  We investigate the screening effects of excess electrons in the doped layer
on the mobility of a GaAs two-dimensional electron system (2DES) with a modern
architecture using short-period superlattice (SL) doping. By controlling the
density of excess electrons in the SL with a top gate while keeping the 2DES
density constant with a back gate, we are able to compare 2DESs with the same
density but different degrees of screening using one sample. Using a
field-penetration technique and circuit-model analysis, we determine the
density of states and excess electron density in the SL, quantities directly
linked to the screening capability. The obtained relation between mobility and
excess electron density is consistent with the theory taking into account the
screening by the excess electrons in the SL. The quantum lifetime determined
from Shubnikov-de Haas oscillations is much lower than expected from theory and
did not show a discernible change with excess electron density.
