Provided are a thin film transistor (TFT) substrate, a display device, and a method of forming the TFT. A TFT substrate includes: a first TFT including: a polycrystalline semiconductor (PS) layer, a first gate electrode (GE) overlapping the PS layer, a nitride layer (NL) on the first GE, an oxide layer (OL) on the NL, and a first source electrode and a first drain electrode on the OL, and a second TFT including: a second GE on a same layer as the first GE, a hydrogen collecting layer between the second GE and the NL, an oxide semiconductor (OS) layer on the OL, a second source electrode and a second drain electrode contacting respective sides of the OS layer, wherein the first TFT and the second TFT are disposed on a same substrate, and wherein the NL includes an opening exposing the hydrogen collecting layer of the second TFT.