PURPOSE: To enable a semiconductor device to operate at a high speed by only controlling signal carriers while removing the other carriers from a space where carriers are present.

CONSTITUTION: Electrodes 3 and 5 are set highest in potential, and an electrode 4 is so set in potential as to enable a traveling layer 1 of ballistic electrons conducive to signals to be kept intermediate in potential. A current flowing through the electrodes 3 and 5 is set higher in intensity as compared with a current which flows through an inlet 2. Ballistic electron injected through the inlet 3 are absorbed into the electrode 3, and even ballistic electrons scattered in the traveling layer 1 are quickly absorbed into the electrode 3 or the electrode 5. That is, electrons other than electrons directly conducive to signals are removed in the ballistic electron traveling layer 1. By this setup, by the potential of the electrode 4, a semiconductor device can be protected against an adverse effect caused by the variation of the electrodes 3 and 5 in potential.