The effective electron mobility was measured as a function of surface field in polysilicon thin film transistors having the following three types of gate dielectrics; silicon dioxide deposited by low temperature (350 °C) plasma-enhanced chemical vapor deposition (PECVD), low temperature (400 °C) nitrogen-rich PECVD silicon nitride and high temperature (1050 °C) thermally grown silicon dioxide. At low surface fields, the maximum true effective electron mobility was 40 ± 3 cm2 V-1 s-1 in all devices independent of the type of gate dielectric, indicating that the quality of the interface is the same. However, at high surface fields a stronger degradation of the mobility was observed in devices having the thermally grown silicon dioxide as gate dielectric, indicating the presence of surface roughness within the interfacial region. The polysilicon structure was studied by transmission electron microscopy in order to ensure that our electrical results were not influenced by variations in polysilicon grain size. It was found that the grain size was the same in all devices including those that were oxidized at high temperature. © 1992.