PURPOSE: To enable alternation of the characteristics of a high electron mobility transistor by providing means for introducing electromagnetic wave to the elctron supply layer at the lower controlling electrorde.

CONSTITUTION: A non-doped GaAs channel layer 2 and an n type AlGaAs electron supply layer 3 are sequentially formed on a GaAs substrate 1, and an input/ output electrode 5 and an annular controlling electrode 6 are formed. This semiconductor device is confined in a low temperature container 7 of approx. 77°K, and an electromagnetic wave incident window 8 is formed at the container 7. When the semiconductor device is normally OFF type, electromagnetic wave of shorter wavelength than far infrared ray is emitted to the layer 3, an excitation phenomenon occurs in the layer 3 by the emitting energy, and the elctron density in the electron storage layer is increased. The increased electron density is maintained even after the emission of the electromagnetic wave is stopped as long as the temperature is maintained at low temperature of approx. 77°K.