A semiconductor device comprising: a first conductive layer over a substrate; a first insulating layer over the first conductive layer; an oxide semiconductor layer over the first insulating layer overlapping with the first conductive layer; a second conductive layer in contact with the oxide semiconductor layer; a third conductive layer in contact with the oxide semiconductor layer; a second insulating layer over the oxide semiconductor layer, the second conductive layer and the third conductive layer; and a fourth conductive layer over the second insulating layer and overlaps with a part of the oxide semiconductor layer; and wherein the first conductive layer has a function of controlling an electric field in the oxide semiconductor layer, wherein the second conductive layer functions as one of a source electrode and a drain electrode, wherein the third conductive layer functions as the other of the source electrode and the drain electrode, wherein the fourth conductive layer functions as a gate electrode, wherein the oxide semiconductor layer comprises a first region and a second region, the first region being located over the second region, wherein the first region of the oxide semiconductor layer and the second region of the oxide semiconductor layer contain indium, gallium and zinc, wherein the first region comprises a crystal, and a c-axis of the crystal is substantially perpendicular to a plane of the substrate or a surface of the oxide semiconductor layer, wherein the second region has a lower crystallinity than the first region, and wherein the crystal in the first region in the oxide semiconductor layer is a crystal of In 2 Ga 2 ZnO 7. a first conductive layer over a substrate; a first insulating layer over the first conductive layer; an oxide semiconductor layer over the first insulating layer overlapping with the first conductive layer; a second conductive layer in contact with the oxide semiconductor layer; a third conductive layer in contact with the oxide semiconductor layer; a second insulating layer over the oxide semiconductor layer, the second conductive layer and the third conductive layer; and a fourth conductive layer over the second insulating layer and overlaps with a part of the oxide semiconductor layer; and wherein the first conductive layer has a function of controlling an electric field in the oxide semiconductor layer, wherein the second conductive layer functions as one of a source electrode and a drain electrode, wherein the third conductive layer functions as the other of the source electrode and the drain electrode, wherein the fourth conductive layer functions as a gate electrode, wherein the oxide semiconductor layer comprises a first region and a second region, the first region being located over the second region, wherein the first region of the oxide semiconductor layer and the second region of the oxide semiconductor layer contain indium, gallium and zinc, wherein the first region comprises a crystal, and a c-axis of the crystal is substantially perpendicular to a plane of the substrate or a surface of the oxide semiconductor layer, wherein the second region has a lower crystallinity than the first region, and wherein the crystal in the first region in the oxide semiconductor layer is a crystal of In 2 Ga 2 ZnO 7.