PURPOSE: To prevent fluctuation in characteristics from occurring due to etching of a semiconductor surface, by forming side walls of lamination structure on sides of a gate electrode also in a FET of LDD structure.

CONSTITUTION: A semiconductor substrate 1 is provided with an n-type regional channel 2 positioned in the center, and source and drain regions which are composed of n+-type regions 3 adjacent to the right and left sides of the channel 2 and n++-type regions 4 further adjacent to the regions 3. A gate electrode 5 is formed on a substrate main surface of the channel 2 by Schottky junction. When three-layer side walls 10 made of a first laminated film 6 and a second Iaminated film 7 and a third laminated film 9 are formed on sides of the gate electrode 5 so as to form a MESFET of LDD structure, and when plasma etch ing of laminated parts 12 is performed while luminous spectra are monitored, then the etching can be finished at the time of detecting luminous spectrum which is due to silicon nitride in the second laminated film 7.