A thin film transistor with excellent electric characteristics, a display device having the thin film transistor, and a method for manufacturing the thin film transistor and the display device are proposed. The thin film transistor includes a gate insulating film formed over a gate electrode, a microcrystalline semiconductor film formed over the gate insulating film, a buffer layer formed over the microcrystalline semiconductor film, a pair of semiconductor films to which an impurity element imparting one conductivity type is added and which are formed over the buffer layer, and wirings formed over the pair of semiconductor films to which the impurity element imparting one conductivity type is added. A part of the gate insulating film or the entire gate insulating film, and/or a part of the microcrystalline semiconductor or the entire microcrystalline semiconductor includes an impurity element which serves as a donor.