THE FACTORS AFFECTING ON THE GAS SENSING: REVIEW

In this review, we discussed the impact of oxygen vacancy, doping, film thickness, annealing temperature, and grain size on the gas sensing. One of the most common defects in thin films is oxygen vacancies, whose concentration varies based on the growth time and the microstructure of the synthesis films. The vacancies of oxygen considered centers for the accumulation of charge carriers, which reducing their losses resulting from recombination and contributes the increasing the life time of photogenerated charge. In addition, the oxygen vacant sites act as active adsorption centers, increasing the amount of chemisorbed oxygen species (O_2^-,O^-) which pull electrons from the conduction band and lead to the creation of a surface depletion layer. Silver atoms on the WO3 sensor surface enhance oxygen adsorbed, thus increasing the chemically adsorbed species such as O^(2-), therefore Ag doping of the WO3 layer successfully improves NO sensitivity. A thicker layer contains more pores and clusters than a thinner layer. In this case, even reactant gas is depleted oxygen and fresh oxygen are introduced, the length of the delay time required for the oxygen to penetrate the thick layer results in slow reabsorption of oxygen ions. Increasing the surface roughness contributes to providing a large surface area, which allows a greater number gas molecules to interact with the surface of film. The number of cracks in the thin film’s material is related to an increase in resistance, which depends on the layer thickness and applied deformation. Although the cracks may be pathway for gas entrapment, they are also very likely to cause an electrical breakdown that in this area. Annealing in air leads to increased oxygen vacancies generation which enhances gas sensitivity. The grain size has a significant effect on gas sensor based on n-type semiconductor like tin oxide, where electrons pass through potential energy barriers. The number of these barriers, which form at the grain boundaries, decreases as grain size increases.