Effect of electric field on the crystallization of ZnO sol-gel films was investigated by measuring x-ray diffraction (XRD) patterns, photoluminescence (PL) spectra, and current-voltage (I-V) characteristics of n-ZnO/p-Si heterojunctions. When sol-gel ZnO films (undoped and Al-doped) were crystallized at 800 °C in air for 10 min, an electric field was applied between 0 and 2.0 kV/cm. As the electric field was increased, the XRD (002) peak intensity increased and had a maximum value at 1.5 and 2.0 kV/cm for the films on quartz and silicon substrates, respectively. The Hall carrier mobility increased and the carrier concentration decreased for the crystallized undoped and Al-doped films when the electric field was 1.0 kV/cm and 1.5 kV/cm, respectively. As the electric field was increased from 0 to 2.0 kV/cm, the forward current increased and the reverse current decreased for n-ZnO/p-Si heterojunctions. These results indicated that application of electric fields was an effective means to reduce the defect density of ZnO sol-gel films and to improve their heterojunction characteristics.