PURPOSE: To obtain high operation velocity by reducing the forbidden band width of a semiconductor for forming a channel region smaller than that of a semiconductor for forming a source region.

CONSTITUTION: A field effect transistor has a semi-insulating gallium arsenide substrate 1, N type gallium arsenide layer 2, an N-type aluminum gallium arsenide layer 3, a gate electrode 4, a source electrode 5, and a drain electrode 6. In the state that a large energy gap is produced in the hetero boundary on the basis of the forbidden band width of the aluminum gallium arsenide and the forbidden band width of the gallium arsenide and a voltage is applied between the electrodes 5 and 6, the electrode implanted to the channel region from the source is abruptly accelerated at the source side of the channel region.