We have found out that in an IGZO-TFT, the energy barrier, i.e., the conduction band energy relative to the Fermi energy is lowered by electrons flowing from n+ regions under source/drain electrodes to a channel region. We have named this phenomenon “conduction band lowering (CBL) effect”. Owing to this effect, even when the Fermi energy of an IGZO film gets closer to the midgap, a TFT formed using the film is always turned on around V
g = 0 V. In other words, by use of an active layer including an IGZO film with a Fermi energy close to the midgap, the CBL effect makes the threshold voltage of I
d-V
g characteristics converge into a certain low value and the characteristics variation can be suppressed.