Recently, oxide semiconductors have become promising alternatives to Si-based semiconductors due to their outstanding properties such as high transparency, possibility of large area fabrication, low leakage current, and low process temperature. Based on these advantages, oxide semiconductors have been used in a variety of applications from the display area to solar cells and sensors. Among numerous types of applications, phototransistors for photo-sensing, which detect the shape, state, and movement of objects, have been attracted attention because of the necessity in all new electronic devices due to the trend of automation such as smart windows and smart glass. However, oxide phototransistor could hardly detect visible light because of its wide band gap of more than 3 eV. Therefore, extra process such as addition of an absorption layer or modification of layers is essential in sensing visible light using oxide phototransistor. These phototransistors have greatly improved photo detectability compared to the conventional oxide phototransistor. Especially, these phototransistors are found to have sudden changes in electrical characteristics under the red light (wavelength of 635 nm) which was not absorbed by the conventional oxide phototransistor. These techniques are expected to be new approaches to improve the photo detectability of the oxide phototransistor up to the visible light region.