  We realize p-p'-p junctions in few-layer black phosphorus (BP) devices, and
use magneto-transport measurements to study the equilibration and transmission
of edge states at the interfaces of regions with different charge densities. We
observe both full equilibration, where all edge channels equilibrate and are
equally partitioned at the interfaces, and partial equilibration, where only
equilibration only takes place among modes of the same spin polarization.
Furthermore, the inner p'-region with low-doping level in the junction can
function as a filter for highly doped p-regions which demonstrates gate-tunable
transmission of edge channels.
