Very strong capacitance‐voltage dispersive effects are observed in mercury/indium phosphide Schottky diodes. These effects are related to a partially compensated region at the surface for which the Debye length is relatively large. The small‐signal capacitance is indicative of a critical dielectric relaxation time exceeding the period of the ac probe. This effect may be used to characterize the majority‐carrier profile in lightly doped or low mobility semiconductors.