An Al-doped ZnO/p-Si heterojunction is fabricated by a laser molecular beam epitaxy technique. The abnormally high ideality factors (n 〉〉 2) of the prepared heterojunction are observed in the interim bias voltage range. A theoretical model is proposed to understand the much higher ideality factor of the special heterojunction diode. The ZnO:Al film shows metal-like conductivity with the electrical resistivity about 6.56 * 10-4 Ω·cm at room temperature. The temperature dependence of the photovoltage indicates that the photovoltaic effect of the Al-doped ZnO based heterojunction can be changed by the intrinsic metal-semiconductor transition at 120 K.