ZnO films (5 nm and 20 nm) have been grown on GaN single-crystal substrates by thermal atomic layer deposition (ALD) and the electrical properties of n-GaN Schottky contacts modified by such ultrathin ZnO films have been characterized. Compared with 5-nm-thick ZnO, 20-nm-thick ZnO exhibited a better rectifying nature. The average barrier height and ideality factor at room temperature were extracted to be 0.64 eV and 2.33 eV, and 1.01 eV and 1.16 eV, for 5-nm- and 20-nm-thick ZnO, respectively. These results indicate that both the barrier height and ideality factor were altered effectively by changing the ZnO thickness. The temperature-dependent reverse current–voltage (I–V) characteristics revealed that tunneling was dominant for the 5-nm-thick ZnO. A laterally inhomogeneous barrier was appropriate to explain the forward I–V characteristics for both samples. Based on the parallel conductance method and forward I–V data, a lower interface state density was observed for 20-nm-thick ZnO, implying improved interface quality. These results suggest that the electrical properties of n-GaN Schottky contacts could be easily modulated by changing the ZnO thickness.