Undoped and Ga-doped ZnO crystals were grown on Zn-polar ZnO substrates by cold-wall chemical vapor transport (CVT). High growth rates exceeding 100 mu m/h have been achieved. X-ray rocking curve and Hall effect measurements revealed that CO2 in the reaction tube of the cold-wall CVT system improves the structural and electrical properties of ZnO crystals. Ga-doped ZnO crystals were grown by using ZnO powder mixed with Ga2O3 powder as a source material. The carrier concentration depends on the Ga2O3 content of the source material and is controlled between 7x10(16) and 3x10(19) cm(-3).