  We introduce an elementary model for the electrostatic self-consistent
potential in a two-dimensional electron gas. By considering the perpendicular
degree of freedom arising from the electron tunneling out of the system plane,
we predict a threshold carrier density above which this effect is relevant. The
predicted value agrees remarkably well with the onset for the insulator to
quasi-metallic transition recently observed in several experiments in SiO2-Si
and AlGaAs-GaAs heterojunctions.
