Gallium doped Zinc Oxide (GZO) nanocrystalline films were synthesized by RF magnetron sputtering and their gas sensing properties were investigated. X-ray Diffraction study revealed the preferential orientation along c-axis for both doped and undoped films. X-ray Photoelectron Spectroscopy study showed a higher level of oxygen vacancies in GZO films. Investigation of gas sensing characteristics confirmed that among the doped films, 3% GZO exhibited the highest sensitivity for H2S at an operating temperature of 300 degrees C. The enhanced sensitivity of GZO for H2S could be attributed to the combined effect of excess oxygen vacancies due to Ga3+ substitution and the large adsorption energy of Ga favorable for chemi-sorption. Presence of negative capacitance in the low frequency region of impedance spectra could be due to the accumulation of ions at the metal/semiconductor interface facilitated by excess oxygen vacancies and interface charge states in GZO. (C) 2018 Elsevier B.V. All rights reserved.