  We have investigated the electronic structure of charged bilayer and trilayer
phoshporene using first-principles, density-functional-theory calculations. We
find that the effective dielectric constant for an external electric field
applied perpendicular to phosphorene layers increases with the charge density
and is twice as large as in an undoped system if the electron density is around
$5\times10^{13}$cm$^{-2}$. It is known that if few-layer phosphorene is placed
under such an electric field, the electron band gap decreases and if the
strength of the electric field is further increased, the band gap closes. We
show that the electronic screening due to added charge carriers reduces the
amount of this reduction in the band gap and increases the critical strength of
the electric field for gap closure. If the electron density is around
$4\times10^{13}$cm$^{-2}$, for example, this critical field for trilayer
phosphorene is 40\% higher than that for a charge-neutral system. The results
are directly relevant to experiments on few-layer phosphorene with top and
bottom electrical gates and / or with chemical dopants.
