  High-mobility layered semiconductors have the potential to enable the
next-generation electronics and computing. This paper demonstrates that the
ultrahigh electron mobility observed in the layered semiconductor Bi$_2$O$_2$Se
originates from an incipient ferroelectric transition that endows the material
with a robust protection against mobility degradation by Coulomb scattering.
Based on first-principles calculations of electron-phonon interaction and
ionized impurity scattering, it is shown that the electron mobility of
Bi$_2$O$_2$Se can reach 10$^4$ to 10$^6$ cm$^2$V$^{-1}$s$^{-1}$ over a wide
range of realistic doping concentrations. Furthermore, a small elastic strain
of 1.7% can drive the material toward a unique interlayer ferroelectric
transition, resulting in a large increase in the dielectric permittivity and a
giant enhancement of the low-temperature electron mobility by more than an
order of magnitude. These results establish a new route to realize
high-mobility layered semiconductors via phase and dielectric engineering.
