  We report experiment and theory on an ambipolar gate-controlled Si-vacuum
field effect transistor (FET) where we study electron and hole (low-temperature
2D) transport in the same device simply by changing the external gate voltage
to tune the system from being a 2D electron system at positive gate voltage to
a 2D hole system at negative gate voltage. The electron (hole) conductivity
manifests strong (moderate) metallic temperature dependence with the
conductivity decreasing by a factor of 8 (2) between 0.3 K and 4.2 K with the
peak electron mobility ($\sim 18$ m$^2$/Vs) being roughly 20 times larger than
the peak hole mobility (in the same sample). Our theory explains the data well
using RPA screening of background Coulomb disorder, establishing that the
observed metallicity is a direct consequence of the strong temperature
dependence of the effective screened disorder.
