In this study, two types of ZnO nanorods (NRs) were hydrothermally grown on Si substrates by controlling the location of the Ga dopant. The growth behavior of ZnO NRs grown on the Ga-doped ZnO (GZO) seed layer (NR1s) was affected by the particle size controlled by the Ga dopant, while that of ZnO NRs grown in a Ga-doped aqueous solution (NR2s) was affected by complexes caused by the Ga dopant in the aqueous solution. The photoresponsivity of the ZnO NR1s overall increased with increasing concentration of Ga dopant due to the large surface area related to the density of ZnO NR1s. However, in case of the ZnO NR2s, the photoresponsivity of the undoped ZnO (UZO) NR2s was larger than that of the GZO NR2s due to the larger surface area for oxygen molecule adsorption in the UZO NR2s. Finally, under a 3-mM Ga doping concentration, the photoresponsivity of the ZnO NR1s was approximately 11.5 times higher than that of the ZnO NR2s, indicating that the ZnO NR1s are promising candidates for use in ultraviolet (UV) photodetector applications. These results indicate that when the ZnO NRs are grown using the hydrothermal method, doping at the seed layer is more suitable for high-performance UV photodetectors.