect that the sensing area dimensions, control gate capacitance and bias point have on the sensitivity.