An X-ray image sensor fabricated using eight-mask steps. A thin film transistor (TFT) having a gate electrode, a first insulation layer, pure and doped amorphous silicon layers, and source and drain electrodes is on a substrate. An island-shaped first insulation layer, semiconductor layer, and ground line are also formed. A second insulation layer having a first drain contact hole and a ground line contact hole covers the TFT, the substrate, and the ground line. An auxiliary drain electrode on the second insulation layer contacts the drain electrode through the first drain contact hole. A capacitor electrode on the second insulation layer contacts the ground line through the ground line contact hole. A third insulation layer having a second drain contact hole that exposes the auxiliary drain is on the second insulation layer, the auxiliary drain electrode, and the capacitor electrode. A pixel electrode on the third insulation layer contacts the auxiliary drain electrode through the second drain contact hole.