This study presents tungsten nitride (WN) as an effective hydrogen (H) barrier for source/drain (S/D) contacts in top-gate self-aligned (TG-SA) amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs). It was confirmed that the H could diffuse into the channel through the S/D contacts, affecting the carrier concentration (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N_C</i>) in the channel. By incorporating the WN as a H-barrier in the S/D contact stack, the fabricated TG-SA a-IGZO TFTs could maintain their electrical properties without any degradation even after forming gas anneal (FGA) at a temperature of 300°C.