A comparative study of Schottky diode hydrogen gas sensors based on Pd/WO3/Si and Pd/WO3/ZnO/Si structure is presented in this work. Atomic force microscopy and X-ray photoelectron spectroscopy reveal that the WO3 sensing layer grown on ZnO has a rougher surface and better stoichiometric composition than the one grown on the Si substrate. Analysis of the I-V characteristics and dynamic response of the two sensors when exposed to different hydrogen concentrations and various temperatures indicate that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10 times higher sensitivity, and shorter response and recovery times (105 s and 25 s, respectively) towards 10,000-ppm H-2/air at 423 K. Study on the energy band diagram of the diode suggests that the barrier height is modulated by the WO3/ZnO heterojunction, which could be verified by the symmetrical sensing properties of the Pd/WO3/ZnO/Si gas sensor with respect to applied voltage. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.