  We report electrical transport measurements on GaAs/AlGaAs based
electron-hole bilayers. These systems are expected to make a transition from a
pair of weakly coupled two-dimensional systems to a strongly coupled exciton
system as the barrier between the layers is reduced. Once excitons form,
phenomena such as Bose-Einstein condensation of excitons could be observed. In
our devices, electrons and holes are confined in double quantum wells, and
carriers in the devices are induced with top and bottom gates leading to
variable density in each layer. Separate contact to each layer allows Coulomb
drag transport measurements where current is driven in one layer while voltage
is measured in the other. Coulomb drag is sensitive to interlayer coupling and
has been predicted to provide a strong signature of exciton condensation. Drag
measurement on EHBLs with a 30 nm barrier are consistent with drag between two
weakly coupled 2D Fermi systems where the drag decreases as the temperature is
reduced. When the barrier is reduced to 20 nm, we observe a consistent increase
in the drag resistance as the temperature is reduced. These results indicate
the onset of a much stronger coupling between the electrons and holes which
leads to exciton formation and possibly phenomena related to exciton
condensation.
