A thin film transistor device, disposed on a substrate, includes a gate electrode, a semiconductor channel layer, a gate insulating layer disposed between the gate electrode and the semiconductor channel layer, a source electrode and a drain electrode disposed at two opposite sides of the semiconductor channel layer and partially overlapping the semiconductor channel layer, respectively, a capacitor electrode at least partially overlapping the gate electrode, and a capacitor dielectric layer disposed between the capacitor electrode and the gate electrode. The capacitor electrode, the gate electrode and the capacitor dielectric layer form a capacitor device.