A thin film transistor including a gate, a gate insulating layer, a semiconductor layer, a source electrode and a drain electrode is provided. The gate is disposed over a substrate, wherein the gate comprises at least one layer of aluminum-yttrium alloy nitride. The gate insulating layer is formed over the substrate to cover the gate. The semiconductor layer is disposed over the gate insulating layer above the gate. The source electrode and the drain electrode are disposed over the semiconductor layer.