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Carrier transport in heavily doped extremely thin silicon- on-insulator (ETSOI) diffusion layers with SOI thickness of less than 10 nm was thoroughly studied. We found that electron mobility ( μe) in heavily doped ETSOI diffusion layer is totally different from μe in heavily doped bulk Si. In ETSOI diffusion layers with SOI thickness ranging from 5 nm to 10 nm μe is enhanced, compared with μe in heavily doped bulk Si. This enhancement is caused by the reduced number of ions which interact with carriers in ETSOI. On the other hand, in ETSOI with SOI thickness of less than 2 nm μe is degraded, compared with μe in heavily doped bulk Si. The degradation is primary due to the scattering induced by SOI thickness fluctuations. μe in heavily doped ETSOI with SOI thickness of less than 2 nm is further decreased as doping concentration increases, which results from the enhanced potential fluctuations by Coulomb potentials made by randomly distributed ions.