A semiconductor device has a so-called SOI structure in which an element is constituted by a semiconductor layer on an insulating surface, and the semiconductor layer is extremely thin as 5 nm to 30 nm. The semiconductor device is provided with a field effect transistor that includes in addition to such a semiconductor layer, a gate insulating layer with a thickness of 2 nm to 20 nm and a gate electrode, and a channel length is ten times or more and less than 40 times the thickness of the semiconductor layer. When the semiconductor layer is formed to be thin, the semiconductor device operates so as not to be easily influenced by a concentration of impurity imparting one conductivity type added to a channel formation region.