  Atomically thin (two-dimensional, 2D) semiconductors have shown great
potential as the fundamental building blocks for next-generation electronics.
However, all the 2D semiconductors that have been experimentally made so far
have room-temperature electron mobility lower than that of bulk silicon, which
is not understood. Here, by using first-principles calculations and
reformulating the transport equations to isolate and quantify contributions of
different mobility-determining factors, we show that the universally low
mobility of 2D semiconductors originates from the high 'density of
scatterings,' which is intrinsic to the 2D material with a parabolic electron
band. The density of scatterings characterizes the density of phonons that can
interact with the electrons and can be fully determined from the electron and
phonon band structures without knowledge of electron-phonon coupling strength.
Our work reveals the underlying physics limiting the electron mobility of 2D
semiconductors and offers a descriptor to quickly assess the mobility.
