A semiconductor device which includes an oxide semiconductor layer, a source electrode and a drain electrode electrically connected to the oxide semiconductor layer, a gate insulating layer covering the oxide semiconductor layer, the source electrode, and the drain electrode, and a gate electrode over the gate insulating layer is provided. The thickness of the oxide semiconductor layer is greater than or equal to 1 nm and less than or equal to 10 nm. The gate insulating layer satisfies a relation where ∈r/d is greater than or equal to 0.08 (nm−1) and less than or equal to 7.9 (nm−1) when the relative permittivity of a material used for the gate insulating layer is ∈r and the thickness of the gate insulating layer is d. The distance between the source electrode and the drain electrode is greater than or equal to 10 nm and less than or equal to 1 μm.