An X-ray detector constructed as an exemplary embodiment of the present invention includes a semiconductor layer, a data line including a source electrode covering a first portion of the semiconductor layer, a drain electrode disposed opposite to the source electrode, a first lower electrode formed on the upper portion of a second portion of the semiconductor layer and a gate insulating layer and elongated from the drain electrode, and a passivation layer formed on the upper portion of one part of the lower electrode including the drain electrode. Further, the second lower electrode is formed approaching the gate electrode. The X-ray detector constructed as the exemplary embodiment of the present invention includes a second lower electrode formed on the passivation layer and placed approaching a gate electrode. The area in which a diode is disposed may be maximized, and the amount of leakage current may be reduced.