In a thin-film transistor (“TFT”) array substrate for an X-ray detector and an X-ray detector having the TFT array substrate, the TFT array substrate includes a gate wiring, a gate insulating layer, an active layer, a data wiring, a photodiode, an organic insulating layer and a bias wiring. The gate wiring is formed on an insulating substrate and includes a gate line and a gate electrode. The gate insulating layer covers the gate wiring. The active layer is formed on the gate insulating layer. The data wiring is formed on the gate insulating layer and includes a data line, source and drain electrodes. The photodiode includes lower and upper electrodes, and a photoconductive layer. The organic insulating layer covers the data wiring and the photodiode. The bias wiring is formed on the organic insulating layer. Thus, an aperture ratio and reliability are enhanced.