  Strain engineering can modulate the material properties of two-dimensional
(2D) semiconductors for electronic and optoelectronic applications. Recent
theory and experiments have found that uniaxial tensile strain can improve the
electron mobility of monolayer MoS$_2$, a 2D semiconductor, but the effects of
biaxial strain on charge transport are not well-understood in 2D
semiconductors. Here, we use biaxial tensile strain on flexible substrates to
probe the electron mobility in monolayer WS$_2$ and MoS$_2$ transistors. This
approach experimentally achieves ~2x higher on-state current and mobility with
~0.3% applied biaxial strain in WS$_2$, the highest mobility improvement at the
lowest strain reported to date. We also examine the mechanisms behind this
improvement through density functional theory simulations, concluding that the
enhancement is primarily due to reduced intervalley electron-phonon scattering.
These results underscore the role of strain engineering 2D semiconductors for
flexible electronics, sensors, integrated circuits, and other optoelectronic
applications.
