  Monolayer transition metal dichalcogenides (MX2) are promising candidates for
future electronics. Although the transport properties (e.g. mobility) at low
electric field have been widely studied, there are limited studies on
high-field properties, which are important for many applications. Particularly,
there is lack of understanding of the physical origins underlying the property
differences across different MX2. Here by combining first-principles
calculations with Monte Carlo simulations, we study the high-field electron
transport in defects-free unstrained and tensilely strained MX2 (M=Mo, W and
X=S, Se). We find that WS2 has the highest peak velocity (due to its smallest
effective mass) that can be reached at the lowest electric field (owing to its
highest mobility). Strain can increase the peak velocity by increasing the
scattering energy. After reaching the peak velocity, most MX2 demonstrates
negative differential mobility (NDM). WS2 shows the largest NDM among
unstrained MX2 due to the strongest effect of electron transfer from the
low-energy small-mass valley to the high-energy large-mass valley. The tensile
strain increases the valley separation, which on one hand suppresses the
electron transfer in WS2, on the other hand allows the electrons to access the
non-parabolic band region of the low-energy valley. The latter effect leads to
an NDM for electrons in the low-energy valley, which can significantly increase
the overall NDM at moderate strain. The valley-separation induced NDM in the
low-energy valley is found to be a general phenomenon. Our work unveils the
physical factors underlying the differences in high-field transport properties
of different MX2, and also identifies the most promising candidate as well as
effective approach for further improvement.
