An electromagnetic wave detector formed of semiconductor materials includes at least one quantum well in which there is provided a fine layer of a material with a gap width that is smaller than that of the quantum well layer. For example, in the case of a GaAlAs/GaAs/GaAlAs, there is provision for a fine layer of InAs. In this way, the difference of energy levels between the two permitted levels is increased and detection of short wavelengths may be accomplished.