Semiconductor gas sensors have attracted much attention, but poor specificity limits their practical applications. In the current work, a universal design with varied n-p core-shell structures was proposed to distinguish NH3 gas from other reducing gases by a p-n competition mechanism. Acting as a model system, devices made of n-ZnO/p-CuxO core-shell nanorod (NR) arrays with variable shell thicknesses prepared by a two-step route demonstrate this unique gas specificity behavior. The transmission electron microscopy results show the outer CuxO shells on the surface of NRs are comprised of major Cu2O and minor CuO crystals. When the shell thickness is greater than a critical value, the NR devices exhibit a p-type response to all the testing reducing gases. When the shell thickness is less than the critical value, only NH3 among the testing reducing gases results in an unusual n-type response, which demonstrates extraordinary discernment to ammonia gas by only observing the trend in the increase or decrease of resistance. A thickness-dependent band structures upon gas sensing is proposed to explain the improved gas specificity. (c) 2018 The Electrochemical Society.