Transparent conducting ZnO films were prepared at substrate temperature 400C with different film thicknesses by nebulizer spray pyrolysis method on glass substrates. XRD studies reveal that the films are polycrystalline in nature having hexagonal crystal structure with preferred grain orientations along (002) and (101) directions. The crystallite size increases along (002) plane with the thickness increase and attains a maximum 109nm for 913nm film thickness. Analysis of structural parameters indicates that the films having thickness 913nm are found to have minimum dislocation density and strain values. The HRSEM measurements show that the surface morphology of the films also changes with film thickness. EDAX estimates the average atomic percentage ratio of Zn and O in the ZnO films. Optical studies reveal the band gap energy decrease from 3.27 to 3.14eV with increase of film thickness. Room temperature PL spectra show the near-band-edge emission and deep-level emission due to the presence of defects in the ZnO thin films. Impedance spectroscopy analysis indicates that grain boundary resistance decreases with the increasing ammonia concentration up to 500ppm and the maximum sensitivity is found to be 1.7 for 500ppm of ammonia.