Abstract: Occurrence of high field domain in the conventional Gunn diode is prevented by covering a solid body such as a semiconductor element partially or wholly by a dielectric member or by a control element such as a metallic layer coupled reactively with the solid body through a dielectric member, whereby a solid state element having a negative differential conductivity is obtained. Such a type of negative-resistance solid state element, together with its various modes of embodimental construction disclosed herein, affords a superior solid state element which is applicable to amplifiers, oscillators, logic memories, and the like of millimeter or submillimeter bands.