  We report on the electronic transport properties of black phosphorus and
analyze them using a two-carrier model in a wide range of pressure up to 2.5
GPa. In semiconducting state at 0.29 GPa, the remarkable non-linear behavior in
the Hall resistance is reasonably reproduced by assuming the coexistence of two
kinds of hole with different densities and mobilities. On the other hand,
two-carrier analyses of the magnetotransport properties above 1.01 GPa suggest
the coexistence of high mobility electron and hole carriers that have almost
the same densities, i.e., nearly compensated semimetallic nature of black
phosphorus. In the semimetallic state, analyses of both the two-carrier model
and quantum oscillations indicate a systematic increase in the carrier
densities as pressure increases. An observed sign inversion of Hall resistivity
at low magnetic fields suggests the existence of high mobility electrons
(\sim105 cm2 V-1 s-1) that is roughly ten times larger than that of holes, in
the semimetallic black phosphorus. We conclude that the extremely large
positive magnetoresistance that has been observed in semimetallic state cannot
be reproduced by a conventional two-carrier model.
