  Black phosphorus (BP), a two-dimensional (2D) van der Waals layered material
composed of phosphorus atoms, has been one of the most actively studied 2D
materials in recent years due to its tunable energy band gap (tunable even to a
negative value) and its highly anisotropic electronic structure. Depending on
the sign of the band gap tuning parameter, few-layer BP can be in a gapped
insulator phase, gapless Dirac semimetal phase, or gapless semi-Dirac
transition point between the two phases. Using the fully anisotropic multiband
Boltzmann transport theory, we systematically study the dc conductivity of
few-layer BP as a function of the carrier density and temperature by varying
the band gap tuning parameter, and determine the characteristic density and
temperature dependence corresponding to each phase.
