Metal-oxide devices change their resistivity in the presence of reducible or oxidation gases. They have been used since 1971  when Taguchi introduced to the market its metal oxide gas sensor based on the SnO2 ceramics. Many published works try to explain the mechanism involved in the gas detection process. Most of these works depend on measuring the change of the resistance as a function of the gas concentration (sensitivity) and type of the gas (selectivity). Essentially, the method detects the sum of different effects (grain, grain boundary and metal–semiconductor contacts) that occur in the surface of the sensor . Among numerous metal oxides that have been investigated such as TiO2, V2O5 , SnO2 , WO3  and ZnO , SnO2 and ZnO had received more attention because of their non toxic, good and stable electrical properties.