A display device can include a first thin film transistor including a first active layer including a first semiconductor material, a first gate electrode overlapping with the first active layer, and a first source electrode and a first drain electrode both electrically connected to the first active layer; a separation insulating layer disposed on the first thin film transistor; and a second thin film transistor disposed on the separation insulating layer and including: a second active layer including a second semiconductor material different from the first semiconductor material, a second gate electrode overlapping with the second active layer, and a second source electrode and a second drain electrode both electrically connected to the second active layer, in which the second active layer of the second thin film transistor has a first thickness and a second thickness greater than the first thickness.