In this work we perform a variational calculation of the electron binding energy in an AlAs GaAs quantum well. We take into account the fact that the minima of the conduction band in those materials occur in points of the Brillouin zone with different symmetries (Γ for GaAs and X for AlAs). Therefore, for thin quantum wells the first subband becomes a hybrid state of those Bloch states in the points Γ and X. The hybridization is included in the variational function by means of contour conditions for the first subband wave function. In the present model we are able to treat impurities anywhere inside the well. At well widths smaller than 37 Å the GaAs layer becomes a barrier for the electrons, so the model is no longer valid. We also compare the results obtained with the Γ- X hybridization with the results obtained by just taking the barrier at the interfaces as the energy differences between the Γ level inside the GaAs layer and the X level across the interface.