The effect of Gallium (Ga) doping on CdS thin film properties and the performance of the corresponding Cu (InGa)Se-2/CdS heterojunction solar cell has been investigated. The CdS thin films were deposited using a conventional chemical bath deposition (CBD) process. For a source of Ga dopant, a gallium nitrate (Ga(NO3)(3)) aqueous solution with Ga concentration from 5 x 10(-4) to 2 x 10(-3) M has been used. Ga doping was carried out by adding gallium nitrate aqueous solution directly to the main CBD solution that contained all the other reactants (i.e., Cd2+, S2-, and NH3). It was found that Ga doping is effective for improving the optical transmittance of CdS films and, thus, increasing the photoelectric current density of the films and the short-circuit current density (J(sc)) of CIGS cells. With the use of a Ga-doped CdS buffer layer, the photovoltaic performance of CIGS cells was improved, primarily because of enhanced J(sc) and fill factor.