▎ 摘　　要

We measure drift velocity in monolayer 8 graphene encapsulated by hexagonal boron nitride (hBN), probing its dependence on carrier density and temperature. Due to the high mobility (>5 x 10(4) cm(2)/V/s) of our samples, the drift velocity begins to saturate at low electric fields (similar to 0.1 V/mu m) at room temperature. Comparing results to a canonical drift velocity model, we extract room-temperature electron saturation velocities ranging from 6 X 10(7) cm/s at a low carrier density of 8 X 10(11) cm(-2) to 2.7 X 10(7) cm/s at a higher density of 4.4 X 10(12) cm(-2). Such drift velocities are much higher than those in silicon (similar to 10(7) cm/s) and in graphene on SiO2, likely due to reduced carrier scattering with surface optical phonons whose energy in hBN (>100 meV) is higher than that in other substrates.