Effective electron mobility has been studied in MOSFETs with ultrathin silicon nitride/oxynitride stacked gate dielectrics formed by rapid thermal chemical vapor deposition. The mobility in these devices is degraded compared to those with SiO2 (the universal mobility curve). Quantitative analysis suggests that the degradation is due to coulombic scattering from both bulk charges in the dielectric and interface trapped charges. Finally, after investigating the impact of process parameters on mobility, it is concluded that interfacial oxynitride grown at higher pressure in nitric oxide is advantageous for achieving thinner effective stack thicknesses and for preserving electron mobility. (C) 2002 Elsevier Science Ltd. All rights reserved.