ZnO films were prepared on p-type silicon wafers by pyrolysis of an ultrasonically nebulized zinc acetate solution. Aluminum electrodes were deposited on the as-prepared ZnO film and at the backside of the p-Si wafer, which forms a n-ZnO/p-Si hetero-junction. It is found that by applying a forward bias of about 6 V in vacuum, S-type negative resistance was observed for the structure. The negative resistance was preserved after the hetero-junction was exposed to air. (C) 2003 Elsevier Science B.V. All rights reserved.