A thin film transistor (TFT) array substrate and an organic light-emitting diode display employing the same are disclosed. In one aspect, the substrate includes at least one TFT, the TFT including a substrate and a semiconductor pattern comprising a source region, a channel region, and a drain region. The TFT also includes a gate insulating layer covering the semiconductor pattern, a side gate electrode electrically insulated from the semiconductor pattern and formed over at least one side of the channel region, and a top gate electrode formed over the gate insulating layer so as to partially overlap the semiconductor pattern, the side gate electrode and the top gate electrode electrically connected to each other.