A structure with which the zero current of a field effect transistor using a conductor-semiconductor junction can be reduced is provided. A floating electrode (102) including a conductor or a semiconductor and being enclosed by an insulator (104) is formed between a semiconductor layer (101) and a gate (105) so as to cross the semiconductor layer (101) and the floating electrode (102) is charged, whereby carriers are prevented from flowing from a source electrode (103a) or a drain electrode (103b). Accordingly, a sufficiently low carrier concentration can be kept in the semiconductor layer (101) and thus the zero current can be reduced.