This paper deals with the development of state-of-the-art metal oxide semiconductor (MOX) gas sensors based on ultra-low power (ULP) consumption micro-machined hot plates. Several gas sensors have been fabricated by means of a wafer level technological process, and deeply functionally and morphologically characterized. Details on device design and on the fabrication processes are provided. Results of functional characterizations towards different gases at different working conditions will be reported. The capability of detecting volatile organic compounds (VOC) down to few parts per billion (ppb), will be shown. Outcomes of morphological sensing layer characterization will show the well controlled nano-structured thin film of tin oxide.