The low gas response of sensor based on Ni0.9Zn0.1O (p-type semiconductor) compared to the others (n-type semiconductors),can be explained by the theory supported by the Hübner et al.’s calculation which states that the gas response of p-type oxide semiconductors is the square root of that of n-type oxide semiconductors with the same morphology [51]. Moreover, owing to its high conductivity (Rair= 131 ) compared to others (Rair= 750 kfor ZnO and Rair= 60 M for Ni0.9Zn0.1O/ZnO), the variation of the resistance at the surface of the particle is negligible because the Debye length is very short and the relative charge carrier change is small when adsorbed gas exchanges electrons. The high gas response of the sensor based on composite material(Ni0.9Zn0.1O/ZnO) for CO specifically can be explained by the formation of heterojunction in one hand but moreover by the catalytic activity of both component (ZnO and Ni0.9Zn0.1O) on the oxidationof CO in other hand [52–54]. This cumulative CO oxidation catalytic activity should favor more adsorption of CO on the surface via the active sites thus enhancing the sensor response. This will not be the case for H2 and NO2.