A characterization of low frequency noise in submicron N-MOSFETs is presented. For large devices, it is found that 1/f noise results from carrier number fluctuations. The slow oxide interface trap density deduced from noise data is found around 10<sup>16</sup> eV<sup>-1</sup> cm<sup>-3</sup> in agreement with state-of-the-art gate oxides. Submicron devices present R.T.S noise and exhibit three independent active traps in saturation range, from weak to strong inversion. All of these traps have been found as acceptor type centers. Their activity ranges, their maximum of activities and their positions in the oxide from the Si-SiO<sub>2</sub> interface have been obtained by the study of emission and capture times against gate voltage. It is shown existing overlap in trap activities for particular gate bias ranges. This overlap is confirmed by the observation of multi level R.T.S in time and frequency domains. For each trap, the number of R.T.S events is explained using the trap occupation probability. Finally, the global R.T.S behavior of devices, including the whole trap activities from weak to strong inversion, could be described using the simple R.T.S model classically used for a single oxide trap. This global study shows a simple method to determine R.T.S impact, and describes perfectly multi-trap activity.