  A first order phase transition between a BCS phase and an insulating Mott
phase for a gas of spatially separated electrons and holes with tunable Coulomb
interaction and variable density is predicted. In the framework of a BCS-like
mean-field approach and a Landau expansion in terms of the pairing order
parameter the phase diagram is studied. This indicates several phases and phase
transitions, including an electron-hole plasma at low density and weak
interaction, an intermediate BCS phase with Cooper pairs and an electron-hole
plasma at high density and weak interaction. The insulating Mott phase appears
for the strong interaction and low temperatures. We briefly discuss the
possibilities to realize these phases in realistic systems such as coupled
quantum wells and graphene double layers.
