This work reports the first nanocrystalline SnON (7.6% nitrogen content) nanosheet n-type Field-Effect Transistor (nFET) with the transistor’s effective mobility (µeff) as high as 357 and 325 cm2/V-s at electron density (Qe) of 5 × 1012cm−2and an ultra-thin body thickness (Tbody) of 7 nm and 5 nm, respectively. At the same Tbodyand Qe, these µeffvalues are significantly higher than those of single-crystalline Si, InGaAs, thin-body Si-on-Insulator (SOI), two-dimensional (2D) MoS2and WS2. The new discovery of a slower µeffdecay rate at high Qethan that of the SiO2/bulk-Si universal curve was found, owing to a one order of magnitude lower effective field (Eeff) by more than 10 times higher dielectric constant (κ) in the channel material, which keeps the electron wave-function away from the gate-oxide/semiconductor interface and lowers the gate-oxide surface scattering. In addition, the high µeffis also due to the overlapped large radius s-orbitals, low 0.29 moeffective mass (me*) and low polar optical phonon scattering. SnON nFETs with record-breaking µeffand quasi-2D thickness enable a potential monolithic three-dimensional (3D) integrated circuit (IC) and embedded memory for 3D biological brain-mimicking structures.