  The intrinsic carrier transport dynamics in phosphorene is theoretically
examined. Utilizing a density functional theory treatment, the low-field
mobility and the saturation velocity are characterized for both electrons and
holes in the monolayer and bilayer structures. The analysis clearly elucidates
the crystal orientation dependence manifested through the anisotropic band
structure and the carrier-phonon scattering rates. In the monolayer, the hole
mobility in the armchair direction is estimated to be approximately five times
larger than in the zigzag direction at room temperature (460 cm$^2$/Vs vs. 90
cm$^2$/Vs). The bilayer transport, on the other hand, exhibits a more modest
anisotropy with substantially higher mobilities (1610 cm$^2$/Vs and 760
cm$^2$/Vs, respectively). The calculations on the conduction-band electrons
indicate a comparable dependence while the characteristic values are generally
smaller by about a factor of two. The variation in the saturation velocity is
found to be less pronounced. With the anticipated superior performance and the
diminished anisotropy, few-layer phosphorene offers a promising opportunity
particularly in p-type applications.
