  The formation of the roton-maxon excitation spectrum and the roton
instability effect for a weakly correlated Bose gas of dipolar excitons in a
semiconductor layer are predicted. The stability diagram is calculated.
According to our numerical estimations, the threshold of the roton instability
for Bose-Einstein condensed exciton gas with roton-maxon spectrum is achievable
experimentally, e.g., in GaAs semiconductor layers.
