A semiconductor device includes a channel layer formed on a substrate, an insulating layer formed in contact with the channel layer, an impurity-doped first semiconductor layer formed on an opposite side of the insulating layer from the channel layer, an impurity-doped second semiconductor layer formed on an opposite side of the first semiconductor layer from the insulating layer, and a gate electrode formed on an opposite side of the second semiconductor layer from the first semiconductor layer. A quotient of an impurity density of the first semiconductor layer divided by a relative permittivity of the first semiconductor layer is greater than a quotient of an impurity density of the second semiconductor layer divided by a relative permittivity of the second semiconductor layer.