A theoretical analysis of the surface Fermi level position (E-FS) at n-InP interfaces with different surface state density (N-SS (E)) and surface fixed charge (Q(FC)) representing adsorbed ions or surface delta-doping has been performed in order to understand the InP-based gas sensor behaviour. Furthermore, the in-depth profiles of the potential barrier in equilibrium and under illumination (surface photovoltage) have been rigorously calculated. A U-shaped interface state continuum has been assumed in accordance with the Disorder Induced Gap State model. From the simulated dependencies of E-FS vs. the minimum surface state density N-sso, the movement of E-FS in the energy band gap as well as its pinning position have been investigated. In addition, the analysis of the E-FS sensitivity to the negative and positive Q(FC) has revealed the remarkable charge detection sensitivity of InP interfaces within different dynamic ranges. (C) 2003 Elsevier Science B.V. All rights reserved.