We prepared ZnO/n-Si heterojunctions by depositing ZnO films on n-Si substrates with different resistivities by radio-frequency magnetron sputtering. The microstructure of ZnO film was analyzed by X-ray diffraction and scanning electron microscopy. The current-voltage characteristics and ethanol gas sensing properties of the junctions were investigated at room temperature. It is found that optimization of n-Si substrate resistivity is critical to enhance the ethanol gas sensitivity of ZnO/n-Si heterojunction. The ZnO/n-Si heterojunction with n-Si substrate of 2-3 Omega cm exhibits the best ethanol gas sensing property. The junction shows the sensitivity of 29.41% to 0.24 g/L ethanol gas under +0.52 V forward bias voltage. (C) 2011 Elsevier B.V. All rights reserved.