The two-dimensional electron gas which is formed at the interface in undoped GaAs/AlxGa1-xAs heterostructures by the electric field generated by a top gate is studied. The barrier region consists of an Al0.3Ga0.7As/Al0.5Ga0.5As superlattice to prevent undesirable tunneling through the barrier. Therefore, we are able to achieve high electron densities exceeding 1016 m-2 with sufficient small gate leakage currents in a MISFET like device for the first time. Despite the high electron density in the sample rather high mobilities of about 100 m2 V s-1 can be maintained at low temperatures. The population of the second subband is studied from Shubnikov-de Haas measurements in these devices. The role of intersubband scattering is important for understanding the magneto-transport experiments.