  The presence of finite bandgap and high mobility in semiconductor few-layer
black phosphorus offers an attractive prospect for using this material in
future two-dimensional electronic devices. Here we demonstrate for the first
time fully encapsulated ultrathin (down to bilayer) black phosphorus field
effect transistors in Van der Waals heterostructures to preclude their
stability and degradation problems which have limited their potential for
applications. Introducing monolayer graphene in our device architecture for
one-atom-thick conformal source-drain electrodes enables a chemically inert
boron nitride dielectric to tightly seal the black phosphorus surface. This
architecture, generally applicable for other sensitive two-dimensional
crystals, results in stable transport characteristics which are hysteresis free
and identical both under high vacuum and ambient conditions. Remarkably, our
graphene electrodes lead to contacts not dominated by thermionic emission,
solving the issue of Schottky barrier limited transport in the technologically
relevant two-terminal field effect transistor geometry.
