Influence of generated carriers originated by hot electrons and radiations is described in MOS structures. A holding time degradation in a dynamic RAM is first presented with the two-step impact-ionization model, which is verified by a series of experiments. The impact ionization rate in the high electric field near the drain is also calculated by a two dimensional analysis, which leads to an accurate substrate current estimation. Effects of hot electron injection into the gate oxide, and of trapping are then studied utilizing a stacked gate MOS structure. From those results, design constraints in small geometry MOS LSI's are discussed.