  We have fabricated and characterized a field-effect transistor in which an
electric field is applied through an encapsulated vacuum cavity and induces a
two-dimensional electron system on a hydrogen-passivated Si(111) surface. This
vacuum cavity preserves the ambient sensitive surface and is created via room
temperature contact bonding of two Si substrates. Hall measurements are made on
the H-Si(111) surface prepared in aqueous ammonium fluoride solution. We obtain
electron densities up to $6.5 \times 10^{11}$ cm$^{-2}$ and peak mobilities of
$\sim 8000$ cm$^{2}$/V s at 4.2 K.
