PURPOSE:To obtain a thermionic emitting SIT in which carriers can move at a thermionic speed without being subjected to scattering of a crystal lattice, by forming a gate of a semiconductor having a wider forbidden band than a channel while providing a space between the source and the gate less than the mean free path of the carriers. CONSTITUTION:In a compound semiconductor such as GaAs which cannot provide a good insulation film, a gate is not formed of GaAs but of a mixed crystal having a wider forbidden band than GaAs, for example of Ga1-xAlxAs so that the gate is similar to an insulated gate. Further, as shown in the drawing, a channel 2 is provided between a source 3 and a drain 1 so as to pass between gate regions 4. In the channel 2, the distance from the source 3 to the true gate 4 is smaller than a mean free path of electrons, so that an electrostatic transistor of thermionic-emitting type is obtained. The space between and the thickness of the gate regions and the impurity density in the channel region can be altered to provide an either normally-ON or normally-OFF operating transistor. The impurity is preferably undoped so as to inhibit the implantation of electrons.