We have realized a high‐switching‐speed Si S‐shaped negative‐resistance diode having a thin MIS structure where the thickness of the SiO2layer is less than 100 Å. It has ap‐n‐I(insulator)‐M(metal) structure in contrast with a Shockley diode. The threshold and sustaining voltages are about 20 and 2–3 V, respectively. The regenerative nature of this device may be related to the minority carrier injection into the interface between thenlayer and the insulating layer, and electron trapping in the interface states.