We consider the formation of exciton bound states between electrons and holes in a two-dimensional layer. The medium modifications of two-particle properties are obtained from a systematic Green function approach. Compared with recent calculations for a GaAs heterostructure [Solid State Commun. 108 (1998) 199], where only screening has been considered, the importance of Pauli blocking on the shift of the binding energy of excitons is shown. The ionization degree is evaluated self-consistently for different parameter values of surface density and temperature for a GaAs heterostructure. A Mott transition is found at low temperatures and high densities. (C) 2002 Elsevier Science Ltd. All rights reserved.