Ga-doped ZnO (GZO) thin films with different Ga contents (1, 2, 4, 3, and 5 at.%) were successfully deposited on glass substrates using a spin-coating process. The temperature dependent electrical conductivity and room-temperature Hall effect measurements were performed for the films. The variation of electrical conductivity with Ga content was well explained by considering the change in the inelastic diffusion length of electrons. The critical value of carrier concentration for the metal-insulator transition was estimated as 1.77 x 10(18) cm(-3) for GZO. The critical value of Ga content, where Ga as a donor source becomes ineffective, on the order of 5.25 at.% was obtained.