  The ground state of two-dimensional (2D) electron systems with equal low
densities of electrons and holes in nearby layers is an exciton fluid. We show
that a reservoir for excitons can be established by contacting the two layers
separately and maintaining the chemical potential difference at a value less
than the spatially indirect band gap. Equilibration between the exciton fluid
and the contacts proceeds via a process involving virtual intermediate states
in which an unpaired electron or hole occupies a free carrier state in one of
the 2D layers. We derive an approximate relationship between the
exciton-contact equilibration rate and the electrical conductances between the
contacts and individual 2D layers when the contact chemical potentials align
with the free-carrier bands, and explain how electrical measurements can be
used to measure thermodynamic properties of the exciton fluid.
