The Cohen–Fritzsche–Ovshinsky model for amorphous covalently bonded semiconductors is used to calculate the electrical characteristics of a field-effect structure utilizing an amorphous covalent film as the semiconductor. The effect of surface states on the semiconductor is considered. The analysis shows that the band bending is exponentially dependent on distance from the surface. The modulation of the surface conductance as a function of transverse field, geometry, and surface-state density is calculated. These calculations indicate that the field effect should be measurable for surface-state densities up to 1.5×1014 cm−2 (eV)−1. The results of these calculations should be used in the design of field-effect experiments in order to achieve the maximum sensitivity.