PURPOSE: To decrease scattering probability from a base subband to a first exciting subband and to increase mobility by so deciding a thickness of a quantum well layer as to increase an energy difference between the base subband and the first exciting subband larger than energy of optical phonon.

CONSTITUTION: An n-type AlGaAs layer 13 is laminated on an i-type GaAs buffer layer 11 formed on a semi-insulating GaAs substrate 12. I-type GaAs quantum well layers 14 and an i-type AlGaAs barrier layers 15 are alternately laminated thereon with the layers 14 as a lowermost layer and an uppermost layer. A total thickness of the multiple quantum well is formed thinner than a thickness in which the energy difference between a base suband of a conduction band to be formed on the well and the first exciting subband becomes the same as energy of optical phonon of a bulk of a quantum well layer material. Thus, an optical phonon scattering is reduced, and electron mobility can be increased.