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MOSFET's with ultrathin (5 to 8.5 nm) silicon oxynitride gate film prepared by low-pressure rapid thermal chemical vapor deposition (RTCVD) using SiH/sub 4/, N/sub 2/O and NH/sub 3/ gases, are studied by low-frequency noise measurements (1 Hz up to 5 kHz). The analysis takes into account the correlated mobility fluctuations induced by those of the interfacial oxide charge. The nitrogen concentration, determined from SIMS analysis, varies from 0 to 11% atomic percentage. A comparison of the electrical properties between thermal and silicon oxynitride films is presented. The increasing LF noise signal with nitrogen atomic percentage indicates the presence of a higher density of slow interface traps with increasing nitrogen incorporation. Besides, a higher Coulomb scattering rate due to the nitridation induced interface charge explains reasonably well the degradation of the low field mobility after nitridation.