High-quality ZnO hexagonal tube-like nanostructures grown on p-type GaN heterojunction have been synthesized for sensing applications through a low-cost catalyst-free process by thermal evaporation at 800 degrees C. The morphological, structural, and optical properties of the ZnO heterostructures have been examined. In this study, Pd/ZnO/Pd metal-semiconductor-metal gas sensor has been fabricated based on the ZnO tube-like nanostructures. The sensitivity of ZnO/p-GaN heterostructures is measured at different flow rates (25, 50, 100, and 150 sccm) of H-2 gas at room temperature. The highest response of the ZnO/p-GaN sensor was 1250%, when 150 sccm of H-2 gas was injected. In addition, Pd/Al n-ZnO/p-GaN heterojunction as an ultraviolet photodiode is demonstrated. The current-voltage curve of the heterojunction demonstrates obvious rectifying behavior in the dark and under illumination. For illumination conditions, one light source of wavelength 365 nm and another at 400 nm were used, sweeping the bias voltage from +4 to -4 V. Under UV light at 365 nm the current was almost 12 times greater than that in the dark, while under UV light at 400 nm the current was 2.2 times greater than that in the dark at 3 V. (C) 2013 Elsevier B.V. All rights reserved.