The South Korea international spent the first half of this season at fellow Championship club Portsmouth where he scored three times in 16 matches.
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Yang said: “I’m very happy and excited to join a club with such strong traditions and history.
“When I played against Coventry, I really felt the positive atmosphere within the team, and that made me even more excited about becoming part of this club.
“The Head Coach explained very clearly how he plans to use me and how I could fit into the team, which gave me a lot of confidence that this is the right place for me.
“I want to help the team achieve its goal and to do that, I need to adapt as quickly as possible, show my qualities on the pitch, and prove why I am here.”