Abstract: There is disclosed a heterojunction field effect transistor with an accumulation of majority carriers functioning as a high cut-off frequency device in which the transistor uses the properties of the same type GaAs/Al.sub.x Ga.sub.1-x As N-doped junctions with the GaAs being weakly doped. This is used to produce a high mobility of charges in the accumulation layer and is effected by a structure in which the source and drain regions are partially covered by the gate region which is in turn covered by an insulation layer and thereby reduces the access resistances and increases the transition frequencies.