An X-ray detector including a thin film transistor (TFT) substrate and a photo-diode array layer is disclosed. Each thin film transistor in the TFT substrate includes: a substrate; a gate-electrode on the substrate; a gate insulating layer on the gate-electrode; a semiconductor layer on the gate insulating layer, wherein a portion of the semiconductor layer covers the gate-electrode; an etching stop layer covering the semiconductor layer; a source-electrode and a drain-electrode respectively disposed on the etching stop layer, wherein the source-electrode and the drain-electrode are respectively electrically connected to the semiconductor layer through conductive via-holes each having a base portion at the semiconductor layer, and at least one of the projection areas of the base portions vertically projected on the substrate has a non-overlapping region beyond the projection area of the gate-electrode vertically projected on the substrate; and a passivation layer covering the source-electrode and the drain-electrode.