Properties and energy levels of In-doped ZnO (IZO), Ga-doped ZnO (GZO), and In- and Ga-doped ZnO (IGZO) crystals, were investigated. Metal-doped ZnO with different dopants were prepared by sol-gel processing. Results showed that the lattice crystal of the doped ZnO was distorted and the average grain sizes of the crystals decreased. The resistivity and mobility of all doped samples decreased, but the carrier concentration increased in the presence of In and Ga. The energy levels of all crystals were varied depending on the dopants in ZnO. Although the conduction band energy (E-c) and valence band energy (E-v) of IZO apparently shifted to lower energy levels, the bandgap energy (E-g) varied little. The E-g of GZO was widened with respect to ZnO because its E-v was altered. The E-g of IGZO was widened because the E-c was significantly shifted by In and the E-v was mainly varied by Ga.