Current-voltage (I-V), ac capacitance-voltage (C-V), and conductance-voltage (G-V) measurements have been made of the effect of magnetic fields up to 15 T on tunneling at 1.6 K from accumulation layers on n-type GaAs. The samples used are n - GaAs"AlxGa1-xAs"n +-GaAs capacitors in which the GaAs/AlxGa1-xAs barrier height is low enough, 1/40.3 eV, and the AlxGa1-xAs is thin enough, 1/420 nm, that direct tunneling of electrons occurs. Magnetic freezeout of carriers in n - GaAs for H4 T can be determined from C-V curves. For n+-GaAs biased positive, VG>0, a quantized accumulation layer forms at the n - GaAs/AlxGa1-xAs interface. Structure is observed in I-V, C-V, and G-V curves due to tunneling from Landau levels in the accumulation layer. Two samples are compared in detail. In one sample the Landau-level structure is relatively simple. In the other, spin splitting is observed in the first three Landau levels. The dependence of the surface concentration of electrons on VG is determined from tunneling currents at fixed VG and varying magnetic field. A second subband in the accumulation layer begins to be populated when the surface concentration 7×1011/cm2. This is the first example of a system in which it is possible to use tunneling to examine the formation of an accumulation layer in a semiconductor, going from depletion through flat-band into accumulation, by applying a bias voltage. © 1985 The American Physical Society.