  Two-dimensional (2D) semiconductors are promising channel materials for
next-generation field-effect transistors (FETs) thanks to their unique
mechanical properties and enhanced electrostatic control. However, the
performance of these devices can be strongly limited by the scattering
processes between carriers and phonons, usually occurring at high rates in 2D
materials. Here, we use quantum transport simulations calibrated on
first-principle computations to report on dissipative transport in antimonene
and arsenene $n$-type FETs at the scaling limit. We show that the widely-used
approximations of either ballistic transport or simple acoustic deformation
potential scattering result in large overestimation of the ON current, due to
neglecting the dominant intervalley and optical phonon scattering processes. We
additionally investigate valley engineering strategy [Nano Lett. \textbf{19},
3723 (2019)] to improve the device performance by removing the valley
degeneracy and suppressing most of the intervalley scattering channels via an
uniaxial strain along the zigzag direction. The method is applicable to other
similar 2D semiconductors characterized by multivalley transport.
