  Two-dimensional (2D) materials have shown great potential in applications as
transistors, where thermal dissipation becomes crucial because of the
increasing energy density. Although thermal conductivity of 2D materials has
been extensively studied, interactions between nonequilibrium electrons and
phonons, which can be strong when high electric fields and heat current
coexist, are not considered. In this work, we systematically study the electron
drag effect, where nonequilibrium electrons impart momenta to phonons and
influence the thermal conductivity, in 2D semiconductors using ab initio
simulations. We find that, at room temperature, electron drag can significantly
increase thermal conductivity by decreasing phonon-electron scattering in 2D
semiconductors, while its impact in three-dimensional (3D) semiconductors is
negligible. We attribute this difference to the large electron-phonon
scattering phase space and higher contribution to thermal conductivity by
drag-active phonons. Our work elucidates the fundamental physics underlying
coupled electron-phonon transport in materials of various dimensionalities.
