  High-frequency (HF) conductivity ($\sigma_{hf}$) measured by an acoustical
method has been studied in GaAs/AlGaAs heterostructures in a linear and
nonlinear regime on acoustic power. It has been shown that in the quantum Hall
regime at magnetic fields corresponding to the middle of the Hall plateaus the
HF conductivity is determined by the sum of the conductivity of 2-dimensional
electrons in the high-mobility channel and the hopping conductivity of the
electrons in the doped thick AlGaAs layer. The dependence of these
conductivities on a temperature is analyzed. The width of the Landau level
broadened by the impurity random potential is determined.
